Routing Area Working Group
Internet Engineering Task Force (IETF)                    P. Sarkar, Ed.
Internet-Draft
Request for Comments: 8518                                  Arrcus, Inc.
Updates: 5286 (if approved)                                           U. Chunduri, Ed.
Intended status:
Category: Standards Track                                     Huawei USA
Expires: May 25, 2019
ISSN: 2070-1721                                                 S. Hegde
                                                  Juniper Networks, Inc.
                                                             J. Tantsura
                                                            Apstra, Inc.
                                                              H. Gredler
                                                           RtBrick, Inc.
                                                       November 21, 2018
                                                              March 2019

       Selection of Loop-Free Alternates selection for Multi-Homed Prefixes
               draft-ietf-rtgwg-multihomed-prefix-lfa-09

Abstract

   Deployment experience gained from implementing algorithms to
   determine Loop-Free Alternates (LFAs) for multi-homed prefixes (MHPs)
   has revealed some avenues for potential improvement.  This document
   provides explicit inequalities that can be used to evaluate neighbors
   as a potential alternates for multi-homed prefixes. MHPs.  It also provides detailed criteria
   for evaluating potential alternates for external prefixes advertised
   by OSPF ASBRs.  This documents document updates and
   expands some of the "Routing Aspects" as specified in Section 6 of RFC 5286.

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 RFC8174 [RFC2119] RFC8174 [RFC8174] when, and only when, they
   appear in all capitals, as shown here. 5286 by
   expanding some of the routing aspects.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of six months RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 25, 2019.
   https://www.rfc-editor.org/info/rfc8518.

Copyright Notice

   Copyright (c) 2018 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3 ....................................................3
      1.1. Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   3 ...................................................3
      1.2. Requirements Language ......................................4
   2. LFA inequalities Inequalities for MHPs . . . . . . . . . . . . . . . . . .   4 .......................................4
   3. LFA selection Selection for the multi-homed prefixes  . . . . . . . . .   5 MHPs ..........................................5
      3.1. Improved coverage Coverage with simplified approach Simplified Approach to MHPs  . . .   7 .........7
      3.2. IS-IS ATT Bit considerations  . . . . . . . . . . . . . .   8 Considerations ...............................8
   4. LFA selection Selection for the multi-homed external prefixes . . . . .   9 Multi-Homed External Prefixes .................9
      4.1. IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .   9 ......................................................9
      4.2. OSPF  . . . . . . . . . . . . . . . . . . . . . . . . . .   9 .......................................................9
           4.2.1. Rules to select alternate ASBR  . . . . . . . . . . .   9 Select Alternate ASBRs .....................9
               4.2.1.1. Multiple ASBRs belonging different area . . . . .  11 Belonging to Different Areas ..11
               4.2.1.2. Type 1 and Type 2 costs . . . . . . . . . . . . .  11 Costs ......................11
               4.2.1.3.  RFC1583compatibility RFC1583Compatibility is set Set to enabled  . . . . .  11 "Enabled" .....11
               4.2.1.4. Type 7 routes . . . . . . . . . . . . . . . . . .  11 Routes ................................12
           4.2.2. Inequalities to be applied Be Applied for alternate Alternate ASBR
               selection . . . . . . . . . . . . . . . . . . . . . .  12
                  Selection ..........................................12
               4.2.2.1. Forwarding address set Address Set to non-zero value  . . . .  12 Non-zero Value .....12
               4.2.2.2. ASBRs advertising type1 Advertising Type 1 and type2 cost  . . . . .  13 Type 2 Costs ....13
   5. LFA Extended Procedures . . . . . . . . . . . . . . . . . . .  13 ........................................14
      5.1. Links with IGP MAX_METRIC . . . . . . . . . . . . . . . .  13 .................................14
      5.2.  Multi Topology MT Considerations . . . . . . . . . . . . . .  14 .........................................15
   6. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15 ............................................15
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .  15
   9. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. ........................................16
   8. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1. .....................................................16
      8.1. Normative References . . . . . . . . . . . . . . . . . .  16
     10.2. ......................................16
      8.2. Informative References . . . . . . . . . . . . . . . . .  16 ....................................16
   Acknowledgements ..................................................18
   Contributors ......................................................18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18 ................................................19

1.  Introduction

   A framework for the development of IP fast-reroute Fast Reroute (FRR) mechanisms
   is detailed in [RFC5714].  The use of LFAs for IP Fast Reroute FRR is specified in
   [RFC5286].  If a prefix is advertised by more than one
   router router, that
   prefix is called as multi-homed a "multi-homed prefix (MHP). (MHP)".  MHPs generally occur
   for prefixes obtained from outside the routing domain by multiple
   routers, for subnets on links where the subnet is announced from
   multiple ends of the link, and for prefixes advertised by multiple
   routers to provide resiliency.

