    Internet Engineering Task Force (IETF)                    P. Sarkar, Ed.
    Request for Comments: 8518                                  Arrcus, Inc.
    Updates: 5286                                           U. Chunduri, Ed.
    Category: Standards Track                                     Huawei USA
    ISSN: 2070-1721                                                 S. Hegde
                                                      Juniper Networks, Inc.
                                                                 J. Tantsura
                                                                Apstra, Inc.
                                                                  H. Gredler
                                                               RtBrick, Inc.
                                                                  March 2019

           Selection of Loop-Free Alternates for Multi-Homed Prefixes

    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 potential alternates for MHPs.  It also provides detailed criteria
       for evaluating potential alternates for external prefixes advertised
       by OSPF ASBRs.  This document updates Section 6 of RFC 5286 by
       expanding some of the routing aspects.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 7841.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       https://www.rfc-editor.org/info/rfc8518.

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    RFC 8518                 LFA Selection for MHPs               March 2019

    Copyright Notice

       Copyright (c) 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
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1. Introduction ....................................................3
          1.1. Acronyms ...................................................4
          1.2. Requirements Language ......................................4
       2. LFA Inequalities for MHPs .......................................4
       3. LFA Selection for MHPs ..........................................6
          3.1. Improved Coverage with Simplified Approach to MHPs .........7
          3.2. IS-IS ATT Bit Considerations ...............................9
       4. LFA Selection for Multi-Homed External Prefixes ................10
          4.1. IS-IS .....................................................10
          4.2. OSPF ......................................................10
               4.2.1. Rules to Select Alternate ASBRs ....................10
                   4.2.1.1. Multiple ASBRs Belonging to Different Areas ..12
                   4.2.1.2. Type 1 and Type 2 Costs ......................12
                   4.2.1.3. RFC1583Compatibility is Set to "Enabled" .....12
                   4.2.1.4. Type 7 Routes ................................13
               4.2.2. Inequalities to Be Applied for Alternate ASBR
                      Selection ..........................................13
                   4.2.2.1. Forwarding Address Set to Non-zero Value .....13
                   4.2.2.2. ASBRs Advertising Type 1 and Type 2 Costs ....14
       5. LFA Extended Procedures ........................................15
          5.1. Links with IGP MAX_METRIC .................................15
          5.2. MT Considerations .........................................16
       6. IANA Considerations ............................................16
       7. Security Considerations ........................................17
       8. References .....................................................17
          8.1. Normative References ......................................17
          8.2. Informative References ....................................17
       Acknowledgements ..................................................19
       Contributors ......................................................19
       Authors' Addresses ................................................20

    Sarkar, et al.               Standards Track                    [Page 2]

    RFC 8518                 LFA Selection for MHPs               March 2019

    1.  Introduction

       A framework for the development of IP Fast Reroute (FRR) mechanisms
       is detailed in [RFC5714].  The use of LFAs for IP FRR is specified in
       [RFC5286].  If a prefix is advertised by more than one router, that
       prefix is called a "multi-homed prefix (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 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 and additional
       considerations to [RFC5286] to address a few coverage and operational
       observations.  These observations are concerned with 1) the IS-IS ATT
       (attach) bit in the Level 1 (L1) area, 2) links provisioned with
       MAX_METRIC (see Section 5.1) for traffic engineering (TE) purposes,
       and 3) multi-topology (MT) IGP deployments.  These are elaborated in
       detail in Sections 3.2 and 5.

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

    Sarkar, et al.               Standards Track                    [Page 3]

    RFC 8518                 LFA Selection for MHPs               March 2019

    1.1.  Acronyms

       AF      -  Address Family

       ATT     -  IS-IS Attach Bit

       ECMP    -  Equal-Cost Multipath

       FRR     -  Fast Reroute

       IGP     -  Interior Gateway Protocol

       IS-IS   -  Intermediate System to Intermediate System

       LFA     -  Loop-Free Alternate

       LSP     -  Link State PDU (IS-IS)

       MHP     -  Multi-Homed Prefix

       MT      -  Multi-Topology

       OSPF    -  Open Shortest Path First

       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 for MHPs

       This document proposes the following set of LFA inequalities for
       selecting the most appropriate LFAs for MHPs.  Distance_opt(X,Y)
       (called "D_opt(X,Y)" in this document) is defined in [RFC5714] and is
       nothing but the metric sum of the shortest path from X to Y.
       Cost(X,Y), introduced in this 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.

    Sarkar, et al.               Standards Track                    [Page 4]

    RFC 8518                 LFA Selection for MHPs               March 2019

       Link-Protecting LFAs:
          A neighbor N can provide 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-Protecting + 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.

