Imported debug from /usr/lib/site-python/debug.pyc draft-ietf-rtgwg-multihomed-prefix-lfa-09 - Selection of Loop-Free Alternates for Multi-Homed Prefixes
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Versions: (draft-psarkar-rtgwg-multihomed-prefix-lfa) 00 01 02 03 04 05 06 07 08 09

Routing Area Working Group                                P. Sarkar, Ed.
Internet-Draft                                              Arrcus, Inc.
Updates: 5286 (if approved)                             U. Chunduri, Ed.
Intended status: Standards Track                              Huawei USA
Expires: May 25, 2019                                           S. Hegde
                                                  Juniper Networks, Inc.
                                                             J. Tantsura
                                                            Apstra, Inc.
                                                              H. Gredler
                                                           RtBrick, Inc.
                                                       November 21, 2018


        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 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.  It also provides
   detailed criteria for evaluating potential alternates for external
   prefixes advertised by OSPF ASBRs.  This documents 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.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   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.

Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  LFA inequalities for MHPs . . . . . . . . . . . . . . . . . .   4
   3.  LFA selection for the multi-homed prefixes  . . . . . . . . .   5
     3.1.  Improved coverage with simplified approach to MHPs  . . .   7
     3.2.  IS-IS ATT Bit considerations  . . . . . . . . . . . . . .   8
   4.  LFA selection for the multi-homed external prefixes . . . . .   9
     4.1.  IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  OSPF  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Rules to select alternate ASBR  . . . . . . . . . . .   9
         4.2.1.1.  Multiple ASBRs belonging different area . . . . .  11
         4.2.1.2.  Type 1 and Type 2 costs . . . . . . . . . . . . .  11
         4.2.1.3.  RFC1583compatibility is set to enabled  . . . . .  11
         4.2.1.4.  Type 7 routes . . . . . . . . . . . . . . . . . .  11
       4.2.2.  Inequalities to be applied for alternate ASBR
               selection . . . . . . . . . . . . . . . . . . . . . .  12
         4.2.2.1.  Forwarding address set to non-zero value  . . . .  12
         4.2.2.2.  ASBRs advertising type1 and type2 cost  . . . . .  13
   5.  LFA Extended Procedures . . . . . . . . . . . . . . . . . . .  13
     5.1.  Links with IGP MAX_METRIC . . . . . . . . . . . . . . . .  13
     5.2.  Multi Topology Considerations . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .  15
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16



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     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

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

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

1.1.  Acronyms

   AF      -  Address Family

   ATT     -  IS-IS Attach Bit

   ECMP    -  Equal Cost Multi Path

   IGP     -  Interior Gateway Protocol



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   IS-IS   -  Intermediate System to Intermediate System

   LFA     -  Loop-Free Alternate

   LSP     -  IS-IS Link State PDU

   OSPF    -  Open Shortest Path First

   MHP     -  Multi-homed Prefix

   MT      -  Multi Topology

   SPF     -  Shortest Path First

2.  LFA 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
   is defined in [RFC5714], which is nothing but the metric sum of the
   shortest path from X to Y and 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



























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Link-Protection:
A neighbor N can provide a loop-free alternate (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 + Downstream-paths-only:
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-Protection:
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 multi-homed prefix being evaluated for
                  computing alternates
   S            - The computing router
   N            - The alternate router being evaluated
   E            - The primary next-hop on 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)    - Cost of reaching the prefix P from prefix
                  originating node X.
   D_opt(X,Y)   - Distance on the shortest path from node X to node
                  Y.

                    Figure 1: LFA inequalities for MHPs

3.  LFA selection for the multi-homed prefixes

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





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     Link-Protection :
     =================
     1. if, in addition to being an alternate neighbor, N is also a prefix-originator of P,
        1.a. Select N as a 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. If LFA inequality condition is met,
             select N as a LFA for prefix P.
        2.b. Else, N is not a LFA for prefix P.

     Link-Protection + Downstream-paths-only :
     =========================================
     1. Evaluate the link-protecting + downstream-only LFA inequality
        for P with the N as the alternate neighbor.
        1.a. If LFA inequality condition is met,
             select N as a LFA for prefix P.
        1.b. Else, N is not a LFA for prefix P.

     Node-Protection :
     =================
     1. if, in addition to being an alternate neighbor, N is also a prefix-originator of P,
        1.a. Select N as a 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. If LFA inequality condition is met,
             select N as a LFA for prefix P.
        2.b. Else, N is not a LFA for prefix P.