   Section 6.1 of [RFC5286] describes a method to determine LFAs for
   MHPs.  This document describes a procedure using explicit
   inequalities that can be used by a computing router to evaluate a
   neighbor as a potential alternate for a an MHP.  The results obtained
   are equivalent to those obtained using the method described in
   Section 6.1 of [RFC5286].

   Section 6.3 of [RFC5286] discusses complications associated with
   computing LFAs for MHPs in OSPF.  This document provides detailed
   criteria for evaluating potential alternates for external prefixes
   advertised by OSPF ASBRs, as well as explicit inequalities.

   This document also provides clarifications, clarifications and additional
   considerations to [RFC5286], [RFC5286] to address a few coverage and operational
   observations.  These observations are in concerned with 1) the area of handling IS-IS attach (ATT) ATT
   (attach) bit in Level-1 the Level 1 (L1) area, 2) links provisioned with
   MAX_METRIC (see Section 5.1) for traffic engineering (TE) purposes purposes,
   and in the area of
   Multi Topology 3) multi-topology (MT) IGP deployments.  These are elaborated in
   detail in Section Sections 3.2 and Section 5.

   This specification uses the same terminology introduced in [RFC5714]
   to represent LFA and builds on the inequalities notation for inequalities used in
   [RFC5286] to compute LFAs for MHPs.

1.1.  Acronyms

   AF      -  Address Family

   ATT     -  IS-IS Attach Bit

   ECMP    -  Equal Cost Multi Path  Equal-Cost Multipath

   FRR     -  Fast Reroute

   IGP     -  Interior Gateway Protocol

   IS-IS   -  Intermediate System to Intermediate System
   LFA     -  Loop-Free Alternate

   LSP     -  IS-IS  Link State PDU (IS-IS)

   MHP     -  Multi-Homed Prefix

   MT      -  Multi-Topology

   OSPF    -  Open Shortest Path First

   MHP     -  Multi-homed Prefix

   MT      -  Multi Topology

   SPF     -  Shortest Path First

1.2.  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.  LFA inequalities Inequalities for MHPs

   This document proposes the following set of LFA inequalities for
   selecting the most appropriate LFAs for MHPs.  D_opt(X,Y) terminology  Distance_opt(X,Y)
   (called "D_opt(X,Y)" in this document) is defined in [RFC5714], which [RFC5714] and is
   nothing but the metric sum of the shortest path from X to Y and Cost(X,Y) Y.
   Cost(X,Y), introduced in this document document, is defined as the metric
   value of prefix Y from the prefix advertising node X.  These LFAs can
   be derived from the inequalities in [RFC5286] combined with the
   observation that D_opt(N,P) = Min (D_opt(N,PO_i) + Cost(PO_i,P)) over
   all PO_i

Link-Protection: PO_i.

   Link-Protecting LFAs:
      A neighbor N can provide a loop-free alternate (LFA) an LFA if and only if

      D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
                                    D_opt(S,PO_best) + Cost(PO_best,P)

Link-Protection

   Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:
      A subset of loop-free alternates are downstream paths that must
      meet a more restrictive condition that is applicable to more
      complex failure scenarios scenarios.

      D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(S,PO_best) + Cost(PO_best,P)

Node-Protection:

   Node-Protecting LFAs:
      For an alternate next-hop next hop N to protect against node failure of a
      primary neighbor E for MHP P, N must be loop-free with respect to
      both E and mhp MHP P.  In other words, N's path to MHP P must not go
      through E (where N is the neighbor providing a loop-free alternate)
      alternate).

      D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
                                    D_opt(E,PO_best) + Cost(PO_best,P)

Where,

      Where:

      P           -  The multi-homed prefix MHP being evaluated for computing alternates

      S           -  The computing router

      N           -  The alternate router being evaluated

      E           -  The primary next-hop next hop on the shortest path from S to
                     prefix P. P

      PO_i        -  The specific prefix-originating router being
                  evaluated.
                     evaluated

      PO_best     -  The prefix-originating router on the shortest path
                     from the computing router S to prefix P. P

      Cost(X,P)   - Cost  The cost of reaching the prefix P from prefix
                     originating node X. X

      D_opt(X,Y)  - Distance  The distance on the shortest path from node X to
                     node
                  Y.

