Link State Vector Routing (LSVR) Working Group
Internet Engineering Task Force (IETF) K. Patel
Internet-Draft
Request for Comments: 0000 Arrcus, Inc.
Intended status:
Category: Standards Track A. Lindem
Expires: 27 July 2025
ISSN: 2070-1721 LabN Consulting, LLC
S. Zandi
LinkedIn
W. Henderickx
Nokia
23 January
June 2025
BGP Link-State Shortest Path First (SPF) Routing
draft-ietf-lsvr-bgp-spf-51
Abstract
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified L3 (Layer 3) Layer 3 (L3) routing. Furthermore, requirements for
operational simplicity has led many of these MSDCs to converge on BGP
as their single routing protocol for both their fabric routing and
their Data
Center Interconnect (DCI) routing. This document describes
extensions to BGP to for use with BGP - Link-State (BGP-LS) distribution
and the Shortest Path First (SPF) algorithm. In doing this, it
allows BGP to be efficiently used as both the underlay protocol and
the overlay protocol in MSDCs.
Status of This Memo
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Internet-Drafts are draft documents valid the IETF community. It has
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This Internet-Draft will expire on 27 July 2025.
https://www.rfc-editor.org/info/rfc0000.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4
1.3. Document Overview . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Base BGP Protocol Relationship . . . . . . . . . . . . . . . 6
3. BGP - Link-State (BGP-LS) Relationship . . . . . . . . . . . . 7
4. BGP SPF Peering Models . . . . . . . . . . . . . . . . . . . 8
4.1. BGP Single-Hop Peering on Network Node Connections . . . 8
4.2. BGP Peering Between Directly-Connected Directly Connected Nodes . . . . . . 8
4.3. BGP Peering in Route-Reflector or Controller Topology . . 9
5. BGP Shortest Path Routing (SPF) Protocol Extensions . . . . . 9
5.1. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . 10
5.1.1. BGP-LS-SPF NLRI TLVs . . . . . . . . . . . . . . . . 10
5.1.2. BGP-LS Attribute . . . . . . . . . . . . . . . . . . 10
5.2. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . 11
5.2.1. Node NLRI Usage . . . . . . . . . . . . . . . . . . . 11
5.2.1.1. BGP-LS-SPF Node NLRI Attribute SPF Status TLV . . 11
5.2.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . 12
5.2.2.1. BGP-LS Link NLRI Address Family Link Descriptor TLV . . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV . . 14
5.2.3. IPv4/IPv6 Prefix NLRI Usage . . . . . . . . . . . . . 15
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV . . . . . . . . . . . . . . . . . . . . . . . . 15
5.2.4. BGP-LS Attribute Sequence Number TLV . . . . . . . . 16
5.3. BGP-LS-SPF End of RIB (EoR) Marker . . . . . . . . . . . 17
5.4. BGP Next-Hop Information . . . . . . . . . . . . . . . . 17
6. Decision Process with the SPF Algorithm . . . . . . . . . . . . . 17
6.1. BGP SPF NLRI Selection . . . . . . . . . . . . . . . . . 18
6.1.1. BGP Self-Originated NLRI . . . . . . . . . . . . . . 19
6.2. Dual Stack Support . . . . . . . . . . . . . . . . . . . 20
6.3. SPF Calculation based Based on BGP-LS-SPF NLRI . . . . . . . . 20
6.4. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 25
6.5. NLRI Advertisement . . . . . . . . . . . . . . . . . . . 25
6.5.1. Link/Prefix Failure Convergence . . . . . . . . . . . 25
6.5.2. Node Failure Convergence . . . . . . . . . . . . . . 26
7. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 26
7.1. Processing of BGP-LS-SPF TLVs . . . . . . . . . . . . . . 26
7.2. Processing of BGP-LS-SPF NLRIs . . . . . . . . . . . . . 28
7.3. Processing of BGP-LS Attribute . . . . . . . . . . . . . 28 Attributes
7.4. BGP-LS-SPF Link State Link-State NLRI Database Synchronization . . . 28
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
8.1. BGP-LS-SPF Allocation in the SAFI Parameters Values Registry . . . . 28
8.2. BGP-LS-SPF Assignments to in the BGP-LS NLRI and Attribute TLV
TLVs Registry . . . . . . . . . . . . . . . . . . . . . . . . 28
8.3. BGP-LS-SPF Node NLRI Attribute SPF Status TLV Status
Registry . . . . . . . . . . . . . . . . . . . . . . . . 29
8.4. BGP-LS-SPF Link NLRI Attribute SPF Status TLV Status
Registry . . . . . . . . . . . . . . . . . . . . . . . . 30
8.5. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV Status
Registry . . . . . . . . . . . . . . . . . . . . . . . . 30
8.6. Assignment in the BGP Error (Notification) Code Assignment . . . . . . . . 31 Codes Registry
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10. Management Considerations . . . . . . . . . . . . . . . . . . 32
10.1. Configuration . . . . . . . . . . . . . . . . . . . . . 32
10.2. Link Metric Configuration . . . . . . . . . . . . . . . 32
10.3. Unnumbered Link Configuration . . . . . . . . . . . . . 32
10.4. Adjacency End-of-RIB (EOR) Marker Requirement . . . . . 32
10.5. backoff-config . . . . . . . . . . . . . . . . . . . . . 33
10.6. BGP-LS-SPF NLRI Readvertisement Delay . . . . . . . . . 33
10.7. Operational Data . . . . . . . . . . . . . . . . . . . . 33
10.8. BGP-LS-SPF Address Family Session Isolation . . . . . . 33
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 34
12.
10.9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
13.
10.10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 35
14.
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1.
11.1. Normative References . . . . . . . . . . . . . . . . . . 35
14.2. Informational
11.2. Informative References . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified L3 (Layer 3) Layer 3 (L3) routing. Furthermore, requirements for
operational simplicity has led many of these MSDCs to converge on BGP
[RFC4271] as their single routing protocol for both their fabric routing
and their Data Center Interconnect (DCI) routing [RFC7938]. This document
describes an alternative solution which that leverages BGP-
LS BGP-LS [RFC9552] and
the Shortest Path First (SPF) algorithm used by Internal Gateway
Protocols (IGPs).
This document leverages both the BGP protocol [RFC4271] and the BGP-
LS [RFC9552] extensions. BGP-LS
extensions [RFC9552]. The relationship, relationship as well as the scope of
changes is are described respectively in Section Sections 2 and Section 3. 3, respectively. The
modifications to [RFC4271] for BGP SPF described herein only apply to
IPv4 and IPv6 as underlay unicast Subsequent Address Families Family
Identifiers (SAFIs). Operations for any other BGP SAFIs are outside
the scope of this document.
This solution avails the benefits of both BGP and SPF-based IGPs.
These include TCP-based flow-control, no periodic link-state refresh,
and completely incremental NLRI Network Layer Reachability Information
(NLRI) advertisement. These advantages can reduce the overhead in
MSDCs where there is a high degree of Equal
Cost Multi-Path Equal-Cost Multipath (ECMP) load-balancing.
load balancing. Additionally, using an SPF-
based SPF-based computation can
support fast convergence and the computation of Loop-Free
Alternatives (LFAs). The SPF LFA extensions defined in [RFC5286] can
be similarly applied to BGP SPF calculations. However, the details
are a matter of implementation detail and out of scope for this
document. Furthermore, a BGP-based solution lends itself to multiple
peering models including those incorporating route- route reflectors
[RFC4456] or controllers.
1.1. Terminology
This specification reuses terms defined in section Section 1.1 of [RFC4271].
Additionally, this document introduces the following terms:
BGP SPF Routing Domain: A set of BGP routers that are under a single
administrative domain and that exchange link-state information
using the BGP-LS-SPF SAFI and compute routes using that use BGP SPF SPF, as
described herein.
BGP-LS-SPF NLRI: This refers to The BGP-LS Network Layer Reachability Information
(NLRI) that is being advertised in the BGP-LS-SPF SAFI
(Section 5.1) and is being used for BGP SPF route computation.
Dijkstra Algorithm: An algorithm for computing the shortest path
from a given node in a graph to every other node in the graph.
Prefix NLRI: In the context of BGP SPF, this term refers to both or
either the IPv4
Topology Prefix NLRI and/or the IPv6 Topology Prefix NLRI.
1.2. BGP Shortest Path First (SPF) Motivation
Given that [RFC7938] already describes how BGP could be used as the
sole routing protocol in an MSDC, one might question the motivation
for defining an alternative BGP deployment model when a mature
solution exists. For both alternatives, BGP offers the operational
benefits of a single routing protocol as opposed to the combination
of an IGP for the underlay and BGP as an the overlay. However, BGP SPF
offers some unique advantages above and beyond standard BGP path-
vector routing. With BGP SPF, the simple single-hop peering model
recommended in section Section 5.2.1 of [RFC7938] is augmented with peering
models requiring fewer BGP sessions.
A primary advantage is that all BGP speakers in the BGP SPF routing
domain have a complete view of the topology. This allows support for
ECMP, IP fast-reroute (e.g., Loop-Free Alternatives) Alternatives (LFAs) [RFC5286],
Shared Risk Link Groups (SRLGs) [RFC4202], and other routing
enhancements without advertisement of additional BGP paths [RFC7911]
or other extensions.
With the BGP SPF decision process as defined in Section 6, NLRI
changes can be disseminated throughout the BGP routing domain much
more rapidly. The added advantage of BGP using TCP for reliable
transport leverages TCP's inherent flow-control and guaranteed in-
order delivery.