                Figure 2: Rules for selecting LFA for MHPs

   In case an alternate neighbor N is also one of the 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 N MUST be chosen as a valid LFA for prefix P, without evaluating
   any of the inequalities in Figure 1 as long as 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 LFA inequality specified in Figure 1 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, as mentioned
   in Figure 1, once for each remote node that originated the prefix.
   In case the inequality is satisfied by the neighbor N router S MUST
   choose neighbor N, as one of the valid LFAs for the prefix P.




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

3.1.  Improved coverage with simplified approach to MHPs

   LFA base specification [RFC5286] Section 6.1 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 and
   the same has been elaborated with appropriate inequalities in the
   above section.  However, [RFC5286] Section 6.1 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 equal cost multi path
   (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 the below example network presented in Figure 3.  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: MHP with same ECMP Next-hop

   In the above network a prefix P, is advertised from both Node E and
   Node F.  With simplified approach taken as specified in [RFC5286]
   Section 6.1, prefix P will get only link protection 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



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   E (link protection) and inherit the better alternative to prefix P
   and here it is A.

   However, in the below example network presented in Figure 4, prefix P
   has an ECMP through both node E and node F with cost 20.  Though it
   has 2 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 corresponding LFAs of each primary
   next-hop calculated for the router advertising the same respectively.
   In the below diagram that would be node E's and node F's LFA i.e.,
   node N1 and node N2 respectively.


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

                Figure 4: MHP with different ECMP Next-hops

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

3.2.  IS-IS ATT Bit considerations

   Per [RFC1195] a default route needs to be added in Level1 (L1) router
   to the closest reachable Level1/Level2 (L1/L2) router in the network
   advertising 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 base LFA specification
   [RFC5286] does not specify any procedure for computing LFA for a
   default route in IS-IS L1 area.  This document specifies, a node can
   consider a default route is being advertised from the border L1/L2



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   router where 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 default route as this would be
   similar to ECMP MHP example as described in Section 3.1.
   Considerations as specified in Section 3 and Section 3.1 are
   applicable for default routes, if the default route is considered as
   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].

4.  LFA selection for the multi-homed external prefixes

   Redistribution of external routes into IGP is required in case of two
   different networks getting merged into one or 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 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 hereafter)
   redistributing the external routes for LFA selection as defined in
   [RFC5286] and leverage the advantage of having multiple re-
   distributing nodes in the network.

4.1.  IS-IS

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

4.2.  OSPF

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

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

4.2.1.  Rules to select alternate ASBR

   The process to select an alternate ASBR is best explained using the
   rules below.  The below process is applied when primary ASBR for the




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   concerned prefix is chosen and there is an alternate ASBR originating
   same prefix.

1. If RFC1583Compatibility is disabled

          1a. if primary ASBR and alternate ASBR belong to intra-area
                  non-backbone go to step 2.
          1b. If primary ASBR and alternate ASBR belong to
                  intra-area backbone and/or inter-area path go
                  to step 2.
          1c. for other paths, skip this alternate ASBR and
                  consider next ASBR.

2. Compare cost types (type 1/type 2) advertised by alternate ASBR and
      by the primary ASBR
          2a. If not the same type skip alternate ASBR and
                  consider next ASBR.
          2b. If same proceed to step 3.

3.If cost types are type 1, compare costs advertised by alternate ASBR
      and by the primary ASBR
             3a. If costs are the same then program ECMP Fast ReRoute (FRR) and return.
             3b. else go to step 5..

4  If cost types are type 2, compare costs advertised by alternate ASBR
      and by the primary ASBR
             4a. If costs are different, skip alternate ASBR and
                     consider next ASBR.
             4b. If cost are the same, proceed to step 4c to compare
                     cost to reach ASBR/forwarding address.
             4c. If cost to reach ASBR/forwarding address are also same
                     program ECMP FRR and return.
             4d. If cost to reach ASBR/forwarding address are different
                     go to step 5.

