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Versions: (draft-litkowski-rtgwg-lfa-manageability) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7916

Routing Area Working Group                                  S. Litkowski
Internet-Draft                                               B. Decraene
Intended status: Standards Track                                  Orange
Expires: July 26, 2014                                       C. Filsfils
                                                                 K. Raza
                                                           Cisco Systems
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                               P. Sarkar
                                                        Juniper Networks
                                                        January 22, 2014


             Operational management of Loop Free Alternates
                 draft-ietf-rtgwg-lfa-manageability-01

Abstract

   Loop Free Alternates (LFA), as defined in RFC 5286 is an IP Fast
   ReRoute (IP FRR) mechanism enabling traffic protection for IP traffic
   (and MPLS LDP traffic by extension).  Following first deployment
   experiences, this document provides operational feedback on LFA,
   highlights some limitations, and proposes a set of refinements to
   address those limitations.  It also proposes required management
   specifications.

   This proposal is also applicable to remote LFA solution.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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 http://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
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on July 26, 2014.

Copyright Notice

   Copyright (c) 2014 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
   (http://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
   2.  Operational issues with default LFA tie breakers  . . . . . .   3
     2.1.  Case 1: Edge router protecting core failures  . . . . . .   3
     2.2.  Case 2: Edge router choosen to protect core failures
           while core LFA exists . . . . . . . . . . . . . . . . . .   5
     2.3.  Case 3: suboptimal core alternate choice  . . . . . . . .   5
     2.4.  Case 4: ISIS overload bit on LFA computing node . . . . .   6
   3.  Configuration requirements  . . . . . . . . . . . . . . . . .   7
     3.1.  LFA enabling/disabling scope  . . . . . . . . . . . . . .   7
     3.2.  Policy based LFA selection  . . . . . . . . . . . . . . .   8
       3.2.1.  Connected vs remote alternates  . . . . . . . . . . .   8
       3.2.2.  Mandatory criteria  . . . . . . . . . . . . . . . . .   9
       3.2.3.  Enhanced criteria . . . . . . . . . . . . . . . . . .   9
       3.2.4.  Retrieving alternate path attributes  . . . . . . . .  10
       3.2.5.  Details on criterions . . . . . . . . . . . . . . . .  11
   4.  Operational aspects . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  ISIS overload bit on LFA computing node . . . . . . . . .  16
     4.2.  Manual triggering of FRR  . . . . . . . . . . . . . . . .  17
     4.3.  Required local information  . . . . . . . . . . . . . . .  17
     4.4.  Coverage monitoring . . . . . . . . . . . . . . . . . . .  17
     4.5.  Deterministic and documents LFA selection behavior  . . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20



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1.  Introduction

   Following the first deployments of Loop Free Alternates (LFA), this
   document provides feedback to the community about the management of
   LFA.

      Section 2 provides real uses cases illustrating some limitations
      and suboptimal behavior.

      Section 3 proposes requirements for activation granularity and
      policy based selection of the alternate.

      Section 4 express requirements for the operational management of
      LFA.

2.  Operational issues with default LFA tie breakers

   [RFC5286] introduces the notion of tie breakers when selecting the
   LFA among multiple candidate alternate next-hops.  When multiple LFA
   exist, RFC 5286 has favored the selection of the LFA providing the
   best coverage of the failure cases.  While this is indeed a goal,
   this is one among multiple and in some deployment this lead to the
   selection of a suboptimal LFA.  The following sections details real
   use cases of such limitations.

   Note that the use case of per-prefix LFA is assumed throughout this
   analysis.

2.1.  Case 1: Edge router protecting core failures






















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       R1 --------- R2 ---------- R3 --------- R4
       |      1           100           1       |
       |                                        |
       | 100                                    | 100
       |                                        |
       |      1           100           1       |
       R5 --------- R6 ---------- R7 --------- R8 -- R9 - PE1
       |             |            |             |
       | 5k          | 5k         | 5k          | 5k
       |             |            |             |
       +--- n*PEx ---+            +---- PE2 ----+
                                         |
                                         |
                                        PEy

                           Figure 1

   Rx routers are core routers using n*10G links.  PEs are connected
   using links with lower bandwidth.