Another primary advantage is a potential reduction in NLRI
advertisement. With standard BGP path-vector routing, a single link
failure may impact 100s or 1000s of prefixes and result in the
withdrawal or re-advertisement readvertisement of the attendant NLRI. With BGP SPF,
only the BGP speakers originating the link NLRI need to withdraw the
corresponding BGP-LS-SPF Link NLRI. Additionally, the changed NLRI
is advertised immediately as opposed to normal BGP where it is only
advertised after the best route selection. These advantages provide
NLRI dissemination throughout the BGP SPF routing domain with
efficiencies similar to link-state protocols.
With controller and route-reflector peering models, BGP SPF
advertisement and distributed computation require a minimal number of
sessions and copies of the NLRI since as only the latest version of the
NLRI from the originator is required (see Section 4). Given that
verification of whether or not to advertise a link (with a BGP-LS-SPF
Link NLRI) is done outside of BGP, each BGP speaker only needs as
many sessions and copies of the NLRI as required for redundancy.
Additionally, a controller could inject topology (i.e., BGP-LS-SPF
NLRI) that is learned outside the BGP SPF routing domain.
Given that BGP-LS NLRI is already defined [RFC9552], this
functionality can be reused for BGP-LS-SPF NLRI.
Another advantage of BGP SPF is that both IPv6 and IPv4 can be
supported using the BGP-LS-SPF SAFI with the same BGP-LS-SPF Link
NLRIs. In many MSDC fabrics, the IPv4 and IPv6 topologies are
congruent (refer to Section 5.2.2). Although However, beyond the scope of
this document, BGP-LS-SPF NLRI multi-topology extensions could be
defined to support separate IPv4, IPv6, unicast, and multicast
topologies while sharing the same NLRI.
Finally, the BGP SPF topology can be used as an underlay for other
BGP SAFIs (using the existing model) and realize all the above
advantages.
1.3. Document Overview
The
This document begins with sections Section 2 defining the precise relationship
that BGP SPF has with both the base BGP protocol [RFC4271]
(Section 2) and Section 3
defining the BGP - Link-State (BGP-LS) extensions [RFC9552]
(Section 3). [RFC9552]. The BGP
peering models, models as well as their respective trade-offs are then
discussed in Section 4. The remaining sections, which make up the
bulk of the document, define the protocol enhancements necessary to
support BGP SPF including BGP-LS Extensions extensions (Section 5), replacement
of the base BGP decision process with the SPF computation
(Section 6), and BGP SPF error handling (Section 7).
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Base BGP Protocol Relationship
With the exception of the decision process, the BGP SPF extensions
leverage the BGP protocol [RFC4271] without change. This includes
the BGP protocol Finite State Machine, BGP messages and their
encodings, the processing of BGP messages, BGP attributes and path
attributes, BGP NLRI encodings, and any error handling defined in
[RFC4271], [RFC4760], and [RFC7606].
Due to the changes to in the decision process, there are mechanisms and
encodings that are no longer applicable. Unless explicitly specified
in the context of BGP SPF, all optional path attributes SHOULD NOT be
advertised. If received, all path attributes MUST be accepted,
validated, and propagated consistent consistently with the BGP protocol
[RFC4271], even if not needed by BGP SPF.
Section 9.1 of [RFC4271] defines the decision process that is used to
select routes for subsequent advertisement by applying the policies
in the local Policy Information Base (PIB) to the routes stored in
its Adj-RIBs-In. The output of the Decision Process is the set of
routes that are announced by a BGP speaker to its peers. These
selected routes are stored by a BGP speaker in the speaker's Adj-
RIBs-Out
RIBs-Out, according to policy.
The BGP SPF extension fundamentally changes the decision process, as
described herein. Specifically:
1. BGP advertisements are readvertised to neighbors immediately
without waiting or dependence on the route computation computation, as
specified in phase 3 of the base BGP decision process. Multiple
peering models are supported supported, as specified in Section 4.
2. Determining the degree of preference for BGP routes for the SPF
calculation as described in phase 1 of the base BGP decision
process is replaced with the mechanisms in Section 6.1.
3. Phase 2 of the base BGP protocol decision process is replaced
with the Shortest Path First (SPF) SPF algorithm, also known as the Dijkstra algorithm.
3. BGP - Link-State (BGP-LS) Relationship
[RFC9552] describes a mechanism by which link-state and Traffic
Engineering (TE) information can be collected from networks and
shared with external entities using BGP. This is achieved by
defining NLRI NLRIs that are advertised using the BGP-LS AFI. The BGP-LS
extensions defined in [RFC9552] make use of the decision process
defined in [RFC4271]. Rather than reusing the BGP-LS SAFI, the BGP-LS-SPF BGP-
LS-SPF SAFI (Section 5.1) is introduced to ensure backward
compatibility for the BGP-LS SAFI usage.
The "BGP-LS NLRI and Attribute TLVs" registry [RFC9552] is shared
between the BGP-LS SAFI and the BGP-LS-SPF SAFI. However, the TLVs
defined in this document may not be applicable to the BGP-LS SAFI.
As specified in Section 5.1 of [RFC9552], the presence of unknown or
unexpected TLVs is required to not result in so that the NLRI or the BGP-LS Attribute being will
not be considered malformed (section (Section 5.2 of [RFC9552]). The list of
BGP-LS TLVs applicable to the BGP-LS-SPF SAFI are described in
Section 5.2. By default, the usage of other BGP-LS TLVs or
extensions are ignored for the BGP-LS-SPF SAFI. However, this
doesn't preclude the usage specification of these TLVs for the BGP-
LS-SPF SAFI in future documents.
4. BGP SPF Peering Models
Depending on the topology, scaling, capabilities of the BGP speakers,
and redundancy requirements, various peering models are supported.
The only requirement is that all BGP speakers in the BGP SPF routing
domain adhere to this specification.
The choice of the deployment model is up to the operator and their
requirements and policies. Deployment model choice is out of scope
for this document and is discussed in [I-D.ietf-lsvr-applicability].
The sub-sections below describe several BGP SPF deployment models.
However, this doesn't preclude other deployment models.
4.1. BGP Single-Hop Peering on Network Node Connections
The simplest peering model is the one where EBGP External BGP (EBGP)
single-hop sessions are established over direct point-to-point links
interconnecting the nodes in the BGP SPF routing domain. Once the
single-hop BGP session has been established and the Multi-Protocol Multiprotocol
Extensions Capability capabilities have been exchanged with the BGP-LS-SPF AFI/SAFI has been exchanged AFI/
SAFI [RFC4760] for the corresponding session, then the link is
considered up and available from a BGP SPF
perspective perspective, and the
corresponding BGP-LS-SPF Link NLRI is advertised.
An End-of-RIB (EoR) Marker marker (Section 5.3) for the BGP-LS-SPF SAFI MAY
be required from a peer prior to advertising the BGP-LS-SPF Link NLRI
for the corresponding link to that peer. When required, the default
wait indefinitely for the EoR Marker marker prior to advertising the BGP-LS-
SPF Link NLRI. Refer to Section 10.4.
A failure to consistently configure the use of the EoR marker can
result in transient micro-loops and dropped traffic due to incomplete
forwarding state.
If the session goes down, the corresponding Link NLRI NLRIs are withdrawn.
Topologically, this would be equivalent to the peering model in
[RFC7938] where there is a BGP session on every link in the data
center switch fabric. The content of the Link NLRI is described in
Section 5.2.2.
4.2. BGP Peering Between Directly-Connected Directly Connected Nodes
In this model, BGP speakers peer with all directly-connected directly connected nodes
but the sessions may be between loopback addresses (i.e., two-hop
sessions)
sessions), and the direct connection discovery and liveness detection
for the interconnecting links are independent of the BGP protocol.
The BFD Bidirectional Forwarding Detection (BFD) protocol [RFC5880] is
RECOMMENDED for liveness detection. Usage of other liveness
connection mechanisms is outside the scope of this document.
Consequently, there is a single BGP session even if there are
multiple direct connections between BGP speakers. The BGP-
LS-SPF BGP-LS-SPF
Link NLRI is advertised as long as a BGP session has been
established, the BGP-LS-SPF AFI/SAFI capability has been exchanged
[RFC4760], the link is operational as determined using liveness
detection mechanisms, and, optionally, the EoR Marker marker has been
received as described in the Section 5.3. This is much like the previous
peering model only model, except peering is between loopback addresses and the
interconnecting links can be unnumbered. However, since there are
BGP sessions between every directly-connected directly connected node in the BGP SPF
routing domain, there is a reduction in BGP sessions when there are
parallel links between nodes. Hence, this peering model is
RECOMMENDED over the single-hop peering model Section 4.1.
4.3. BGP Peering in Route-Reflector or Controller Topology
In this model, BGP speakers peer solely with one or more Route
Reflectors route
reflectors [RFC4456] or controllers. As in the previous model,
direct connection discovery and liveness detection for those links in
the BGP SPF routing domain are done outside of the BGP protocol.
BGP-LS-SPF Link NLRI is advertised as long as the corresponding link
is considered up and available as per the chosen liveness detection
mechanism (The (thus, the BFD protocol [RFC5880] is RECOMMENDED).
This peering model, known as sparse peering, "sparse peering", allows for fewer BGP
sessions and, consequently, fewer instances of the same NLRI received
from multiple peers. Ideally, the route-reflectors route reflectors or controller BGP
sessions would be on directly-connected directly connected links to avoid dependence on
another routing protocol for session connectivity. However, multi-
hop peering is not precluded. The number of BGP sessions is
dependent on the redundancy requirements and the stability of the BGP
sessions.
The controller may use constraints to determine when to advertise
BGP-LS-SPF NLRI for BGP-LS peers. For example, a controller may
delay advertisement of a link between two peers the until the EoR
marker Section 5.3 has been received from both BGP peers and the BGP-LS BGP-
LS Link NLRI for the link(s) between the two nodes have has been received
from both BGP peers.