                    Figure 1: LFA inequalities for MHPs Y

3.  LFA selection Selection for the multi-homed prefixes MHPs

   To compute a valid LFA for a given MHP P, a computing router S MUST MUST,
   for each alternate neighbor N, follow one of the appropriate
   procedures below, for each alternate
   neighbor N and below once for each remote node that originated the prefix
   P.

     Link-Protection :
     =================

   Link-Protecting LFAs:

   1. if,  If, in addition to being an alternate neighbor, N is also a prefix-originator
       prefix originator of P,
        1.a.

       A.  Select N as a an LFA for prefix P (irrespective of the metric
           advertised by N for the prefix P).

   2.  Else, evaluate the link-protecting LFA inequality for P with
        the N as
       the alternate neighbor.
        2.a.

       A.  If the LFA inequality condition is met, select N as a an LFA
           for prefix P.
        2.b.

       B.  Else, N is not a an LFA for prefix P.

     Link-Protection

   Link-Protecting + Downstream-paths-only :
     ========================================= LFAs:

   1.  Evaluate the link-protecting + downstream-only downstream-paths-only LFA
       inequality for P with the N as the alternate neighbor.
        1.a.

       A.  If the LFA inequality condition is met, select N as a an LFA
           for prefix P.
        1.b.

       B.  Else, N is not a an LFA for prefix P.

     Node-Protection :
     =================

   Node-Protecting LFAs:

   1. if,  If, in addition to being an alternate neighbor, N is also a prefix-originator
       prefix originator of P,
        1.a.

       A.  Select N as a an LFA for prefix P (irrespective of the metric
           advertised by N for the prefix P).

   2.  Else, evaluate the appropriate node-protecting LFA inequality for
       P with the N as the alternate neighbor.
        2.a.

       A.  If the LFA inequality condition is met, select N as a an LFA
           for prefix P.
        2.b.

       B.  Else, N is not a an LFA for prefix P.

                Figure 2: Rules for selecting LFA for MHPs

   In case

   If an alternate neighbor N is also one of the prefix-originators prefix originators of
   prefix P, N being a prefix-originator it is guaranteed that N will not loop back packets destined
   for prefix P to computing router S.
   So  Therefore, N MUST be chosen as a
   valid LFA for prefix P, P without evaluating any of the inequalities in Figure 1
   Section 2 as long as a downstream-paths-only LFA is not desired.  To
   ensure such a neighbor N also provides a downstream-paths-only LFA,
   router S MUST also evaluate the
   downstream-only downstream-paths-only LFA inequality
   specified in Figure 1 Section 2 for neighbor N and ensure router N satisfies
   the inequality.

   However, if N is not a prefix-originator prefix originator of P, the computing router
   MUST evaluate one of the corresponding LFA inequalities, as mentioned inequalities defined in Figure 1,
   Section 2 once for each remote node that originated the prefix.
   In case  If
   the inequality is satisfied by the neighbor N N, router S MUST choose
   neighbor N, N as one of the valid LFAs for the prefix P.

   For more specific rules rules, please refer to the later sections of this
   document. Section 4.

3.1.  Improved coverage Coverage with simplified approach Simplified Approach to MHPs

   Section 6.1 of the LFA base specification [RFC5286] Section 6.1 recommends that a
   router computes the alternate next-hop next hop for an IGP MHP by considering
   alternate paths via all routers that have announced that prefix and
   the prefix.  The
   same has been elaborated with appropriate inequalities in the
   above
   previous section.  However, [RFC5286] Section 6.1 of [RFC5286] also allows for
   the router to simplify the MHP calculation by assuming that the MHP
   is solely attached to the router that was its pre-failure optimal
   point of attachment, at the expense of potentially lower coverage.
   If an implementation chooses to simplify the MHP calculation by
   assuming that the MHP is solely attached to the router that was its pre-
   failure
   pre-failure optimal point of attachment, the procedure described in
   this memo can potentially improve coverage for equal cost multi path
   (ECMP) ECMP MHPs without
   incurring extra computational cost.

   This document improves the above approach to provide loop-free
   alternatives without any additional cost for ECMP MHPs as described
   through
   in the below example network presented in Figure 3. 1.  The approach specified
   here may also be applicable for handling default routes as explained
   in Section 3.2.