   In figure 1, let us consider the traffic flowing from PE1 to PEx.
   The nominal path is R9-R8-R7-R6-PEx.  Let us consider the failure of
   link R7-R8.  For R8, R4 is not an LFA and the only available LFA is
   PE2.

   When the core link R8-R7 fails, R8 switches all traffic destined to
   all the PEx towards the edge node PE2.  Hence an edge node and edge
   links are used to protect the failure of a core link.  Typically,
   edge links have less capacity than core links and congestion may
   occur on PE2 links.  Note that although PE2 was not directly affected
   by the failure, its links become congested and its traffic will
   suffer from the congestion.

   In summary, in case of failure, the impact on customer traffic is:

   o  From PE2 point of view :

      *  without LFA: no impact

      *  with LFA: traffic is partially dropped (but possibly
         prioritized by a QoS mechanism).  It must be highlighted that
         in such situation, traffic not affected by the failure may be
         affected by the congestion.

   o  From R8 point of view:

      *  without LFA: traffic is totally dropped until convergence
         occurs.



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      *  with LFA: traffic is partially dropped (but possibly
         prioritized by a QoS mechanism).

   Besides the congestion aspects of using an Edge router as an
   alternate to protect a core failure, a service provider may consider
   this as a bad routing design and would like to prevent it.

2.2.  Case 2: Edge router choosen to protect core failures while core
      LFA exists

       R1 --------- R2 ------------ R3 --------- R4
       |      1           100       |     1     |
       |                            |           |
       | 100                        | 30        | 30
       |                            |           |
       |     1        50        50  |    10     |
       R5 -------- R6 ---- R10 ---- R7 -------- R8 --- R9 - PE1
       |            |         \                 |
       | 5000       | 5000     \ 5000           | 5000
       |            |           \               |
       +--- n*PEx --+            +----- PE2 ----+
                                         |
                                         |
                                        PEy

                         Figure 2

   Rx routers are core routers meshed with n*10G links.  PEs are meshed
   using links with lower bandwidth.

   In the figure 2, let us consider the traffic coming from PE1 to PEx.
   Nominal path is R9-R8-R7-R6-PEx.  Let us consider the failure of the
   link R7-R8.  For R8, R4 is a link-protecting LFA and PE2 is a node-
   protecting LFA.  PE2 is chosen as best LFA due to its better
   protection type.  Just like in case 1, this may lead to congestion on
   PE2 links upon LFA activation.

2.3.  Case 3: suboptimal core alternate choice













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               +--- PE3 --+
              /            \
        1000 /              \ 1000
            /                \
    +----- R1 ---------------- R2 ----+
    |      |       500         |      |
    | 10   |                   |      | 10
    |      |                   |      |
    R5     | 10                | 10   R7
    |      |                   |      |
    | 10   |                   |      | 10
    |      |       500         |      |
    +---- R3 ---------------- R4 -----+
            \                 /
             \               /
              \             /
               +--- PE1 ---+

               Figure 3

   Rx routers are core routers.  R1-R2 and R3-R4 links are 1G links.
   All others inter Rx links are 10G links.

   In the figure above, let us consider the failure of link R1-R3.  For
   destination PE3, R3 has two possible alternates:

   o  R4, which is node-protecting

   o  R5, which is link-protecting

   R4 is chosen as best LFA due to its better protection type.  However,
   it may not be desirable to use R4 for bandwidth capacity reason.  A
   service provider may prefer to use high bandwidth links as prefered
   LFA.  In this example, prefering shortest path over protection type
   may achieve the expected behavior, but in cases where metric are not
   reflecting bandwidth, it would not work and some other criteria would
   need to be involved when selecting the best LFA.

2.4.  Case 4: ISIS overload bit on LFA computing node












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       P1       P2
       |   \  /   |
    50 | 50 \/ 50 | 50
       |    /\    |
       PE1-+  +-- PE2
        \        /
      45 \      / 45
          -PE3-+
          (OL set)

               Figure 4

   In the figure above, PE3 has its overload bit set (permanently, for
   design reason) and wants to protect traffic using LFA for destination
   PE2.