5. BGP Shortest Path Routing (SPF) Protocol Extensions
5.1. BGP-LS Shortest Path Routing (SPF) SAFI
This document introduces the BGP-LS-SPF SAFI with a value of 80. The
SPF-based decision process (Section 6) applies only to the BGP-LS-SPF
SAFI and MUST NOT be used with other combinations of the BGP-LS AFI
(16388). In order for two BGP speakers to exchange BGP-LS-SPF NLRI,
they MUST exchange the Multiprotocol Extensions Capability capabilities [RFC4760] to
ensure that they are both capable of properly processing such an
NLRI. This is done with AFI 16388 / SAFI 80. The BGP-LS-SPF SAFI is
used to advertise IPv4 and IPv6 prefix information in a format
facilitating an SPF-based decision process.
5.1.1. BGP-LS-SPF NLRI TLVs
All the TLVs defined for BGP-LS [RFC9552] are applicable and can be
used with the BGP-LS-SPF SAFI to describe links, nodes, and prefixes
comprising BGP-SPF LSDB BGP SPF Link-State Database (LSDB) information.
The NLRI and comprising TLVs MUST be encoded as specified in section
Section 5.1 of [RFC9552]. TLVs specified as mandatory in [RFC9552]
are considered mandatory for the BGP-LS-SPF SAFI as well. If a
mandatory TLV is not present, the NLRI MUST NOT be used in the BGP
SPF route calculation. All the other TLVs are considered as optional
TLVs. Documents specifying usage of optional TLV TLVs for BGP SPF MUST
address backward compatibility.
5.1.2. BGP-LS Attribute
The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly the exact same
format
of as the BGP-LS AFI [RFC9552]. In other words, all the TLVs
used in the BGP-LS attribute of the BGP-LS AFI are applicable and are
used for the BGP-LS attribute of the BGP-LS-SPF SAFI. This attribute
is an optional, non-transitive BGP attribute that is used to carry
link, node, and prefix properties and attributes. The BGP-LS
attribute is a set of TLVs.
All the TLVs defined for the BGP-LS Attribute [RFC9552] are
applicable and can be used with the BGP-LS-SPF SAFI to carry link,
node, and prefix properties and attributes.
The BGP-LS attribute may potentially be quite large depending on the
amount of link-state information associated with a single BGP-LS-SPF
NLRI. The BGP specification [RFC4271] mandates a maximum BGP message
size of 4096 octets. It is RECOMMENDED that an implementation
support [RFC8654] in order to accommodate a greater amount of
information within the BGP-LS Attribute. BGP speakers MUST ensure
that they limit the TLVs included in the BGP-LS Attribute to ensure
that a BGP update message for a single BGP-LS-SPF NLRI does not cross
the maximum limit for a BGP message. The determination of the types
of TLVs to be included by the BGP speaker originating the attribute
is outside the scope of this document. If, due to the limits on the
maximum size of an UPDATE message, a single route doesn't fit into
the message, the BGP speaker MUST NOT advertise the route to its peer
and MAY choose to log an error locally [RFC4271].
5.2. Extensions to BGP-LS
[RFC9552] describes a mechanism by which link-state and TE
information can be collected from IGPs and shared with external
components using the BGP protocol. It describes both the definition
of the BGP-LS NLRI that advertise links, nodes, and prefixes
comprising IGP link-state information and the definition of a BGP
path attribute (BGP-LS attribute) that carries link, node, and prefix
properties and attributes, such as the link and prefix metric or
auxiliary Router-IDs of nodes, etc. This document extends the usage
of BGP-LS NLRI for the purpose of BGP SPF calculation via
advertisement in the BGP-LS-SPF SAFI.
The protocol identifier specified in the Protocol-ID field [RFC9552]
represents the origin of the advertised NLRI. For Node NLRI and Link
NLRI, the specified Protocol-ID MUST be the direct protocol (4).
Node or Link NLRI with a Protocol-ID other than the direct protocol
is considered malformed. For Prefix NLRI, the specified Protocol-ID
MUST be the origin of the prefix. The local Local and remote node
descriptors Remote Node
Descriptors for all NLRI MUST include the BGP Router-ID (TLV 516)
[RFC9086] and the AS Number Autonomous System (TLV 512) number [RFC9552]. The
BGP Confederation Member (TLV 517) [RFC9086] is not applicable.
5.2.1. Node NLRI Usage
The Node NLRI MUST be advertised unconditionally by all routers in
the BGP SPF routing domain.
5.2.1.1. BGP-LS-SPF Node NLRI Attribute SPF Status TLV
A BGP-LS Attribute SPF Status TLV of the BGP-LS-SPF Node NLRI is
defined to indicate the status of the node with respect to the BGP
SPF calculation. This is used to rapidly take a node out of service
(refer to Section 6.5.2) or to indicate that the node is not to be
used for transit (i.e., non-local) traffic (refer to Section 6.3).
If the SPF Status TLV is not included with the Node NLRI, the node is
considered to be up and is available for transit traffic. A single
TLV type is shared by the Node, Link, and Prefix NLRI. The TLV type
is 1184.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
SPF Status Values:
+=======+=======================================================+
| Value | Description |
+=======+=======================================================+
| 0 - | Reserved |
+-------+-------------------------------------------------------+
| 1 - | Node unreachable with respect to BGP SPF |
+-------+-------------------------------------------------------+
| 2 - | Node does not support transit with respect to BGP SPF |
+-------+-------------------------------------------------------+
| 3-254 - Undefined | Unassigned |
+-------+-------------------------------------------------------+
| 255 - | Reserved |
+-------+-------------------------------------------------------+
Table 1: SPF Status Values
If a BGP speaker received the Node NLRI but the SPF Status TLV is not
received, then any previously received SPF status information is
considered as implicitly withdrawn withdrawn, and the NLRI is propagated to
other BGP speakers. A BGP speaker receiving a BGP Update containing
an SPF Status TLV in the BGP-LS attribute [RFC9552] with an unknown
value SHOULD be advertised to other BGP speakers and MUST ignore the
Status TLV with an unknown value in the SPF computation. An
implementation MAY log this condition for further analysis. If the
SPF Status TLV contains a reserved value (0 or 255) 255), the TLV is
considered malformed and is handled as described in Section 7.1.
5.2.2. Link NLRI Usage
The criteria for advertisement of Link NLRI NLRIs are discussed in
Section 4.
Link NLRI is advertised with unique local Local and remote node descriptors Remote Node Descriptors
dependent on the IP addressing. For IPv4 links, the link's local
IPv4 interface address (TLV 259) and remote IPv4 neighbor address
(TLV 260) addresses are used. For IPv6 links, the local IPv6 interface address
(TLV 261) and remote IPv6 neighbor address (TLV 262)
addresses are used
(Section 5.2.2 of [RFC9552]). IPv6 links without global IPv6
addresses are considered unnumbered links and are handled as
described below. For links supporting having both IPv4 and IPv6 addresses,
both sets of descriptors MAY be included in the same Link NLRI.
For unnumbered links, the Link Local/Remote Identifiers (TLV 258) are
used. The Link Remote Identifier isn't normally exchanged in BGP BGP,
and discovering the Link Remote Identifier is beyond the scope of
this document. If the Link Remote Identifier is unknown, a Link
Remote Identifier of 0 MUST be advertised. When 0 is advertised and
there are parallel unnumbered links between a pair of BGP speakers,
there may be transient intervals where the BGP speakers don't agree
on which of the parallel unnumbered links are operational. For this
reason, it is RECOMMENDED that the Link Remote Identifiers be known
(e.g., discovered using alternate mechanisms or configured) in the
presence of parallel unnumbered links.
The link descriptors are described in table Table 4 of [RFC9552].
Additionally, the Address Family Link Descriptor TLV is defined to
determine whether an unnumbered link can be used in the IPv4 SPF, the
IPv6, or both (refer to Section 5.2.2.1).
For a link to be used in SPF computation for a given address family,
i.e., IPv4 or IPv6, both routers connecting the link MUST have
matching addresses (i.e., router interface addresses must be on the
same subnet for numbered interfaces interfaces, and the local/remote link
identifiers (Section 6.3) must match for unnumbered interfaces).
The IGP metric attribute TLV Metric (TLV 1095) MUST be advertised. If a BGP speaker
receives a Link NLRI without an IGP metric Metric attribute TLV, then it
MUST consider the received NLRI as a malformed (refer to Section 7).
The BGP SPF metric length is 4 octets. A metric is associated with
the output side of each router interface. This metric is
configurable by the system administrator. The lower the metric, the
more likely the interface is to be used to forward data traffic. One
possible default for the metric would be to give each interface a
metric of 1 making it effectively a hop count.
The usage of other link attribute TLVs is beyond the scope of this
document.
5.2.2.1. BGP-LS Link NLRI Address Family Link Descriptor TLV
For unnumbered links, the address family cannot be ascertained from
the endpoint link descriptors. Hence, the Address Family (AF) Link
Descriptor SHOULD be included with the Link Local/Remote Identifiers
TLV for unnumbered links, so that the link can be used in the
respective address family SPF. If the Address Family Link Descriptor
is not present for an unnumbered link, the link will not be used in
the SPF computation for either address family. If the Address Family
Link Descriptor is present for a numbered link, the link descriptor
will be ignored. If the Address Family Link Descriptor TLV contains
an undefined value (3-254), the link descriptor will be ignored. If
the Address Family Link Descriptor TLV contains a reserved value (0
or 255) 255), the TLV is considered malformed and is handled as described
in Section 7.1.
