[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-psarkar-rtgwg-multihomed-prefix-lfa) 00 01 02 03 04 05 06 07 08

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: April 19, 2019                                         S. Hegde
                                                  Juniper Networks, Inc.
                                                             J. Tantsura
                                                            Apstra, Inc.
                                                              H. Gredler
                                                           RtBrick, Inc.
                                                        October 16, 2018


                 LFA selection for Multi-Homed Prefixes
               draft-ietf-rtgwg-multihomed-prefix-lfa-08

Abstract

   This document shares experience gained from implementing algorithms
   to determine Loop-Free Alternates (LFAs) for multi-homed prefixes.
   In particular, 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




Sarkar, et al.           Expires April 19, 2019                 [Page 1]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   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 April 19, 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  . . .   6
     3.2.  IS-IS ATT Bit considerations  . . . . . . . . . . . . . .   8
   4.  LFA selection for the multi-homed external prefixes . . . . .   8
     4.1.  IS-IS . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  OSPF  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
       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  . . . . .  12
   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



Sarkar, et al.           Expires April 19, 2019                 [Page 2]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


     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].  Section 6.1 of [RFC5286] describes a method
   to determine LFAs for multi-homed prefixes (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 multi-homed prefix.  The results obtained are equivalent to
   those obtained using the method described in Section 6.1 of
   [RFC5286].  However, some may find this formulation useful.

   Section 6.3 of [RFC5286] discusses complications associated with
   computing LFAs for multi-homed prefixes 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 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

   IS-IS   -  Intermediate System to Intermediate System

   LFA     -  Loop-Free Alternate




Sarkar, et al.           Expires April 19, 2019                 [Page 3]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   LSP     -  IS-IS Link State PDU

   OSPF    -  Open Shortest Path First

   MHP     -  Multi-homed Prefix

   MT      -  Multi Topology

   SPF     -  Shortest Path First PDU

2.  LFA inequalities for MHPs

   This document proposes the following set of LFA inequalities for
   selecting the most appropriate LFAs for multi-homed prefixes (MHPs).
   They 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:
   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,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





Sarkar, et al.           Expires April 19, 2019                 [Page 4]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


3.  LFA selection for the multi-homed prefixes

   To compute a valid LFA for a given multi-homed prefix P, a computing
   router S MUST follow one of the appropriate procedures below, for
   each alternate neighbor N.

        Link-Protection :
        =================
        1. If alternate neighbor N is also 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 alternate neighbor N is also 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.




Sarkar, et al.           Expires April 19, 2019                 [Page 5]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   However, if N is not a prefix-originator of P, the computing router
   SHOULD 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.

   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 multi-homed prefix 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 multi-homed prefix 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 multi-
   homed prefix 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





Sarkar, et al.           Expires April 19, 2019                 [Page 6]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   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
   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.







Sarkar, et al.           Expires April 19, 2019                 [Page 7]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


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
   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 given for each
   primary next-hop associated with default route.  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 similar
   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].



Sarkar, et al.           Expires April 19, 2019                 [Page 8]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


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










































Sarkar, et al.           Expires April 19, 2019                 [Page 9]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


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 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 route p-bit,
                  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






Sarkar, et al.           Expires April 19, 2019                [Page 10]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


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 16.4.1
   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 defined 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 adress
   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
   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 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




Sarkar, et al.           Expires April 19, 2019                [Page 11]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   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

  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

4.2.2.2.  ASBRs advertising type1 and type2 cost








Sarkar, et al.           Expires April 19, 2019                [Page 12]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   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 type 2 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 (as defined for IS-IS in [RFC5305] or as defined in [RFC6987]
   for OSPF).  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.









Sarkar, et al.           Expires April 19, 2019                [Page 13]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


                             +---+  10  +---+  10 +---+
                             | S |------|N1 |-----|D1 |
                             +---+      +---+     +---+
                               |                    |
                            10 |                 10 |
                               |MAX_MET(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.

   In Multi Topology (MT) IGP deployments, for each MT ID, a separate
   shortest path tree (SPT) is built with topology specific adjacencies,
   the LFA principles laid out in [RFC5286] are actually applicable for
   MT IS-IS [RFC5120] LFA SPF.  The primary difference in this case is,



Sarkar, et al.           Expires April 19, 2019                [Page 14]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   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

   Thanks to 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.

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






Sarkar, et al.           Expires April 19, 2019                [Page 15]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


9.  Security Considerations

   Existing OSPF security considerations and stronger authentication and
   manual key management mechanisms are specified in [RFC7474] SHOULD be
   considered for OSPF deployments.  Security concerns for IS-IS are
   addressed in [RFC5304] and [RFC5310].  Further security analysis for
   IS-IS protocol is done in [RFC7645] SHOULD be considered for IS-IS
   deployments.  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>.

   [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>.







Sarkar, et al.           Expires April 19, 2019                [Page 16]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


   [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>.





Sarkar, et al.           Expires April 19, 2019                [Page 17]


Internet-Draft   LFA selection for Multi-Homed Prefixes     October 2018


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















Sarkar, et al.           Expires April 19, 2019                [Page 18]

Html markup produced by rfcmarkup 1.127, available from https://tools.ietf.org/tools/rfcmarkup/