   On PE3, the loopfree condition is not satisified : 100 !< 45 + 45.
   PE1 is thus not considered as an LFA.  However thanks to the overload
   bit set on PE3, we know that PE1 is loopfree so PE1 is an LFA to
   reach PE2.

   In case of overload condition set on a node, LFA behavior must be
   clarified.

3.  Configuration requirements

   Controlling best alternate and LFA activation granularity is a
   requirement for Service Providers.  This section defines
   configuration requirements for LFA.

3.1.  LFA enabling/disabling scope

   The granularity of LFA activation should be controlled (as alternate
   nexthop consume memory in forwarding plane).

   An implementation of LFA SHOULD allow its activation with the
   following criteria:

   o  Per address-family : ipv4 unicast, ipv6 unicast, LDP IPv4 unicast,
      LDP IPv6 unicast ...

   o  Per routing context : VRF, virtual/logical router, global routing
      table, ...

   o  Per interface

   o  Per protocol instance, topology, area




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   o  Per prefixes: prefix protection SHOULD have a better priority
      compared to interface protection.  This means that if a specific
      prefix must be protected due to a configuration request, LFA must
      be computed and installed for this prefix even if the primary
      outgoing interface is not configured for protection.

3.2.  Policy based LFA selection

   When multiple alternates exist, LFA selection algorithm is based on
   tie breakers.  Current tie breakers do not provide sufficient control
   on how the best alternate is chosen.  This document proposes an
   enhanced tie breaker allowing service providers to manage all
   specific cases:

   1.  An implementation of LFA SHOULD support policy-based decision for
       determining the best LFA.

   2.  Policy based decision SHOULD be based on multiple criterions,
       with each criteria having a level of preference.

   3.  If the defined policy does not permit to determine a unique best
       LFA, an implementation SHOULD pick only one based on its own
       decision, as a default behavior.  An implementation SHOULD also
       support election of multiple LFAs, for loadbalancing purposes.

   4.  Policy SHOULD be applicable to a protected interface or to a
       specific set of destinations.  In case of application on the
       protected interface, all destinations primarily routed on this
       interface SHOULD use the interface policy.

   5.  It is an implementation choice to reevaluate policy dynamically
       or not (in case of policy change).  If a dynamic approach is
       chosen, the implementation SHOULD recompute the best LFAs and
       reinstall them in FIB, without service disruption.  If a non-
       dynamic approach is chosen, the policy would be taken into
       account upon the next IGP event.  In this case, the
       implementation SHOULD support a command to manually force the
       recomputation/reinstallation of LFAs.

3.2.1.  Connected vs remote alternates

   In addition to direct LFAs, tunnels (e.g. IP, LDP or RSVP-TE) to
   distant routers may be used to complement LFA coverage (tunnel tail
   used as virtual neighbor).  When a router has multiple alternate
   candidates for a specific destination, it may have connected
   alternates and remote alternates reachable via a tunnel.  Connected
   alternates may not always provide an optimal routing path and it may
   be preferable to select a remote alternate over a connected



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   alternate.  The usage of tunnels to extend LFA coverage is described
   in [I-D.ietf-rtgwg-remote-lfa].

   In figure 1, there is no core alternate for R8 to reach PEs located
   behind R6, so R8 is using PE2 as alternate, which may generate
   congestion when FRR is activated.  Instead, we could have a remote
   core alternate for R8 to protect PEs destinations.  For example, a
   tunnel from R8 to R3 would ensure LFA protection without using an
   edge router to protect a core router.

   When selecting the best alternate, the selection algorithm MUST
   consider all available alternates (connected or tunnel).  Especially,
   computation of PQ set ([I-D.ietf-rtgwg-remote-lfa]) SHOULD to be
   performed before best alternate selection.

3.2.2.  Mandatory criteria

   An implementation of LFA MUST support the following criteria:

   o  Non candidate link: A link marked as "non candidate" will never be
      used as LFA.

   o  A primary nexthop being protected by another primary nexthop of
      the same prefix (ECMP case).

   o  Type of protection provided by the alternate: link protection,
      node protection.  In case of node protection preference, an
      implementation SHOULD support fallback to link protection if node
      protection is not available.

   o  Shortest path: lowest IGP metric used to reach the destination.

   o  SRLG (as defined in [RFC5286] Section 3).