Note that an unnumbered link can be used for both the IPv4 and IPv6
SPF computation by advertising separate Address Family Link
Descriptor TLVs for IPv4 and IPv6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1185) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family|
+-+-+-+-+-+-+-+-+
Address Family Values:
+=======+=====================+
| Value | Description |
+=======+=====================+
| 0 - | Reserved |
+-------+---------------------+
| 1 - | IPv4 Address Family |
+-------+---------------------+
| 2 - | IPv6 Address Family |
+-------+---------------------+
| 3-254 - | Undefined |
+-------+---------------------+
| 255 - | Reserved |
+-------+---------------------+
Table 2: Address Family Values
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV
This
The BGP-LS-SPF Attribute TLV of the BGP-LS-SPF Link NLRI is defined
to indicate the status of the link with respect to the BGP SPF
calculation. This is used to expedite convergence for link failures
as discussed in Section 6.5.1. If the SPF Status TLV is not included
with the Link NLRI, the link is considered up and available. The SPF
status is acted upon with the execution of the next SPF calculation
Section 6.3.
(Section 6.3). A single TLV type is shared by the Node, Link, and
Prefix NLRI. The TLV type is 1184.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values:
+=======+==========================================+
| Value | Description |
+=======+==========================================+
| 0 - | Reserved |
+-------+------------------------------------------+
| 1 - | Link Unreachable unreachable with respect to BGP SPF |
+-------+------------------------------------------+
| 2-254 - Undefined | Unassigned |
+-------+------------------------------------------+
| 255 - | Reserved |
+-------+------------------------------------------+
Table 3: BGP Status Values
If a BGP speaker received the Link NLRI but the SPF Status TLV is not
received, then any previously received SPF status information is
considered as implicitly withdrawn withdrawn, and the NLRI is propagated to
other BGP speakers. A BGP speaker receiving a BGP Update containing
an SPF Status TLV in the BGP-LS attribute [RFC9552] with an unknown
value SHOULD be advertised to other BGP speakers and MUST ignore the
SPF Status TLV with an unknown value in the SPF computation. An
implementation MAY log this information for further analysis. If the
SPF Status TLV contains a reserved value (0 or 255) 255), the TLV is
considered malformed and is handled as described in Section 7.1.
5.2.3. IPv4/IPv6 Prefix NLRI Usage
A IPv4/IPv6 Prefix NLRI is advertised with a Local Node Descriptor
and the prefix and length. The Prefix Descriptors Descriptor field includes the IP
Reachability Information TLV (TLV 265) as described in [RFC9552]. The
Prefix Metric TLV (TLV 1155) MUST be advertised to be considered for
route calculation. The IGP Route Tag TLV (TLV 1153) MAY be advertised.
The usage of other BGP-LS attribute TLVs is beyond the scope of this
document.
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV
A BGP-LS Attribute SPF Status TLV of the BGP-LS-SPF Prefix NLRI is
defined to indicate the status of the prefix with respect to the BGP
SPF calculation. This is used to expedite convergence for prefix
unreachability
unreachability, as discussed in Section 6.5.1. If the SPF Status TLV
is not included with the Prefix NLRI, the prefix is considered
reachable. A single TLV type is shared by the Node, Link, and Prefix
NLRI. The TLV type is 1184.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values:
+=======+============================================+
| Value | Description |
+=======+============================================+
| 0 - | Reserved |
+-------+--------------------------------------------+
| 1 - | Prefix Unreachable unreachable with respect to BGP SPF |
+-------+--------------------------------------------+
| 2-254 - Undefined | Unassigned |
+-------+--------------------------------------------+
| 255 - | Reserved |
+-------+--------------------------------------------+
Table 4: BGP Status Values
If a BGP speaker received the Prefix NLRI but the SPF Status TLV is
not received, then any previously received SPF status information is
considered as implicitly withdrawn withdrawn, and the NLRI is propagated to
other BGP speakers. A BGP speaker receiving a BGP Update containing
an SPF Status TLV in the BGP-LS attribute [RFC9552] with an unknown
value SHOULD be advertised to other BGP speakers and MUST ignore the
Status TLV with an unknown value in the SPF computation. An
implementation MAY log this information for further analysis. If the
SPF Status TLV contains a reserved value (0 or 255) 255), the TLV is
considered malformed and is handled as described in Section 7.1.
5.2.4. BGP-LS Attribute Sequence Number TLV
A BGP-LS Attribute Sequence Number TLV of the BGP-LS-SPF NLRI types
is defined to assure the most recent version of a given NLRI is used
in the SPF computation. The Sequence Number TLV is mandatory for
BGP-LS-SPF NLRI. The TLV type 1181 has been assigned by IANA. The
BGP-LS Attribute Sequence Number TLV contains an 8-octet sequence
number. The usage of the Sequence Number TLV is described in
Section 6.1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1181) | Length (8 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (High-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Low-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number: The 64-bit strictly-increasing strictly increasing sequence number MUST
be incremented for every self-originated version of a BGP-LS-SPF
NLRI. BGP speakers implementing this specification MUST use
available mechanisms to preserve the sequence number's strictly
increasing property for the deployed life of the BGP speaker
(including cold restarts). One mechanism for accomplishing this
would be to use the high-order 32 bits of the sequence number as a
wrap/boot count that is incremented any time the BGP router loses its
sequence number state or the low-order 32 bits wrap.
When incrementing the sequence number for each self-originated NLRI,
the sequence number should be treated as an unsigned 64-bit value.
If the lower-order 32-bit value wraps, the higher-order 32-bit value
should be incremented and saved in non-volatile storage. If a BGP
speaker completely loses its sequence number state (e.g., the BGP
speaker hardware is replaced or experiences a cold-start), cold start), the BGP
NLRI selection rules (see Section 6.1) ensure convergence, albeit not
immediately.
If the Sequence Number TLV is not received, then the corresponding
NLRI is considered as malformed and MUST be handled as 'Treat-as- 'treat-as-
withdraw'. An implementation SHOULD log an error for further
analysis.
5.3. BGP-LS-SPF End of RIB (EoR) Marker
The usage of the End-of-RIB (EoR) Marker EoR marker [RFC4724] with the BGP-LS-
SPF BGP-LS-SPF SAFI is
somewhat different than the other BGP SAFIs. Reception of the EoR
marker MAY optionally be expected prior to advertising an
LINK-NLRI a Link NLRI
for a given peer.
5.4. BGP Next-Hop Information
The rules for setting the BGP Next-Hop in the MP_REACH_NLRI attribute
[RFC4760] for the BGP-LS-SPF SAFI follow the rules in section Section 5.5 of
[RFC9552]. All BGP peers that support SPF extensions will locally
compute the Local-RIB Next-Hop as a result of the SPF process.
Hence, the use of the MP_REACH_NLRI Next-Hop as a tiebreaker in the
standard BGP path decision processing is not applicable.
6. Decision Process with the SPF Algorithm
The Decision Process described in [RFC4271] takes place in three
distinct phases. The Phase 1 decision function of the Decision
Process is responsible for calculating the degree of preference for
each route received from a BGP speaker's peer. The Phase 2 decision
function is invoked on completion of the Phase 1 decision function
and is responsible for choosing the best route out of all those
available for each distinct destination, destination and for installing each
chosen route into the Local-RIB. The combination of the Phase 1 and
2 decision functions is characterized as a Path Vector algorithm.
The SPF-based Decision process Process replaces the BGP Decision process Process
described in [RFC4271]. Since BGP-LS-SPF NLRI always contains the
local node descriptor
Local Node Descriptor as described in Section 5.2, each NLRI is
uniquely originated by a single BGP speaker in the BGP SPF routing
domain (the BGP node matching the NLRI's Node Descriptors).
Instances of the same NLRI originated by multiple BGP speakers would
be indicative of a configuration error or a masquerading attack
(refer to Section 9). These selected Node NLRI NLRIs and their Link/Prefix
NLRI Link/
Prefix NLRIs are used to build a directed graph during the SPF
computation as described below. The best routes for BGP prefixes are
installed in the RIB as a result of the SPF process.
When BGP-LS-SPF NLRI is received, all that is required is to
determine whether it is the most recent by examining the Node-ID and
sequence number as described in Section 6.1. If the received NLRI
has changed, it is advertised to other BGP-LS-SPF peers. If the
attributes have changed (other than the sequence number), a BGP SPF
calculation is triggered. However, a changed NLRI MAY be advertised
immediately to other peers and prior to any SPF calculation. Note
that the BGP MinASOriginationIntervalTimer [RFC4271] timer is not
applicable to the BGP-LS-SPF SAFI. The
MinRouteAdvertisementIntervalTimer is applicable with a suggested
default of 5 seconds consistent with Internal BGP (IBGP) (refer to
section
Section 10 of [RFC4271]). The scheduling of the SPF calculation, as
described in Section 6.3, is an implementation and/or configuration
matter. Scheduling MAY be dampened consistent with the SPF back-off Back-Off
Delay algorithm specified in [RFC8405].
The Phase 3 decision function of the Decision Process [RFC4271] is
also simplified since because under normal SPF operation, a BGP speaker
MUST advertise the changed NLRIs to all BGP peers with the BGP-LS-SPF AFI/
SAFI
AFI/SAFI and install the changed routes in the GLOBAL-RIB. The only
exception
exceptions are unchanged NLRIs or stale NLRIs, i.e., an NLRI received
with a less recent (numerically smaller) sequence number.
6.1. BGP SPF NLRI Selection
For all BGP-LS-SPF NLRIs, the selection rules for phase Phase 1 of the BGP
decision process, section process (see Section 9.1.1 [RFC4271], of [RFC4271]) no longer apply.
1. NLRI NLRIs self-originated from directly-connected directly connected BGP SPF peers are
preferred. This condition can be determined by comparing the BGP
Identifiers in the received Local Node Descriptor and the BGP
OPEN message for an active BGP session. This rule assures that a
stale NLRI is updated even if a BGP SPF router loses its sequence
number state due to a cold-start. cold start. Note that once the BGP session
goes down, the NLRI received is no longer considered as being
from a directly connected BGP SPF peer.