                         5   +---+  8   +---+  5  +---+
                       +-----| S |------| A |-----| B |
                       |     +---+      +---+     +---+
                       |       |                    |
                       |     5 |                  5 |
                       |       |                    |
                     +---+ 5 +---+   4 +---+  1    +---+
                     | C |---| E |-----| M |-------| F |
                     +---+   +---+     +---+       +---+
                               |   10           5    |
                               +-----------P---------+

                   Figure 3: 1: MHP with same Same ECMP Next-hop Next Hop

   In the above network Figure 1, a prefix P, P is advertised from both Node node E and
   Node node F.
   With a simplified approach taken as specified in [RFC5286] Section 6.1, 6.1 of
   [RFC5286], prefix P will get only link protection a link-protecting LFA through the
   neighbor C while a node protection node-protection path is available through neighbor
   A.  In this scenario, E and F both are pre-failure optimal points of
   attachment and share the same primary next-hop. next hop.  Hence, an
   implementation MAY compare the kind of protection A provides to F
   (link-and-node
   (link and node protection) with the kind of protection C provides to
   E (link protection) and inherit the better alternative to prefix P
   and here it P.
   In this case, the better alternative is A.

   However, in the below example network presented in Figure 4, 2, prefix P has
   an ECMP through both node E and node F with cost 20.  Though it has 2
   two pre-failure optimal points of attachment, the primary next-hop next hop to
   each pre-failure optimal point of attachment is different.  In this
   case, prefix P MUST inherit the corresponding LFAs of each primary
   next-hop
   next hop calculated for the router advertising the same respectively. same.  In the below diagram
   Figure 2, that would be the LFA for node E's E and node F's LFA F, i.e., node N1
   and node N2 N2, respectively.

                                           4      +----+
                               +------------------| N2 |
                               |                  +----+
                               |                    | 4
                        10   +---+         3      +---+
                      +------| S |----------------| B |
                      |      +---+                +---+
                      |        |                    |
                      |     10 |                  1 |
                      |        |                    |
                   +----+ 5  +---+        16       +---+
                   | N1 |----| E |-----------------| F |
                   +----+    +---+                 +---+
                               |   10          16    |
                               +-----------P---------+

                Figure 4: 2: MHP with different Different ECMP Next-hops Next Hops

   In summary, if there are multiple pre-failure points of attachment
   for a MHP an MHP, and the primary next-hop next hop of a an MHP is the same as that of
   the primary next-hop next hop of the router that was the pre-failure optimal
   point of attachment, an implementation MAY provide a better
   protection to the MHP without incurring any additional computation
   cost.

3.2.  IS-IS ATT Bit considerations Considerations

   Per [RFC1195] [RFC1195], a default route needs to be added in Level1 the Level 1 (L1)
   router to the closest reachable Level1/Level2 Level 1 / Level 2 (L1/L2) router in
   the network advertising the ATT (attach) bit in its LSP-0 fragment.
   All L1 routers in the area would do this during the decision process
   with the next- next hop of the default route set to the adjacent router
   through which the closest L1/L2 router is reachable.  The base LFA base
   specification [RFC5286] does not specify any procedure for computing
   LFA for a default route in the IS-IS L1 area.  This document specifies,
   specifies that a node can consider a default route is being
   advertised from the border L1/L2 router where the ATT bit is set, set and
   can do LFA computation for that default route.  But, when multiple
   ECMP L1/L2 routers are reachable in an L1 area area, corresponding best
   LFAs SHOULD be computed for each primary next-hop next hop associated with the
   default route as this would be similar to the ECMP MHP example as
   described in Section 3.1.  Considerations as specified in Section Sections 3 and Section
   3.1 are applicable for default routes, routes if the default route is
   considered as an ECMP MHP.  Note that, that this document doesn't alter any
   ECMP handling rules or computation of LFAs for ECMP in general as
   laid out in [RFC5286].

4.  LFA selection Selection for the multi-homed external prefixes Multi-Homed External Prefixes

   Redistribution of external routes into IGP is required in case of 1) when two
   different networks getting get merged into one or 2) during protocol
   migrations.  External routes could be distributed into an IGP domain
   via multiple nodes to avoid a single point of failure.

   During LFA calculation, alternate LFA next-hops next hops to reach the best
   ASBR could be used as LFA for the routes redistributed via that ASBR.
   When there is no LFA available to the best ASBR, it may be desirable
   to consider the other ASBRs (referred to as alternate ASBR "alternate ASBRs"
   hereafter) redistributing the external routes for LFA selection as
   defined in [RFC5286] and leverage the advantage of having multiple re-
   distributing
   redistributing nodes in the network.