3.2.3.  Enhanced criteria

   An implementation of LFA SHOULD support the following enhanced
   criteria:

   o  Downstreamness of a neighbor : preference of a downstream path
      over a non downstream path SHOULD be configurable.

   o  Link coloring with : include, exclude and preference based system.

   o  Link Bandwidth.

   o  Neighbor preference.




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3.2.4.  Retrieving alternate path attributes

   The policy to select the best alternate evaluate multiple criterions
   (e.g. metric, SRLG, link colors ...) which first need to be computed
   for each alternate.. In order to compare the different alternate
   path, a router must retrieve the attributes of each alternate path.
   The alternate path is composed of two distinct parts : PLR to
   alternate and alternate to destination.

3.2.4.1.  Connected alternate

   For alternate path using a connected alternate :

   o  attributes from PLR to alternate path are retrieved from the
      interface connected to the alternate.

   o  attributes from alternate to destination path are retrieved from
      SPF rooted at the alternate.  As the alternate is a connected
      alternate, the SPF has already been computed to find the
      alternate, so there is no need of additional computation.

3.2.4.2.  Remote alternate

   For alternate path using a remote alternate (tunnel) :

   o  attributes from the PLR to alternate path are retrieved using the
      PLR's primary SPF if P space is used or using the neighbor's SPF
      if extended P space is used, combined with the attributes of the
      link(s) to reach that neighbor.  In both cases, no additional SPF
      is required.

   o  attributes from alternate to destination path are retrieved from
      SPF rooted at the remote alternate.  An additional forward SPF is
      required for each remote alternate as indicated in
      [I-D.psarkar-rtgwg-rlfa-node-protection] section 3.2..

   The number of remote alternates may be very high, simulations shown
   that hundred's of PQs may exist for a single interface being
   protected.  Running a forward SPF for every PQ-node in the network is
   not scalable.

   To handle this situation, it is needed to limit the number of remote
   alternates to be evaluated to a finite number before collecting
   alternate path attributes and running the policy evaluation.  [I-D
   .psarkar-rtgwg-rlfa-node-protection] Section 2.3.3 provides a way to
   reduce the number of PQ to be evaluated.





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                  Link            Remote              Remote
                  alternate       alternate           alternate
                 -------------  ------------------   -------------
   Alternates    |  LFA      |  |   rLFA (PQs)   |   |  Static   |
   sources       |           |  |                |   |  tunnels  |
                 -------------  ------------------   -------------
                      |                   |                  |
                      |                   |                  |
                      |        ----------------------        |
                      |        |  Prune some PQs    |        |
                      |        | (sorting strategy) |        |
                      |        ----------------------        |
                      |                   |                  |
                      |                   |                  |
                  ------------------------------------------------
                  |          Collect alternate attributes        |
                  ------------------------------------------------
                                          |
                                          |
                               -------------------------
                               |    Evaluate policy    |
                               -------------------------
                                          |
                                          |
                                   Best alternates

3.2.5.  Details on criterions

3.2.5.1.  LFA and ECMP

           10
      PE2 - PE3
       |     |
    50 |  5  | 50
       P1----P2
       \\    //
    50  \\  // 50
         PE1

       Figure 5

   Links between P1 and PE1 are L1 and L2, links between P2 and PE1 are
   L3 and L4

   In the figure above, primary path from PE1 to PE2 is through P1 using
   ECMP on two parallel links L1 and L2.  In case of standard ECMP
   behavior, if L1 is failing, postconvergence nexthop would become L2
   and there would be no longer ECMP.  If LFA is activated, as stated in



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   [RFC5286] Section 3.4., "alternate next-hops may themselves also be
   primary next-hops, but need not be" and "alternate next-hops should
   maximize the coverage of the failure cases".  In this scenario there
   is no alternate providing node protection, LFA will so prefer L2 as
   alternate to protect L1 which makes sense compared to postconvergence
   behavior.