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

       Node-Protecting LFAs:
          For an alternate 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 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).

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

          Where:

          P           -  The MHP being evaluated for computing alternates

          S           -  The computing router

          N           -  The alternate router being evaluated

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

          PO_i        -  The specific prefix-originating router being
                         evaluated

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

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

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

    Sarkar, et al.               Standards Track                    [Page 5]

    RFC 8518                 LFA Selection for MHPs               March 2019

    3.  LFA Selection for MHPs

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

       Link-Protecting LFAs:

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

           A.  Select N as 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 N as
           the alternate neighbor.

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

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

       Link-Protecting + Downstream-paths-only LFAs:

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

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

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

       Node-Protecting LFAs:

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

           A.  Select N as 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 N as the alternate neighbor.

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

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

    Sarkar, et al.               Standards Track                    [Page 6]

    RFC 8518                 LFA Selection for MHPs               March 2019

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

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

       For more specific rules, please refer to Section 4.

    3.1.  Improved Coverage with Simplified Approach to MHPs

       Section 6.1 of the LFA base specification [RFC5286] recommends that a
       router computes the alternate next hop for an IGP MHP by considering
       alternate paths via all routers that have announced that prefix.  The
       same has been elaborated with appropriate inequalities in the
       previous section.  However, 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 optimal point of attachment, the procedure described in
       this memo can potentially improve coverage for 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
       in the example network presented in Figure 1.  The approach specified
       here may also be applicable for handling default routes as explained
       in Section 3.2.

    Sarkar, et al.               Standards Track                    [Page 7]

    RFC 8518                 LFA Selection for MHPs               March 2019

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

                       Figure 1: MHP with Same ECMP Next Hop

       In Figure 1, a prefix P is advertised from both node E and node F.
       With a simplified approach taken as specified in Section 6.1 of
       [RFC5286], prefix P will get only a link-protecting LFA through the
       neighbor C while a 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.  Hence, an
       implementation MAY compare the kind of protection A provides to F
       (link and node protection) with the kind of protection C provides to
       E (link protection) and inherit the better alternative to prefix P.
       In this case, the better alternative is A.

       However, in the example network presented in Figure 2, prefix P has
       an ECMP through both node E and node F with cost 20.  Though it has
       two pre-failure optimal points of attachment, the primary 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 calculated for the router advertising the same.  In
       Figure 2, that would be the LFA for node E and node F, i.e., node N1
       and node N2, respectively.

    Sarkar, et al.               Standards Track                    [Page 8]

    RFC 8518                 LFA Selection for MHPs               March 2019

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

                    Figure 2: MHP with Different ECMP Next Hops

       In summary, if there are multiple pre-failure points of attachment
       for an MHP, and the primary next hop of an MHP is the same as that of
       the primary 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

       Per [RFC1195], a default route needs to be added in the Level 1 (L1)
       router to the closest reachable 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 hop of the default route set to the adjacent router
       through which the closest L1/L2 router is reachable.  The 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 that a node can consider a default route is being
       advertised from the border L1/L2 router where the ATT bit is set and
       can do LFA computation for that default route.  But, when multiple
       ECMP L1/L2 routers are reachable in an L1 area, corresponding best
       LFAs SHOULD be computed for each primary next hop associated with the
       default route as this would be similar to the ECMP MHP example
       described in Section 3.1.  Considerations specified in Sections 3 and
       3.1 are applicable for default routes if the default route is
       considered an ECMP MHP.  Note that this document doesn't alter any
       ECMP handling rules or computation of LFAs for ECMP in general as
       laid out in [RFC5286].

    Sarkar, et al.               Standards Track                    [Page 9]

    RFC 8518                 LFA Selection for MHPs               March 2019

    4.  LFA Selection for Multi-Homed External Prefixes

       Redistribution of external routes into IGP is required 1) when two
       different networks get merged into one or 2) during protocol
       migrations.

       During LFA calculation, alternate LFA 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 ASBRs"
       hereafter) redistributing the external routes for LFA selection as
       defined in [RFC5286] and leverage the advantage of having multiple
       redistributing nodes in the network.

    4.1.  IS-IS

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

    4.2.  OSPF

       The LFA base specification [RFC5286] describes mechanisms to apply
       inequalities to find the loop-free alternate neighbor.  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 alternate loop-
       free ASBR evaluation.  These inequalities are based on those in
       [RFC5286].