4.1.  IS-IS

   LFA evaluation for multi-homed external prefixes in IS-IS is the same to
   as the multi-homed internal prefixes.  Inequalities described in
   Section 2 would also apply to multi-homed external prefixes.

4.2.  OSPF

   Loop Free Alternates

   The LFA base specification [RFC5286] describes mechanisms to apply
   inequalities to find the loop free loop-free alternate neighbor.  For the
   selection of alternate ASBR for LFA consideration, additional  Additional
   rules have to be applied in selecting the alternate ASBR for LFA
   consideration due to the external route calculation rules imposed by
   [RFC2328].

   This document defines inequalities specifically for the alternate
   loop-free loop-
   free ASBR evaluation, evaluation.  These inequalities are based on those in
   [RFC5286].

4.2.1.  Rules to select alternate ASBR Select Alternate ASBRs

   The process to select an alternate ASBR is best explained using the
   rules below.  The below process below is applied when a primary ASBR for
   the concerned prefix is chosen and there is an alternate ASBR
   originating the same prefix.

   1.  If RFC1583Compatibility is disabled

          1a. if disabled:

       A.  If primary ASBR and alternate ASBR belong to intra-area
                  non-backbone non-
           backbone, go to step 2.
          1b.

       B.  If primary ASBR and alternate ASBR belong to intra-area
           backbone and/or inter-area path path, go to step 2.
          1c. for

       C.  For other paths, skip this alternate ASBR and consider next
           ASBR.

   2.  Compare cost types (type 1/type 1 / type 2) advertised by alternate ASBR
       and
      by the primary ASBR
          2a. ASBR:

       A.  If not the same type type, skip alternate ASBR and consider next
           ASBR.
          2b.

       B.  If same the same, proceed to step 3.

3.If

   3.  If cost types are type 1, compare costs advertised by alternate
       ASBR and by the primary ASBR
             3a. ASBR:

       A.  If costs are the same same, then program ECMP Fast ReRoute (FRR) FRR and return.
             3b. else

       B.  Else, go to step 5..

4 5.

   4.  If cost types are type 2, compare costs advertised by alternate
       ASBR and by the primary ASBR
             4a. ASBR:

       A.  If costs are different, skip alternate ASBR and consider next
           ASBR.
             4b.

       B.  If cost costs are the same, proceed to step 4c 4C to compare
                     cost costs to
           reach ASBR/forwarding address.
             4c.

       C.  If cost costs to reach ASBR/forwarding address are also same the same,
           program ECMP FRR and return.
             4d.

       D.  If cost costs to reach ASBR/forwarding address are different different, go
           to step 5.

   5. If  Compare route type types (type 5/type 5 and type 7)
           5a. for alternate ASBR and
       primary ASBR:

       A.  If route type is types are the same, check if the route p-bit and the
           forwarding address field for routes from both ASBRs match.
           If p-bit and forwarding address matches match, proceed to step 6.  If
           not, skip this alternate ASBR and consider next ASBR.
           5b.

       B.  If route type is types are not the same, skip this alternate ASBR and
           consider next alternate ASBR.

   6.  Apply inequality on the alternate ASBR.

           Figure 5: Rules for selecting alternate ASBR in OSPF

4.2.1.1.  Multiple ASBRs belonging different area Belonging to Different Areas

   When "RFC1583compatibility" RFC1583Compatibility is set to disabled, "disabled", OSPF [RFC2328]
   defines certain rules of preference to choose the ASBRs.  While
   selecting an alternate ASBR for loop evaluation for LFA, these rules
   should be applied to ensure that the alternate neighbor does not
   cause looping.

   When there are multiple ASBRs belonging to different area areas
   advertising the same prefix, pruning rules as defined in [RFC2328] section Section 16.4
   of [RFC2328] are applied.  The alternate ASBRs pruned using above these
   rules are not considered for LFA evaluation.