   Considering a different scenario using figure 5, where L1 and L2 are
   configured as a layer 3 bundle using a local feature, as well as L3/
   L4 being a second layer 3 bundle.  Layer 3 bundles are configured as
   if a link in the bundle is failing, the traffic must be rerouted out
   of the bundle.  Layer 3 bundles are generally introduced to increase
   bandwidth between nodes.  In nominal situation, ECMP is still
   available from PE1 to PE2, but if L1 is failing, postconvergence
   nexthop would become ECMP on L3 and L4.  In this case, LFA behavior
   SHOULD be adapted in order to reflect the bandwidth requirement.

   We would expect the following FIB entry on PE1 :


       On PE1 : PE2 +--> ECMP -> L1
                    |     |
                    |     +----> L2
                    |
                    +--> LFA(ECMP) -> L3
                          |
                          +---------> L4


   If L1 or L2 is failing, traffic must be switched on the LFA ECMP
   bundle rather than using the other primary nexthop.

   As mentioned in [RFC5286] Section 3.4., protecting a link within an
   ECMP by another primary nexthop is not a MUST.  Moreover, we already
   presented in this document, that maximizing the coverage of the
   failure case may not be the right approach and policy based choice of
   alternate may be preferred.

   An implementation SHOULD permit to prefer a primary nexthop by
   another primary nexthop with the possibility to deactivate this
   criteria.  An implementation SHOULD permit to use an ECMP bundle as a
   LFA.

3.2.5.2.  SRLG

   [RFC5286] Section 3. proposes to reuse GMPLS IGP extensions to encode
   SRLGs ([RFC4205] and [RFC4203]).  The section is also describing the
   algorithm to compute SRLG protection.



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   When SRLG protection is computed, and implementation SHOULD permit to
   :

   o  Exclude alternates violating SRLG.

   o  Maintain a preference system between alternates based on number of
      SRLG violations : more violations = less preference.

   When applying SRLG criteria, the SRLG violation check SHOULD be
   performed on source to alternate as well as alternate to destination
   paths.  In the case of remote LFA, PQ to destination path attributes
   would be retrieved from SPT rooted at PQ.

3.2.5.3.  Link coloring

   Link coloring is a powerful system to control the choice of
   alternates.  Protecting interfaces are tagged with colors.  Protected
   interfaces are configured to include some colors with a preference
   level, and exclude others.

   Link color information SHOULD be signalled in the IGP.  How
   signalling is done is out of scope of the document but it may be
   useful to reuse existing admin-groups from traffic-engineering
   extensions.

                  PE2
                  |   +---- P4
                  |  /
         PE1 ---- P1 --------- P2
                  |      10Gb
              1Gb |
                  |
                  P3

                Figure 5

   Example : P1 router is connected to three P routers and two PEs.

   P1 is configured to protect the P1-P4 link.  We assume that given the
   topology, all neighbors are candidate LFA.  We would like to enforce
   a policy in the network where only a core router may protect against
   the failure of a core link, and where high capacity links are
   prefered.

   In this example, we can use the proposed link coloring by:

   o  Marking PEs links with color RED




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   o  Marking 10Gb CORE link with color BLUE

   o  Marking 1Gb CORE link with color YELLOW

   o  Configured the protected interface P1->P4 with :

      *  Include BLUE, preference 200

      *  Include YELLOW, preference 100

      *  Exclude RED

   Using this, PE links will never be used to protect against P1-P4 link
   failure and 10Gb link will be be preferred.

   The main advantage of this solution is that it can easily be
   duplicated on other interfaces and other nodes without change.  A
   Service Provider has only to define the color system (associate color
   with a significance), as it is done already for TE affinities or BGP
   communities.

   An implementation of link coloring:

   o  SHOULD support multiple include and exclude colors on a single
      protected interface.

   o  SHOULD provide a level of preference between included colors.

   o  SHOULD support multiple colors configuration on a single
      protecting interface.