    4.2.1.  Rules to Select Alternate ASBRs

       The process to select an alternate ASBR is best explained using the
       rules below.  The 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:

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

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

    Sarkar, et al.               Standards Track                   [Page 10]

    RFC 8518                 LFA Selection for MHPs               March 2019

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

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

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

           B.  If the same, proceed to step 3.

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

           A.  If costs are the same, then program ECMP FRR and return.

           B.  Else, go to step 5.

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

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

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

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

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

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

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

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

       6.  Apply inequality on alternate ASBR.

    Sarkar, et al.               Standards Track                   [Page 11]

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    4.2.1.1.  Multiple ASBRs Belonging to Different Areas

       When RFC1583Compatibility is set to "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 areas
       advertising the same prefix, pruning rules as defined in Section 16.4
       of [RFC2328] are applied.  The alternate ASBRs pruned using these
       rules are not considered for LFA evaluation.

    4.2.1.2.  Type 1 and Type 2 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, 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, as described in 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 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 costs.

    4.2.1.3.  RFC1583Compatibility is Set to "Enabled"

       When RFC1583Compatibility is set to "enabled", multiple ASBRs
       belonging to different 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
       the external Link State Advertisement (LSA).

    Sarkar, et al.               Standards Track                   [Page 12]

    RFC 8518                 LFA Selection for MHPs               March 2019

    4.2.1.4.  Type 7 Routes

       Type 5 routes always get preference over type 7, and the alternate
       ASBRs chosen for LFA calculation should belong to the same type.
       Among 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 for Alternate ASBR Selection

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

    4.2.2.1.  Forwarding Address Set to Non-zero Value

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

       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-Protecting + Downstream-paths-only LFAs:

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

       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:

          P           -  The MHP being evaluated for computing alternates

          S           -  The computing router

          N           -  The alternate router being evaluated

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

          PO_i        -  The specific prefix-originating router being
                         evaluated

    Sarkar, et al.               Standards Track                   [Page 13]

    RFC 8518                 LFA Selection for MHPs               March 2019

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

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

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

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

    4.2.2.2.  ASBRs Advertising Type 1 and Type 2 Costs

       Similar to the inequalities defined in Section 2, the following
       inequalities are defined for type 1 and 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-Protecting + Downstream-paths-only LFAs:

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

       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:

          P           -  The MHP being evaluated for computing alternates

          S           -  The computing router

          N           -  The alternate router being evaluated

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

          PO_i        -  The specific prefix-originating router being
                         evaluated

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

    Sarkar, et al.               Standards Track                   [Page 14]

    RFC 8518                 LFA Selection for MHPs               March 2019

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

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

    5.  LFA Extended Procedures

       This section explains additional considerations to the LFA base
       specification [RFC5286].

    5.1.  Links with IGP MAX_METRIC

       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, described below, where the only potential alternate
       available 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 called "maximum link
       metric" when defined for IS-IS in [RFC5305] and "MaxLinkMetric" when
       defined for OSPF in [RFC6987].

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

                        Figure 3: Link with IGP MAX_METRIC

       In the simple example network in Figure 3, all the 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, 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, i.e., from N2 to S for a specific
       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

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       N2 could be excluded as a potential alternative because of the
       current exclusions specified in Sections 3.5 and 3.6 of [RFC5286].
       But, the primary traffic destined to D2 continues to use the 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 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.  MT Considerations

       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 IGP deployments, for each MT-ID, a separate
       shortest path tree (SPT) is built with 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 identifying the eligible set of neighbors for each LFA
       computation; this is done per MT-ID.  The eligible set for each MT-ID
       is determined by the presence of IGP adjacency from 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-
       instance OSPFv3 [RFC5838].

       However, for MT IS-IS, if a "standard unicast topology" is used with
       MT-ID #0 [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 having the same address families provisioned on all the
       links and all the nodes of the network with MT-ID #0.  Here, with a
       single-decision process, both IPv4 and IPv6 next hops are computed
       for all the prefixes in the 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 IANA actions.

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    7.  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 Keying and Authentication for Routing
       Protocols (KARP).  This document does not change any of the discussed
       protocol specifications (i.e., [RFC1195], [RFC5120], [RFC2328], and
       [RFC5838]); therefore, the security considerations of the LFA base
       specification [RFC5286] continue to apply.

    8.  References

    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>.

    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>.

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    RFC 8518                 LFA Selection for MHPs               March 2019

       [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>.

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    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

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    Authors' Addresses

       Pushpasis Sarkar (editor)
       Arrcus, Inc.

       Email: pushpasis.ietf@gmail.com

       Uma Chunduri (editor)
       Huawei USA
       2330 Central Expressway
       Santa Clara, CA  95050
       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

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