4.2.1.2.  Type 1 and Type 2 costs Costs

   If there are multiple ASBRs not pruned via the rules described in
   Section 4.2.1.1, the cost type advertised by the ASBRs is compared.
   ASBRs advertising type 1 costs are preferred preferred, and the type 2 costs
   are pruned.  If two ASBRs advertise the same type 2 cost, the
   alternate ASBRs are considered along with their cost to reach the
   ASBR/forwarding address for evaluation.  If the two ASBRs have the
   same type 2 cost as well as the same cost to reach the ASBR, ECMP FRR
   is programmed.  When there are multiple ASBRs advertising the same
   type 2 cost for the prefix, primary Autonomous System (AS) external
   route calculation calculation, as described in
   [RFC2328] section Section 16.4.1 of [RFC2328],
   selects the route with the lowest type 2 cost.  ASBRs advertising a
   different type 2 cost (higher cost) are not considered for LFA
   evaluation.  Alternate ASBRs advertising a type 2 cost for the prefix
   but are not chosen as primary due to a higher cost to reach ASBR are
   considered for LFA evaluation.  The inequalities for evaluating
   alternate ASBR for type 1 and type 2 costs are same, as the alternate
   ASBRs with different type 2 costs are pruned and the evaluation is
   based on ASBRS with equal type 2 cost ASBRS. costs.

4.2.1.3.  RFC1583compatibility  RFC1583Compatibility is set Set to enabled "Enabled"

   When RFC1583Compatibility is set to enabled, "enabled", multiple ASBRs
   belonging to different area areas advertising the same prefix are chosen
   based on cost and hence are valid alternate ASBRs for the LFA
   evaluation.  The inequalities described in Section 4.2.2 are
   applicable based on forwarding address and cost type advertised in External
   the external Link State Advertisement (LSA).

4.2.1.4.  Type 7 routes Routes

   Type 5 routes always get preference over Type 7 type 7, and the alternate
   ASBRs chosen for LFA calculation should belong to the same type.
   Among
   Type type 7 routes, routes with the p-bit and forwarding address set
   have a higher preference than routes without these attributes.
   Alternate ASBRs selected for LFA comparison should have the same
   p-bit and forwarding address attributes.

4.2.2.  Inequalities to be applied Be Applied for alternate Alternate ASBR selection Selection

   The alternate ASBRs selected using above the mechanism described in
   Section 4.2.1, 4.2.1 are evaluated for Loop free loop-free criteria using below
   inequalities. the
   inequalities below.

4.2.2.1.  Forwarding address set Address Set to non-zero value Non-zero Value

   Similar to the inequalities as defined in Figure 1, Section 2, the following
   inequalities are defined when the forwarding address is a non-zero
   value.

  Link-Protection:

   Link-Protecting LFAs:

      F_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
                                    F_opt(S,PO_best) + Cost(PO_best,P)

  Link-Protection

   Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:

      F_opt(N,PO_i)+ Cost(PO_i,P) < F_opt(S,PO_best) + Cost(PO_best,P)

  Node-Protection:

   Node-Protecting LFAs:

      F_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
                                    F_opt(E,PO_best) + Cost(PO_best,P)

  Where,

      Where:

      P           -  The multi-homed prefix MHP being evaluated for computing alternates

      S           -  The computing router

      N           -  The alternate router being evaluated

      E           -  The primary next-hop next hop on the shortest path from S to
                     prefix P. P

      PO_i        -  The specific prefix-originating router being
                            evaluated.
                     evaluated
      PO_best     -  The prefix-originating router on the shortest path
                     from the computing router S to prefix P. P

      Cost(X,Y)   - External  The external cost for Y as advertised by X

      F_opt(X,Y)  - Distance  The distance on the shortest path from node X to Forwarding
                     the forwarding address specified by ASBR Y. Y

      D_opt(X,Y)  - Distance  The distance on the shortest path from node X to
                     node Y.

    Figure 6: LFA inequality definition when forwarding address is non-
                                   zero Y

4.2.2.2.  ASBRs advertising type1 Advertising Type 1 and type2 cost Type 2 Costs

   Similar to the inequalities as defined in Figure 1, Section 2, the following
   inequlities
   inequalities are defined for type1 type 1 and type2 cost.

   Link-Protection: type 2 costs.

   Link-Protecting LFAs:

      D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
                                    D_opt(S,PO_best) + Cost(PO_best,P)

   Link-Protection

   Link-Protecting + Downstream-paths-only: Downstream-paths-only LFAs:

      D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(S,PO_best) + Cost(PO_best,P)

   Node-Protection:

   Node-Protecting LFAs:

      D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,E) +
                                    D_opt(E,PO_best) + Cost(PO_best,P)

   Where,

      Where:

      P           -  The multi-homed prefix MHP being evaluated for computing alternates

      S           -  The computing router

      N           -  The alternate router being evaluated

      E           -  The primary next-hop next hop on the shortest path from S to
                     prefix P. P

      PO_i        -  The specific prefix-originating router being
                             evaluated.
                     evaluated

      PO_best     -  The prefix-originating router on the shortest path
                     from the computing router S to prefix P. P
      Cost(X,Y)   - External  The external cost for Y as advertised by X. X

      D_opt(X,Y)  - Distance  The distance on the shortest path from node X to
                     node Y.