2. Consistent with base BGP [RFC4271], an NLRI received from a peer
will always replace the same NLRI received from that peer.
Coupled with rule #1, this will ensure that any stale NLRI in the
BGP SPF routing domain will be updated.
3. The NLRI with the most recent Sequence Number TLV, i.e., the
highest sequence number is selected.
4. The NLRI received from the BGP speaker with the numerically
larger BGP Identifier is preferred.
When a BGP speaker completely loses its sequence number state, e.g.,
due to a cold start, or in the unlikely possibility that a 64-bit
sequence number wraps, the BGP routing domain will still converge.
This is due to the fact that BGP speakers adjacent to the router
always accept self-originated NLRI NLRIs from the associated speaker as
more recent (rule #1). When a BGP speaker reestablishes a connection
with its peers, any existing sessions are taken down and stale NLRI NLRIs
are replaced. The adjacent BGP speakers update their NLRI
advertisements and advertise to their neighbors until the BGP routing
domain has converged.
The modified SPF Decision Process performs an SPF calculation rooted
at the local BGP speaker using the metrics from the Link Attribute
IGP Metric TLV (1095) (TLV 1095) and the Prefix Attribute Prefix Metric TLV
(1155) (TLV
1155) [RFC9552]. These metrics are considered consistently across
the BGP SPF domain. As a result, any other BGP attributes that would
influence the BGP decision process defined in [RFC4271] including
ORIGIN, MULTI_EXIT_DISC, and LOCAL_PREF attributes are ignored by the
SPF algorithm. The Next Hop in the MP_REACH_NLRI attribute [RFC4760]
is discussed in Section 5.4. The AS_PATH and AS4_PATH [RFC6793] attributes
[RFC6793] are preserved and used for loop detection [RFC4271]. They
are ignored during the SPF computation for BGP-LS-SPF NLRIs.
6.1.1. BGP Self-Originated NLRI
Node, Link,
Nodes, Links, or Prefix NLRI NLRIs with Node Descriptors matching the
local BGP speaker are considered self-originated. When self-originated a self-
originated NLRI is received and it doesn't match the local node's
NLRI content (including the sequence number), special processing is
required.
* If a self-originated NLRI is received and the sequence number is
more recent (i.e., greater than the local node's sequence number
for the NLRI), the NLRI sequence number is advanced to one greater
than the received sequence number number, and the NLRI is readvertised to
all peers.
* If a self-originated NLRI is received and the sequence number is
the same as the local node's sequence number but the attributes
differ, the NLRI sequence number is advanced to one greater than
the received sequence number number, and the NLRI is readvertised to all
peers.
The above actions are performed immediately when the first instance
of a newer self-originated NLRI is received. In this case, the newer
instance is considered to be a stale instance that was advertised by
the local node prior to a restart where the NLRI state was lost.
However, if subsequent newer self-originated NLRI is received for the
same Node, Link, or Prefix NLRI, the readvertisement or withdrawal is
delayed by BGP_LS_SPF_SELF_READVERTISEMENT_DELAY (default 5) seconds
since it is likely being advertised by a misconfigured or rogue BGP
speaker (refer to Section 9).
6.2. Dual Stack Support
The SPF-based decision process operates on Node, Link, and Prefix
NLRIs that support both IPv4 and IPv6 addresses. Whether to run a
single SPF computation or multiple SPF computations for separate AFs
is an implementation and/or policy matter. Normally, IPv4 next-hops
are calculated for IPv4 prefixes prefixes, and IPv6 next-hops are calculated
for IPv6 prefixes.
6.3. SPF Calculation based Based on BGP-LS-SPF NLRI
This section details the BGP-LS-SPF local routing information base Routing Information Base
(RIB) calculation. The router uses BGP-LS-SPF Node, Link, and Prefix
NLRI
NLRIs to compute routes using the following algorithm. This
calculation yields the set of routes associated with the BGP SPF
Routing Domain. A router calculates the shortest-path tree using
itself as the root. Optimizations to the BGP-LS-SPF algorithm are
possible but MUST yield the same set of routes. The algorithm below
supports Equal Cost Multi-Path (ECMP) ECMP routes. Weighted Unequal Cost
Multi-Path Unequal-Cost Multipath (UCMP) routes
are out of scope.
The following abstract data structures are defined in order to
specify the algorithm.
*
Local Route Information Base (Local-RIB) - This (Local-RIB): A routing table that
contains reachability information (i.e., next hops) for all
prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF node
reachability. Implementations may choose to implement this with
separate RIBs for each address family and/or Prefix versus Node
reachability.
*
Global Routing Information Base (GLOBAL-RIB) - This is the Routing
Information Base (RIB) (GLOBAL-RIB): The RIB containing the
current routes that are installed in the router's forwarding
plane. This is commonly referred to in networking parlance as
"the RIB".
* Link State
Link-State NLRI Database (LSNDB) - Database (LSNDB): A database of BGP-LS-SPF NLRI NLRIs
that facilitates facilitate access to all Node, Link, and Prefix NLRI.
* NLRIs.
Candidate List (CAN-LIST) - This is a (CAN-LIST): A list of candidate Node NLRIs used
during the BGP SPF calculation. The list is sorted by the cost to
reach the Node NLRI NLRI, with the Node NLRI with that has the lowest
reachability cost at the head of the list. This facilitates the
execution of the Dijkstra algorithm algorithm, where the shortest paths
between the local node and other nodes in the graph are computed.
The CAN-LIST is typically implemented as a heap but other data
structures have been used.
The Dijkstra algorithm consists of the steps below:
1. The current Local-RIB is invalidated, and the CAN-LIST is
initialized to be empty. The Local-RIB is rebuilt during the
course of the SPF computation. The existing routing entries are
preserved for comparison to determine changes that need to be
made to the GLOBAL-RIB in step Step 6. These routes are referred to
as stale routes. "stale routes".
2. The cost of the Local-RIB Node route entry for the computing
router is set to 0. The computing router's Node NLRI is added to
the CAN-LIST (which was previously initialized to be empty in
step
Step 1). The next-hop list is set to the internal loopback next-
hop.
3. The Node NLRI with the lowest cost is removed from the CAN-LIST
for processing. If the BGP-LS Node attribute includes an SPF
Status TLV (refer to Section 5.2.1.1) indicating the node is
unreachable, the Node NLRI is ignored and the next lowest cost lowest-cost
Node NLRI is selected from the CAN-LIST. The Node corresponding
to this NLRI is referred to as the Current-Node. "Current-Node". If the CAN-LIST CAN-
LIST list is empty, the SPF calculation has completed and the
algorithm proceeds to step Step 6.
4. All the Prefix NLRI NLRIs with the same Local Node Descriptors as the
Current-Node are considered for installation. The next-hop(s)
for these Prefix NLRI NLRIs are inherited from the Current-Node. If
the Current-Node is for the local BGP Router, the next-hop for
the prefix is a direct next-hop. The cost for each prefix is the
metric advertised in the Prefix Attribute Prefix Metric TLV
(1155) (TLV
1155) added to the cost to reach the Current-Node. The following
is done for each Prefix NLRI (referred to as the
Current-Prefix): "Current-
Prefix"):
* If the BGP-LS Prefix attribute includes an SPF Status TLV
indicating the prefix is unreachable, the Current-Prefix is
considered unreachable unreachable, and the next Prefix NLRI is examined
in Step 4.
* If the Current-Prefix's corresponding prefix is in the Local-
RIB and the Local-RIB metric is less than the Current-Prefix's
metric, the Current-Prefix does not contribute to the route route,
and the next Prefix NLRI is examined in Step 4.
* If the Current-Prefix's corresponding prefix is not in the
Local-RIB, the prefix is installed with the Current-Node's
next-hops installed as the Local-RIB route's next-hops and the
metric being updated. If the IGP Route Tag TLV (1153) (TLV 1153) is
included in the Current-Prefix's NLRI Attribute, the tag(s)
are is
installed in the current Local-RIB route's tag(s).
* If the Current-Prefix's corresponding prefix is in the Local-
RIB and the cost is less than the Local-RIB route's metric,
the prefix is installed with the Current-Node's next-hops
replacing next-hops,
which replace the Local-RIB route's next-hops and the metric
being
updated updated, and any route tags are removed. If the IGP
Route Tag TLV
(1153) (TLV 1153) is included in the Current-Prefix's NLRI
Attribute, the tag(s) are is installed in the current Local-RIB
route's tag(s).
* If the Current-Prefix's corresponding prefix is in the Local-
RIB and the cost is the same as the Local-RIB route's metric,
the Current-Node's next-hops are merged with the Local-RIB
route's next-hops. The algorithm below supports Equal Cost Multi-Path
(ECMP) ECMP routes.
Some platforms or implementations may have limits on the
number of ECMP routes that can be supported. The setting or
identification of any limitations is outside the scope if this
document. Weighted Unequal Cost Multi-Path UCMP routes are out of scope as well.
5. All the Link NLRI NLRIs with the same Node Identifiers as the Current-
Node are considered for installation. Each link is examined and
is
referred to as the "Current-Link" in the following text as the Current-Link. text. The
cost of the Current-Link is the advertised IGP Metric TLV (1095) (TLV 1095)
from the Link NLRI BGP-LS attribute added to the cost to reach
the Current-Node. If the Current-Node is for the local BGP
Router, the next-hop for the link is a direct next-hop pointing
to the corresponding local interface. For any other Current-
Node, the next-hop(s) for the Current-Link are is inherited from the
Current-Node. The following is done for each link:
a. If the Current-Link's NLRI attribute includes an SPF Status
TLV indicating the link is down, the BGP-LS-SPF Link NLRI is
considered down down, and the next link for the Current-Node is
examined in Step 5.
b. If the Current-Node NLRI attributes includes include the SPF Status
TLV (refer to Section 5.2.1.1) and the status indicates that
the Node doesn't support transit, the next link for the
Current-Node is processed in Step 5.
c. The Current-Link's Remote Node NLRI is accessed (i.e., the
Node NLRI with the same Node identifiers Identifiers as the Current-
Link's Remote Node Descriptors). If it exists, it is
referred to as the Remote-Node "Remote-Node" and the algorithm proceeds
as follows:
* If the Remote-Node's NLRI attribute includes an SPF Status
TLV indicating the node is unreachable, the next link for
the Current-Node is examined in Step 5.