5. If route type (type 5/type 7)
           5a. If route type is 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
                  proceed to step 6.
                  If not, skip this alternate ASBR and consider
                  next ASBR.
           5b. If route type is not 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



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4.2.1.1.  Multiple ASBRs belonging different area

   When "RFC1583compatibility" is set to disabled, OSPF [RFC2328]
   defines certain rules of preference to choose the ASBRs.  While
   selecting 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 advertising
   the same prefix, pruning rules as defined in [RFC2328] section 16.4
   are applied.  The alternate ASBRs pruned using above 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 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 same type 2 cost, the alternate ASBRs
   are considered along with their cost to reach ASBR/forwarding address
   for evaluation.  If the two ASBRs have same type 2 cost as well as
   same cost to reach ASBR, ECMP FRR is programmed.  When there are
   multiple ASBRs advertising same type 2 cost for the prefix, primary
   Autonomous System (AS) external route calculation as described in
   [RFC2328] section 16.4.1 selects the route with lowest type 2 cost.
   ASBRs advertising different type 2 cost (higher cost) are not
   considered for LFA evaluation.  Alternate ASBRs advertising type 2
   cost for the prefix but are not chosen as primary due to 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 equal type 2 cost ASBRS.

4.2.1.3.  RFC1583compatibility is set to enabled

   When RFC1583Compatibility is set to enabled, multiple ASBRs belonging
   to different area advertising 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 Link State
   Advertisement (LSA).

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 same type.  Among
   Type 7 routes, routes with p-bit and forwarding address set have
   higher preference than routes without these attributes.  Alternate



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   ASBRs selected for LFA comparison should have same p-bit and
   forwarding address attributes.

4.2.2.  Inequalities to be applied for alternate ASBR selection

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

4.2.2.1.  Forwarding address set to non-zero value

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

  Link-Protection:
  F_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
                                F_opt(S,PO_best) + Cost(PO_best,P)

  Link-Protection + Downstream-paths-only:
  F_opt(N,PO_i)+ Cost(PO_i,P) < F_opt(S,PO_best) + Cost(PO_best,P)

  Node-Protection:
  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 multi-homed prefix being evaluated for
                            computing alternates
  S            - The computing router
  N            - The alternate router being evaluated
  E            - The primary next-hop on 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,Y)    - External cost for Y as advertised by X
  F_opt(X,Y)   - Distance on the shortest path from node X to Forwarding
                 address specified by ASBR Y.
  D_opt(X,Y)   - Distance on the shortest path from node X to node Y.


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







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4.2.2.2.  ASBRs advertising type1 and type2 cost

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

   Link-Protection:
   D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(N,S) +
                                 D_opt(S,PO_best) + Cost(PO_best,P)

   Link-Protection + Downstream-paths-only:
   D_opt(N,PO_i)+ Cost(PO_i,P) < D_opt(S,PO_best) + Cost(PO_best,P)

   Node-Protection:
   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 multi-homed prefix being evaluated for
                             computing alternates
   S            - The computing router
   N            - The alternate router being evaluated
   E            - The primary next-hop on 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,Y)    - External cost for Y as advertised by X.
   D_opt(X,Y)   - Distance on the shortest path from node X to node Y.


       Figure 7: LFA inequality definition for type1 and type2 cost

5.  LFA Extended Procedures

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

5.1.  Links with IGP MAX_METRIC

   Section 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 satisfies the basic loop-free condition will not be
   considered as alternative.  This document refers the maximum link
   metric in IGPs as the MAX_METRIC.  MAX_METRIC is defined for IS-IS in




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   [RFC5305], where it is called as "maximum link metric" and defined
   for OSPF in [RFC6987], where it is called as "MaxLinkMetric".


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


                    Figure 8: Link with IGP MAX_METRIC

   In the simple example network, all the link costs 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 Traffic Engineering (TE) requirement of the operator.  At node
   S, D1 is reachable through N1 with cost 20, and D2 is reachable
   through N2 with cost 20.  Even though neighbor N2 satisfies 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 3.5 and 3.6 procedure of
   [RFC5286].  But, as the primary traffic destined to D2 continues to
   use the link and hence irrespective of the reverse metric in this
   case, same link MAY be used as a potential LFA for D1.

   Alternatively, 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.  Multi Topology Considerations

   Section 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.




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   In Multi Topology (MT) 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 which is done per MT ID.  The eligible-set for each MT ID
   is determined by the presence of IGP adjacency from 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 topology" is used with MT-ID #0
   [RFC5286] 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
   same address families provisioned on all the links and all the nodes
   of the network with MT-ID #0.  Here with single decision process both
   IPv4 and IPv6 next-hops are computed for all the prefixes in the
   network and 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.

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.











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   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.  This document does not change any of
   the discussed protocol specifications [RFC1195] [RFC5120] [RFC2328]
   [RFC5838], and the security considerations of the LFA base
   specification [RFC5286] therefore continue to apply.

10.  References

10.1.  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.  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>.




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   [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>.






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   [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>.

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

   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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