       Figure 7: LFA inequality definition for type1 and type2 cost Y

5.  LFA Extended Procedures

   This section explains the additional considerations in various
   aspects as listed below to the base LFA base
   specification [RFC5286].

5.1.  Links with IGP MAX_METRIC

   Section

   Sections 3.5 and 3.6 of [RFC5286] describe procedures for excluding
   nodes and links from use in alternate paths based on the maximum link
   metric.  If these procedures are strictly followed, there are
   situations, as described below, where the only potential alternate
   available which that satisfies the basic loop-free condition will not be
   considered as alternative.  This document refers to the maximum link
   metric in IGPs as the MAX_METRIC.  MAX_METRIC is defined for IS-IS in
   [RFC5305], where it is called as "maximum link
   metric" when defined for IS-IS in [RFC5305] and "MaxLinkMetric" when
   defined for OSPF in [RFC6987], where it is called as "MaxLinkMetric". [RFC6987].

                             +---+  10  +---+  10 +---+
                             | S |------|N1 |-----|D1 |
                             +---+      +---+     +---+
                               |                    |
                            10 |                 10 |
                               |MAX_METRIC(N2 to S) |
                               |                    |
                               |       +---+        |
                               +-------|N2 |--------+
                                       +---+
                                     10  |
                                       +---+
                                       |D2 |
                                       +---+

                    Figure 8: 3: Link with IGP MAX_METRIC

   In the simple example network, network in Figure 3, all the link costs links have a cost
   of 10 in both directions, except for the link between S and N2.  The
   S-N2 link has a cost of 10 in the forward direction direction, i.e., from S to
   N2, and a cost of MAX_METRIC (0xffffff /2^24 - 1 for IS-IS and 0xffff
   for OSPF) in the reverse direction direction, i.e., from N2 to S for a specific end-
   to-end Traffic Engineering (TE)
   end-to-end TE requirement of the operator.  At node S, D1 is
   reachable through N1 with a cost of 20, and D2 is reachable through
   N2 with a cost of 20.  Even though neighbor N2 satisfies the basic
   loop-free condition (inequality 1 of [RFC5286]) for D1, S's neighbor
   N2 could be excluded as a potential alternative because of the
   current exclusions as specified in section Sections 3.5 and 3.6 procedure of [RFC5286].
   But, as the primary traffic destined to D2 continues to use the link and hence link;
   hence, irrespective of the reverse metric in this case, the same link
   MAY be used as a potential LFA for D1.

   Alternatively, the reverse metric of the link MAY be configured with
   MAX_METRIC-1,
   MAX_METRIC-1 so that the link can be used as an alternative while
   meeting the operator's TE requirements and without having to update
   the router to fix this particular issue.

5.2.  Multi Topology  MT Considerations

   Section

   Sections 6.2 and 6.3.2 of [RFC5286] state that multi-topology OSPF
   and IS-IS are out of scope for that specification.  This memo
   clarifies and describes the applicability.

   In Multi Topology (MT) multi-topology IGP deployments, for each MT ID, MT-ID, a separate
   shortest path tree (SPT) is built with topology specific adjacencies, topology-specific adjacencies
   so the LFA principles laid out in [RFC5286] are actually applicable
   for MT IS-IS [RFC5120] LFA SPF.  The primary difference in this case
   is,
   is identifying the eligible-set eligible set of neighbors for each LFA
   computation which
   computation; this is done per MT ID. MT-ID.  The eligible-set eligible set for each MT ID MT-ID
   is determined by the presence of IGP adjacency from Source the source to the
   neighboring node on that MT-ID apart from the administrative
   restrictions and other checks laid out in [RFC5286].  The same is
   also applicable for MT-OSPF [RFC4915] or different AFs in multi multi-
   instance OSPFv3 [RFC5838].