* All the Link NLRI NLRIs corresponding to the Remote-Node are
searched for a Link NLRI pointing to the Current-Node.
Each Remote-Node's Link NLRI (referred to as the Remote-
Link) is examined for Remote Node Descriptors matching the
Current-Node and Link Descriptors matching the Current-
Link.
- For IPv4/IPv6 numbered Link Descriptors Decriptors to match during
the IPv4 SPF computation, the Current-Link's IP4/IPv6
interface address link descriptor MUST match the
Remote-Link IPv4/IPv6 neighbor address link descriptor descriptor,
and the Current-Link's IPv4/IPv6 neighbor address MUST
match the Remote-Link's IPv4/IPv6 interface address.
- For unnumbered links to match during the IPv4 or IPv6
SPF computation, the Current-Link and Remote-Link's
Address Family Link Descriptor TLV must match the
address family of the IPv4 or IPv6 SPF computation, the
Current-Link's Remote Identifier MUST match the Remote-
Link's Local Identifier Identifier, and the Current-Link's Remote
Identifier MUST match the Remote-Link's Local
Identifier. Since the Link's Remote Identifier may not
be known, a value of 0 is considered a wildcard and
will match any Current or Remote Link's Local
Identifier (see TLV 258 [RFC9552]). Address Family
Link Descriptor TLVs for multiple address families may
be advertised so that an unnumbered link can be used in
the SPF computation for multiple address families.
If these conditions are satisfied for one of the Remote-
Node's links, the bi-directional bidirectional connectivity check
succeeds and the Remote-Node may be processed further.
The Remote-Node's Link NLRI providing bi-directional bidirectional
connectivity is referred to as the Remote-Link. If no
Remote-Link is found, the next link for the Current-Node
is examined in Step 5.
* If the Remote-Link NLRI attribute includes an SPF Status
TLV indicating the link is down, the Remote-Link NLRI is
considered down down, and the next link for the Current-Node is
examined in Step 5.
* If the Remote-Node is not on the CAN-LIST, it is inserted
based on the cost. The Remote Node's cost is the cost of
the Current-Node added to the Current-Link's IGP Metric TLV
(1095).
(TLV 1095). The next-hop(s) for the Remote-Node are is
inherited from the Current-Link.
* If the Remote-Node NLRI is already on the CAN-LIST with a
higher cost, it must be removed and reinserted with the
Remote-Node cost based on the Current-Link (as calculated
in the previous step). The next-hop(s) for the Remote-
Node are is inherited from the Current-Link.
* If the Remote-Node NLRI is already on the CAN-LIST with
the same cost, it need not be reinserted on the CAN-LIST.
However, the Current-Link's next-hop(s) must be merged
into the current set of next-hops for the Remote-Node.
* If the Remote-Node NLRI is already on the CAN-LIST with a
lower cost, it need not be reinserted on the CAN-LIST.
d. Return to step Step 3 to process the next lowest cost lowest-cost Node NLRI on
the CAN-LIST.
6. The Local-RIB is examined and changes (adds, deletes, and
modifications) are installed into the GLOBAL-RIB. For each route
in the Local-RIB:
* If the route was added during the current BGP SPF computation,
install the route into the GLOBAL-RIB.
* If the route was modified during the current BGP SPF
computation (e.g., metric, tags, or next-hops), update the
route in the GLOBAL-RIB.
* If the route was not installed during the current BGP SPF
computation, remove the route from the GLOBAL-RIB.
6.4. IPv4/IPv6 Unicast Address Family Interaction
While the BGP-LS-SPF address family and the BGP unicast address
families may install routes into the same device routing tables, tables of the same
device, they operate independently (i.e., "Ships-in-the-Night" "ships-in-the-night" mode).
There is no implicit route redistribution between the BGP-LS-SPF
address family and the BGP unicast address families.
It is RECOMMENDED that BGP-LS-SPF IPv4/IPv6 route computation and
installation be given scheduling priority by default over other BGP
address families as these address families are considered as underlay
SAFIs.
6.5. NLRI Advertisement
6.5.1. Link/Prefix Failure Convergence
A local failure prevents a link from being used in the SPF
calculation due to the IGP bi-directional bidirectional connectivity requirement.
Consequently, local link failures SHOULD always be communicated as
quickly as possible and given priority over other categories of
changes to ensure expeditious propagation and optimal convergence.
According to standard BGP procedures, the link would continue to be
used until the last copy of the BGP-LS-SPF Link NLRI is withdrawn.
In order to avoid this delay, the originator of the Link NLRI SHOULD
advertise a more recent version with an increased Sequence Number TLV
for the BGP-LS-SPF Link NLRI including the SPF Status TLV (refer to
Section 5.2.2.2) indicating the link is down with respect to BGP SPF.
The configurable LinkStatusDownAdvertise timer controls the interval
that the BGP-LS-LINK NLRI is advertised with SPF Status indicating
the link is down prior to withdrawal. If a BGP-LS-SPF Link NLRI has
been advertised with the SPF Status TLV and the link becomes
available in that period, the originator of the BGP-LS-SPF LINK Link NLRI
MUST advertise a more recent version of the BGP-LS-SPF Link NLRI
without the SPF Status TLV in the BGP-LS Link Attributes. The suggested
default value for the LinkStatusDownAdvertise timer is 2 seconds.
Similarly, when a prefix becomes unreachable, a more recent version
of the BGP-LS-SPF Prefix NLRI SHOULD be advertised with the SPF
Status TLV (refer to Section 5.2.3.1) indicating to indicate that the prefix is
unreachable in the BGP-LS Prefix Attributes Attributes, and the prefix will be
considered unreachable with respect to BGP SPF. The configurable
PrefixStatusDownAdvertise timer controls the interval that the BGP-
LS-Prefix NLRI is advertised with SPF Status indicating the prefix is
unreachable prior to withdrawal. If the BGP-LS-SPF Prefix has been
advertised with the SPF Status TLV and the prefix becomes reachable
in that period, the originator of the BGP-LS-SPF Prefix NLRI MUST
advertise a more recent version of the BGP-LS-SPF Prefix NLRI without
the SPF Status TLV in the BGP-LS Prefix Attributes. The suggested
default value for the PrefixStatusDownAdvertise timer is 2 seconds.
6.5.2. Node Failure Convergence
By default, all the NLRI NLRIs advertised by a node are withdrawn when a
session failure is detected [RFC4271]. If fast failure detection
such as BFD [RFC5880] is utilized, and the node is on the fastest
converging path, the most recent versions of BGP-LS-SPF NLRI will be
withdrawn. This may result in older versions of NLRI NLRIs received from
peer(s)
one or more peers on a different path(s) being in the LSNDB until they are
withdrawn. These stale NLRI NLRIs will not delay convergence since the
adjacent nodes detect the link failure and advertise a more recent
NLRI indicating the link is down with respect to BGP SPF (refer to
Section 6.5.1) and the bi-directional bidirectional connectivity check fails during
the BGP SPF calculation (refer to Section 6.3).
7. Error Handling
This section describes the Error Handling error-handling actions, as described in
[RFC7606], that are specific to SAFI BGP-LS-SPF SAFI BGP Update message
processing.
7.1. Processing of BGP-LS-SPF TLVs
When a BGP speaker receives a BGP Update containing a malformed Node
NLRI SPF Status TLV in the BGP-LS Attribute [RFC9552], the
corresponding Node NLRI is considered as malformed and MUST be handled
as 'Treat-as-withdraw'. 'treat-as-withdraw'. An implementation SHOULD log an error
(subject to rate-limiting) rate limiting) for further analysis.
When a BGP speaker receives a BGP Update containing a malformed Link
NLRI SPF Status TLV in the BGP-LS Attribute [RFC9552], the
corresponding Link NLRI is considered as malformed and MUST be handled
as 'Treat-as-withdraw'. 'treat-as-withdraw'. An implementation SHOULD log an error
(subject to rate-limiting) rate limiting) for further analysis.
When a BGP speaker receives a BGP Update containing a malformed
Address Family Link Descriptor TLV in the BGP-LS Attribute [RFC9552],
the corresponding Link NLRI is considered as malformed and MUST be
handled as 'Treat-as-withdraw'. 'treat-as-withdraw'. An implementation SHOULD log an
error (subject to rate-limiting) rate limiting) for further analysis.
When a BGP speaker receives a BGP Update containing a malformed
Prefix NLRI SPF Status TLV in the BGP-LS Attribute [RFC9552], the
corresponding Prefix NLRI is considered as malformed and MUST be handled
as 'Treat-as-withdraw'. 'treat-as-withdraw'. An implementation SHOULD log an error
(subject to rate-limiting) rate limiting) for further analysis.
When a BGP speaker receives a BGP Update containing any a malformed
BGP-LS BGP-
LS Attribute TE and IGP Metric TLV, the corresponding NLRI is
considered as malformed and MUST be handled as 'Treat-as-withdraw' 'treat-as-withdraw'
[RFC7606]. An implementation SHOULD log an error (subject to rate- rate
limiting) for further analysis.
The BGP-LS Attribute consists of Node attribute TLVs, Link attribute
TLVs, and the Prefix attribute TLVs. Node attribute TLVs and their
error handling
error-handling rules are either defined in [RFC9552] or derived from
[RFC5305] and [RFC6119]. If a BGP speaker receives a BGP-LS
Attribute which that is considered malformed based on these error handling error-handling
rules, then it MUST consider the received NLRI as malformed malformed, and the
receiving BGP speaker MUST handle such a malformed NLRI as 'Treat-as- 'treat-as-
withdraw' [RFC7606].
Node Descriptor TLVs and their error handling error-handling rules are defined in
section
Section 5.2.1 of [RFC9552]. Node Attribute TLVs and their error error-
handling rules are either defined in [RFC9552] or derived from
[RFC5305] and [RFC6119].
Link Descriptor TLVs and their error handling error-handling rules are defined in
section
Section 5.2.2 of [RFC9552]. Link Attribute TLVs and their error error-
handling rules are either defined in [RFC9552] or derived from
[RFC5305] and [RFC6119].
Prefix Descriptor TLVs and their error handling error-handling rules are defined in
section
Section 5.2.3 of [RFC9552]. Prefix Attribute TLVs and their error error-
handling rules are either defined in [RFC9552] or derived from
[RFC5130] and [RFC2328].
If a BGP speaker receives NLRI with a Node Descriptor TLV, Link
Descriptor TLV, or Prefix Descriptor TLV that is considered malformed
based on error handling rules defined in the above references, then
it MUST consider the received NLRI as malformed malformed, and the receiving
BGP speaker MUST handle such a malformed NLRI as 'Treat-as-withdraw' 'treat-as-withdraw'
[RFC7606].
When a BGP speaker receives a BGP Update that does not contain any
BGP-LS Attribute, Attributes, it is most likely an indication of 'Attribute
Discard' fault handling handling, and the BGP speaker SHOULD preserve and
propagate the BGP-LS-SPF NLRI as described in Section 8.2.2 of
[RFC9552]. However, NLRI NLRIs without the BGP-LS attribute MUST NOT be
used in the SPF Calculation Section 6.3. calculation (Section 6.3). How this is accomplished
is an implementation matter matter, but one way would be for these NLRI NLRIs not
to be returned in LSNDB lookups.
7.2. Processing of BGP-LS-SPF NLRIs
A BGP speaker supporting the BGP-LS-SPF SAFI MUST perform the
syntactic validation checks of the BGP-LS-SPF NLRI listed in
Section 8.2.2 of [RFC9552] to determine if it is malformed.
7.3. Processing of BGP-LS Attribute Attributes
A BGP speaker supporting the BGP-LS-SPF SAFI MUST perform the
syntactic validation checks of the BGP-LS Attribute listed in
Section 8.2.2 of [RFC9552] to determine if it is malformed.
An implementation SHOULD log an error for further analysis for
problems detected during syntax validation.
7.4. BGP-LS-SPF Link State Link-State NLRI Database Synchronization
While uncommon, there may be situations where the LSNDBs of two BGP
speakers supporting support the BGP-LS-SPF SAFI lose synchronization. In these
situations, the BGP session MUST be reset unless other means of
resynchronization are used (beyond the scope of this document). When
the session is reset, the BGP speaker MUST send a NOTIFICATION
message with the BGP error code "Loss of LSDB Synchronization" as
described in section Section 3 of [RFC4271]. The mechanisms to detect loss
of synchronization are beyond the scope of this document.
8. IANA Considerations
8.1. BGP-LS-SPF Allocation in the SAFI Parameters Values Registry
IANA has assigned value 80 for BGP-LS-SPF from the First Come First
Served range [RFC8126] and listed this document as a reference in the
"SAFI Values" registry within the "Subsequent Address Family
Identifiers (SAFI) Parameters" registry. IANA is requested to update the registration
to reference only to this document. registry group.
8.2. BGP-LS-SPF Assignments to in the BGP-LS NLRI and Attribute TLV TLVs
Registry
IANA has assigned six TLVs for BGP-LS-SPF NLRI in the "BGP-LS NLRI
and Attribute TLV" TLVs" registry. Supported TLV types include the Sequence
Number, SPF
Status TLV type, Status, and Address Family Link Descriptor TLV type, and
Sequence Number TLV type. Descriptor. Deprecated
TLV types include the SPF
Capability TLV type, Capability, IPv4 Link Prefix Length TLV type, Length, and IPv6
Link Prefix Length TLV type.
+==========+=================+=================================+ Length.
+================+=================+============================+
| TLV Code Point | Description | Reference |
| Point | | |
+==========+=================+=================================+
| 1185 | Address Family | Section 5.2.2.1, RFCXXXX ([this |
| | Link Descriptor | document]). |
+----------+-----------------+---------------------------------+
+================+=================+============================+
| 1181 | Sequence Number | RFCXXXX ([this document]), |
| | | Section 5.2.4 of RFC XXXX |
+----------+-----------------+---------------------------------+
+----------------+-----------------+----------------------------+
| 1184 | SPF Status | Section Sections 5.2.1.1, RFCXXXX ([this 5.2.2.2, |
| | | document]), Section 5.2.2.2 and 5.2.3.1 of RFC XXXX |
+----------------+-----------------+----------------------------+
| 1185 | Address Family | Section 5.2.3.1 5.2.2.1 of RFC |
| | Link Descriptor | XXXX |
+----------+-----------------+---------------------------------+
+----------------+-----------------+----------------------------+
Table 1: 5: NLRI Attribute TLVs
The early allocation assignments for the TLV types SPF Capability
(1180), IPv4 Link Prefix Length (1182), and IPv6 Link Prefix Length
(1183) are no longer required and are to be have been deprecated.
8.3. BGP-LS-SPF Node NLRI Attribute SPF Status TLV Status Registry
IANA is requested to create has created the "BGP-LS-SPF Node NLRI Attribute SPF Status TLV
Status" Registry registry for status values in a new BGP SPF within the "BGP Shortest Path
First (BGP SPF)" registry group. Initial values for this registry
are provided below. Future assignments are to be made using the
Expert Review registration policy [RFC8126] with guidance for Designated Experts
designated experts as per section Section 7.2 of [RFC9552].
+========+==========================================+
| Values | Description |
+========+==========================================+
| 0 | Reserved |
+--------+------------------------------------------+
| 1 | Node unreachable with respect to BGP SPF |
+--------+------------------------------------------+
| 2 | Node does not support transit traffic |
| | with respect to BGP SPF |
+--------+------------------------------------------+
| 3-254 | Unassigned |
+--------+------------------------------------------+
| 255 | Reserved |
+--------+------------------------------------------+
Table 2: 6: BGP-LS-SPF Node NLRI Attribute SPF
Status TLV Status Registry Assignments
8.4. BGP-LS-SPF Link NLRI Attribute SPF Status TLV Status Registry
IANA is requested to create has created the "BGP-LS-SPF Link NLRI Attribute SPF Status TLV
Status" Registry registry for status values in a new within the BGP SPF Shortest Path First
(BGP SPF)" registry group. Initial values for this registry are
provided below. Future assignments are to be made using the IETF
Review registration policy [RFC8126].
+=======+==========================================+
| Value | Description |
+=======+==========================================+
| 0 | Reserved |
+-------+------------------------------------------+
| 1 | Link unreachable with respect to BGP SPF |
+-------+------------------------------------------+
| 2-254 | Unassigned |
+-------+------------------------------------------+
| 255 | Reserved |
+-------+------------------------------------------+
Table 3: 7: BGP-LS-SPF Link NLRI Attribute SPF
Status TLV Status Registry Assignments
8.5. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV Status Registry
IANA is requested to create has created the "BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV
Status" Registry registry for status values in a new BGP SPF within the "BGP Shortest Path
First (BGP SPF)" registry group. Initial values for this registry
are provided below. Future assignments are to be made using the IETF
Review registration policy [RFC8126].
+=======+============================================+
| Value | Description |
+=======+============================================+
| 0 | Reserved |
+-------+--------------------------------------------+
| 1 | Prefix unreachable with respect to BGP SPF |
+-------+--------------------------------------------+
| 2-254 | Unassigned |
+-------+--------------------------------------------+
| 255 | Reserved |
+-------+--------------------------------------------+
Table 4: 8: BGP-LS-SPF Prefix NLRI Attribute SPF
Status TLV Status Registry Assignments
8.6. Assignment in the BGP Error (Notification) Code Assignment Codes Registry
IANA is requested to assign a TBD code has assigned value 9 for "Loss Loss of LSDB
Synchronization" Synchronization in the BGP
"BGP Error (Notification) Codes" registry in within the "Border Gateway
Protocol (BGP) Parameters" registry group.
9. Security Considerations
This document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This
document does not change the underlying security issues inherent in
the BGP protocol [RFC4271]. The Security Considerations security considerations discussed in
[RFC4271] apply to the BGP SPF functionality as well. The analysis
of the security issues for BGP mentioned in [RFC4272] and [RFC6952]
also applies to this document. The threats and security
considerations are the similar to the BGP IPv4 Unicast SAFI and IPv6
Unicast SAFI when utilized in similar deployments, e.g., [RFC7938].
The analysis of Generic Threats generic threats to Routing Protocols done routing protocols in [RFC4593] is
also worth noting.
As the modifications for BGP SPF described in this document for BGP SPF apply to
IPv4 Unicast and IPv6 Unicast as underlay SAFIs in a single BGP SPF
Routing Domain, the BGP security solutions described in [RFC6811] and
[RFC8205] are out of scope as they are meant to apply for inter-
domain BGP BGP, where multiple BGP Routing Domains are typically
involved. The BGP-LS-SPF SAFI NLRI NLRIs described in this document are
typically advertised between External BGP (EBGP) EBGP or Internal BGP (IBGP) IBGP speakers under a single
administrative domain.