   However

   However, for MT IS-IS, if a "standard unicast topology" is used with
   MT-ID #0
   [RFC5286] [RFC5120] and both IPv4 [RFC5305] and IPv6 routes/AFs
   [RFC5308] are present, then the condition of network congruency is
   applicable for LFA computation as well.  Network congruency here
   refers to, to having the same address families provisioned on all the
   links and all the nodes of the network with MT-ID #0.  Here  Here, with single decision process a
   single-decision process, both IPv4 and IPv6 next-hops next hops are computed
   for all the prefixes in the
   network and similarly network.  Similarly, with one LFA
   computation from all eligible neighbors per [RFC5286], all potential
   alternatives can be computed.

6.  IANA Considerations

   This document has no actions for IANA. IANA actions.

7.  Acknowledgements

   Authors acknowledge Alia Atlas and Salih K A for their useful
   feedback and inputs.  Thanks to Stewart Bryant for being document
   shepherd and providing detailed review comments.  Thanks to Elwyn
   Davies for reviewing and providing feedback as part of Gen-art
   review.  Thanks to Alvaro Retena, Adam Roach, Ben Campbell, Benjamin
   Kaduk and sponsoring Routing Area Director Martin Vigoureux for
   providing detailed feedback and suggestions.

8.  Contributing Authors

   The following people contributed substantially to the content of this
   document and should be considered co-authors.

   Chris Bowers
   Juniper Networks, Inc.
   1194 N. Mathilda Ave,
   Sunnyvale, CA 94089, USA

   Email: cbowers@juniper.net

   Bruno Decraene
   Orange,
   France

   Email: bruno.decraene@orange.com

9.  Security Considerations

   The existing OSPF security considerations continue to apply, as do
   the recommended manual key management mechanisms specified in
   [RFC7474].  The existing security considerations for IS-IS also
   continue to apply, as specified in [RFC5304] and [RFC5310] and
   extended by [RFC7645] for KARP. Keying and Authentication for Routing
   Protocols (KARP).  This document does not change any of the discussed
   protocol specifications [RFC1195] [RFC5120] [RFC2328]
   [RFC5838], (i.e., [RFC1195], [RFC5120], [RFC2328], and
   [RFC5838]); therefore, the security considerations of the LFA base
   specification [RFC5286] therefore continue to apply.

10.

8.  References

10.1.

8.1.  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>.

   [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>.

   [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>.

10.2.

8.2.  Informative References

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <https://www.rfc-editor.org/info/rfc4915>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [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>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework",
              RFC 5714, DOI 10.17487/RFC5714, January 2010,
              <https://www.rfc-editor.org/info/rfc5714>.

   [RFC5838]  Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
              R. Aggarwal, "Support of Address Families in OSPFv3",
              RFC 5838, DOI 10.17487/RFC5838, April 2010,
              <https://www.rfc-editor.org/info/rfc5838>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <https://www.rfc-editor.org/info/rfc6987>.

   [RFC7474]  Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
              "Security Extension for OSPFv2 When Using Manual Key
              Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
              <https://www.rfc-editor.org/info/rfc7474>.

   [RFC7645]  Chunduri, U., Tian, A., and W. Lu, "The Keying and
              Authentication for Routing Protocol (KARP) IS-IS Security
              Analysis", RFC 7645, DOI 10.17487/RFC7645, September 2015,
              <https://www.rfc-editor.org/info/rfc7645>.

Acknowledgements

   The authors acknowledge Alia Atlas and Salih K.A. for their useful
   feedback and input.  Thanks to Stewart Bryant for being Document
   Shepherd and providing detailed review comments.  Thanks to Elwyn
   Davies for reviewing and providing feedback as part of the Gen-ART
   review.  Thanks to Alvaro Retana, Adam Roach, Ben Campbell, Benjamin
   Kaduk, and sponsoring Routing Area Director Martin Vigoureux for
   providing detailed feedback and suggestions.

Contributors

   The following people contributed substantially to the content of this
   document and should be considered coauthors:

   Chris Bowers
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   United States of America

   Email: cbowers@juniper.net

   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com

Authors' Addresses

   Pushpasis Sarkar (editor)
   Arrcus, Inc.

   Email: pushpasis.ietf@gmail.com

   Uma Chunduri (editor)
   Huawei USA
   2330 Central Expressway
   Santa Clara, CA  95050
   USA
   United States of America

   Email: uma.chunduri@huawei.com

   Shraddha Hegde
   Juniper Networks, Inc.
   Electra, Exora Business Park
   Bangalore, KA  560103
   India

   Email: shraddha@juniper.net

   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com

   Hannes Gredler
   RtBrick, Inc.

   Email: hannes@rtbrick.com