The BGP SPF processing and the BGP-LS-SPF SAFI inherit the encoding
from BGP-LS [RFC9552], and consequently, inherit the security
considerations for BGP-LS associated with encoding. Additionally,
given that the BGP SPF processing is used to install IPv4 and IPv6
Unicast
unicast routes, the BGP SPF processing is vulnerable to attacks to
the routing control plane that aren't applicable to BGP-LS. One
notable Denial-of-Service attack, attack would be to include malformed BGP
attributes in a replicated BGP Update, causing the receiving peer to
treat the advertised BGP-LS-SPF to a withdrawal [RFC7606].
In order to mitigate the risk of peering with BGP speakers
masquerading as legitimate authorized BGP speakers, it is RECOMMENDED
that the TCP Authentication Option (TCP-AO) [RFC5925] be used to
authenticate BGP sessions. If an authorized BGP peer is compromised,
that BGP peer could advertise a modified Node, Link, or Prefix NLRI
which result
that results in misrouting, repeating origination of NLRI, and/or
excessive SPF calculations. When a BGP speaker detects that its
self-originated NLRI is being originated by another BGP speaker, an
appropriate error SHOULD be logged so that the operator can take
corrective action. This exposure is similar to other BGP AFI/SAFIs.
10. Management Considerations
This section includes unique management considerations for the BGP-
LS-SPF address family.
10.1. Configuration
All routers in the BGP SPF Routing Domain are under a single
administrative domain allowing for consistent configuration.
10.2. Link Metric Configuration
For loopback prefixes, it is RECOMMENDED that the metric be 0. For
non-loopback prefixes, the setting of the metric is a local matter
and beyond the scope of this document.
Algorithms such as setting the metric inversely to the link speed as
supported in some IGP implementations MAY be supported. However, the
details of how the metric is computed are beyond the scope of this
document.
Within a BGP SPF Routing Domain, the IGP metrics for all advertised
links SHOULD be configured or defaulted consistently. For example,
if a default metric is used for one router's links, then a similar
metric should be used for all router's links. Similarly, if the link
metric is derived from using the inverse of the link bandwidth on one
router, then this SHOULD be done for all routers routers, and the same
reference bandwidth SHOULD be used to derive the inversely
proportional metric. Failure to do so will result in incorrect
routing based on the link metric.
10.3. Unnumbered Link Configuration
When parallel unnumbered links between BGP-SPF BGP and SPF routers are
included in the BGP SPF routing domain and the Remote Link
Identifiers aren't readily discovered, it is RECOMMENDED that these the
Remote Link Identifiers be configured so that precise NLRI Link NLRI
matching can be done.
10.4. Adjacency End-of-RIB (EOR) Marker Requirement
Depending on the peering model, topology, and convergence
requirements, an End-of-RIB (EoR) Marker Section 5.3 EoR marker (Section 5.3) for the BGP-LS-
SPF BGP-LS-SPF SAFI MAY
be required from the peer prior to advertising a BGP-LS Link NLRI for
the peer. If configuration is supported, this MUST be configurable
at the BGP SPF instance level and MUST be configured consistently
throughout the BGP SPF routing domain.
When this configuration is provided, the default MUST be to wait
indefinitely prior to advertising a BGP-LS link Link NLRI. Configuration
of a timer specifying the maximum time to wait prior to advertisement
MAY be provided.
10.5. backoff-config
In addition to the configuration of the BGP-LS-SPF address family,
implementations SHOULD support the "Shortest Path First (SPF) Back-
Off Back-Off
Delay Algorithm for Link-State IGPs" [RFC8405]. If supported,
configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST be
supported [RFC8405]. Section 6 of [RFC8405] recommends consistent
configuration of these values throughout the IGP routing domain domain, and
this also applies to the BGP SPF Routing Domain.
10.6. BGP-LS-SPF NLRI Readvertisement Delay
The configuration parameter that specifies the delay for
readvertising a more recent instance of a self-originated NLRI when
received more than once in succession is
BGP_LS_SPF_SELF_READVERTISEMENT_DELAY. The default is 5 seconds.
10.7. Operational Data
In order to troubleshoot SPF issues, implementations SHOULD support
an SPF log including entries for previous SPF computations. Each SPF
log entry would include the BGP-LS-SPF NLRI SPF triggering the SPF,
SPF scheduled time, SPF start time time, and SPF end time. Since the size
of the log is finite, implementations SHOULD also maintain counters
for the total number of SPF computations and the total number of SPF
triggering events. Additionally, to troubleshoot troubleshooting should be available
for SPF scheduling and back-off [RFC8405], the current SPF back-off
state, the remaining time-
to-learn, time-to-learn, the remaining hold-down interval,
the last trigger event time, the last SPF time, and the next SPF time should be available.
time.
10.8. BGP-LS-SPF Address Family Session Isolation
In common deployment scenarios, the unicast routes installed during
BGP-LS-SPF AFI/SAFI SPF computation serve as the underlay for other
BGP AFI/SAFIs. To avoid errors encountered in other AFI/SAFIs from
impacting the BGP-LS-SPF AFI/SAFI or vice versa, isolation mechanisms
such as separate BGP instances or separate BGP sessions (e.g., using
different addresses for peering) for BGP SPF Link-State information distribution
information SHOULD be used.
11. Implementation Status
Note RFC Editor: Please remove this section and the associated
references prior to publication.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
The document [I-D.psarkar-lsvr-bgp-spf-impl] contains an
implementation report documenting implementations of BGP Link-State
Short Path First (SPF) routing, i.e., this specification.
12.
10.9. Acknowledgements
The authors would like to thank Sue Hares, Jorge Rabadan, Boris
Hassanov, Dan Frost, Matt Anderson, Fred Baker, Lukas Krattiger,
Yingzhen Qu, and Haibo Wang for their review reviews and comments. Thanks
to Pushpasis Sarkar for discussions on preventing a BGP SPF Router
from being used for non-local traffic (i.e., transit traffic).
The authors extend a special thanks to Eric Rosen for fruitful
discussions on BGP-LS-SPF convergence as compared to IGPs.
The authors would also like extend thanks to thank the following people:
* Alvaro Retana for multiple AD reviews and discussions.
The authors would to thank
* Ketan Talaulikar for an extensive shepherd review.
The authors would like to thank
* Adrian Farrel, Li Zhang, and Jie Dong for WG last call Last Call review
comments.
The authors would to like to thank
* Jim Guichard for his AD review and discussion.
The authors would to like to thank
* David Dong for his IANA review.
The authors would to like to thank
* Joel Halpern for his GENART review.
The authors would to like to thank
* Erik Kline, Eric Vyncke, Mahesh Jethanandani, and Roman Danyliw
for IESG review comments.
The authors would to like to thank
* John Scudder for his detailed IESG review and specifically for
helping align the document with BGP documents.
13.
10.10. Contributors
In addition to the authors listed on the front page, the
The following
co-authors have people contributed substantially to the document. content of this
document and should be considered coauthors:
Derek Yeung
Arrcus, Inc.
Email: derek@arrcus.com
Gunter Van De Velde
Nokia
Email: gunter.van_de_velde@nokia.com
Abhay Roy
Arrcus, Inc.
Email: abhay@arrcus.com
Venu Venugopal
Cisco Systems
Email: venuv@cisco.com
Chaitanya Yadlapalli
AT&T
Email: cy098d@att.com
14.
11. References
14.1.
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4202] Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4202, DOI 10.17487/RFC4202, October 2005,
<https://www.rfc-editor.org/info/rfc4202>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5130] Previdi, S., Shand, M., Ed., and C. Martin, "A Policy
Control Mechanism in IS-IS Using Administrative Tags",
RFC 5130, DOI 10.17487/RFC5130, February 2008,
<https://www.rfc-editor.org/info/rfc5130>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6119] Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
February 2011, <https://www.rfc-editor.org/info/rfc6119>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018,
<https://www.rfc-editor.org/info/rfc8405>.
[RFC8654] Bush, R., Patel, K., and D. Ward, "Extended Message
Support for BGP", RFC 8654, DOI 10.17487/RFC8654, October
2019, <https://www.rfc-editor.org/info/rfc8654>.
[RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
2021, <https://www.rfc-editor.org/info/rfc9086>.
[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.
14.2. Informational
11.2. Informative References
[I-D.ietf-lsvr-applicability]
Patel, K., Lindem, A., Zandi, S., Dawra, G., and J. Dong,
"Usage and Applicability of BGP Link-State Shortest Path
Routing (BGP-SPF) in Data Centers", Work in Progress,
January 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-lsvr-applicability-21>.
[I-D.psarkar-lsvr-bgp-spf-impl]
Sarkar, P., Patel, K., Pallagatti, S., and
sajibasil@gmail.com, "BGP Shortest Path Routing Extension
Implementation Report", Work in Progress, Internet-Draft,
draft-psarkar-lsvr-bgp-spf-impl-02, 23 September 2024,
<https://datatracker.ietf.org/doc/html/draft-psarkar-lsvr-
bgp-spf-impl-02>.
draft-ietf-lsvr-applicability-22>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, DOI 10.17487/RFC4593,
October 2006, <https://www.rfc-editor.org/info/rfc4593>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007,
<https://www.rfc-editor.org/info/rfc4724>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Authors' Addresses
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Acee Lindem
LabN Consulting, LLC
301 Midenhall Way
Cary, NC 27513
United States of America
Email: acee.ietf@gmail.com
Shawn Zandi
LinkedIn
222 2nd Street
San Francisco, CA 94105
United States of America
Email: szandi@linkedin.com
Wim Henderickx
Nokia
copernicuslaan 50
2018 Antwerp
Belgium
Email: wim.henderickx@nokia.com