3.2.5.4.  Bandwidth

   As mentionned in previous sections, not taking into account bandwidth
   of an alternate could lead to congestion during FRR activation.  We
   propose to base the bandwidth criteria on the link speed information
   for the following reason :

   o  if a router S has a set of X destinations primarly forwarded to N,
      using per prefix LFA may lead to have a subset of X protected by a
      neighbor N1, another subset by N2, another subset by Nx ...

   o  S is not aware about traffic flows to each destination and is not
      able to evaluate how much traffic will be sent to N1,N2, ... Nx in
      case of FRR activation.

   Based on this, it is not useful to gather available bandwidth on
   alternate paths, as the router does not know how much bandwidth it



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   requires for protection.  The proposed link speed approach provides a
   good approximation with a small cost as information is easily
   available.

   The bandwidth criteria of the policy framework SHOULD work in two
   ways :

   o  PRUNE : exclude a LFA if link speed to reach it is lower than the
      link speed of the primary nexthop interface.

   o  PREFER : prefer a LFA based on his bandwidth to reach it compared
      to the link speed of the primary nexthop interface.

3.2.5.5.  Neighbor preference

   Rather than tagging interface on each node (using link color) to
   identify neighbor node type (as example), it would be helpful if
   routers could be identified in the IGP.  This would permit a grouped
   processing on multiple nodes.  [I-D.psarkar-isis-node-admin-tag]
   proposes a tagging mechanism for ISIS nodes that may help the node
   identification.  As an implementation need to exclude some specific
   neighbors (see Section 3.2.3), an implementation :

   o  SHOULD be able to give a preference to specific neighbor.

   o  SHOULD be able to give a preference to a group of neighbor.

   o  SHOULD be able to exclude a group of neighbor.

   A specific neighbor may be identified by its interface or IP address
   and group of neighbors may be identified by a marker like SUB-TLV1 in
   TLV135.  As multiple prefixes may be present in TLVs 135, an
   heuristic is required to choose the appropriate one that will
   identify the neighbor and will transport the tag associated with the
   neighbor preference.

   We propose the following algorithm to select the prefix :

   1.  Select the prefix in TLV#135 that is equal to TLV#134 value
       (Router ID) and prefix length is 32.

   2.  Select the prefix in TLV#135 that is equal to TLV#132 value (IP
       Addresses) and prefix length is 32, it must be noted that TLV#132
       may transport multiple addresses and so multiple matches may
       happen.

   3.  If multiple prefixes are matching TLV#132 values, choose the
       highest one.



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   Consider the following network:

                  PE3
                  |
                  |
                  PE2
                  |   +---- P4
                  |  /
         PE1 ---- P1 -------- P2
                  |      10Gb
              1Gb |
                  |
                  P3

             Figure 6



   In the example above, each node is configured with a specific tag
   flooded through the IGP.

   o  PE1,PE3: 200 (non candidate).

   o  PE2: 100 (edge/core).

   o  P1,P2,P3: 50 (core).

   A simple policy could be configured on P1 to choose the best
   alternate for P1->P4 based on router function/role as follows :

   o  criteria 1 -> neighbor preference: exclude tag 100 and 200.

   o  criteria 2 -> bandwidth.

4.  Operational aspects

4.1.  ISIS overload bit on LFA computing node

   In [RFC5286], Section 3.5, the setting of the overload bit condition
   in LFA computation is only taken into account for the case where a
   neighbor has the overload bit set.

   In addition to RFC 5286 inequality 1 Loop-Free Criterion
   (Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)), the
   IS-IS overload bit of the LFA calculating neighbor (S) SHOULD be
   taken into account.  Indeed, if it has the overload bit set, no
   neighbor will loop back to traffic to itself.




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4.2.  Manual triggering of FRR

   Service providers often perform manual link shutdown (using router
   CLI) to perform some network changes/tests.  Especially testing or
   troubleshooting FRR requires to perform the manual shutdown on the
   remote end of the link as generally a local shutdown would not
   trigger FRR.  To enhance such situation, an implementation SHOULD
   support triggering/activating LFA Fast Reroute for a given link when
   a manual shutdown is done.

4.3.  Required local information

   LFA introduction requires some enhancement in standard routing
   information provided by implementations.  Moreover, due to the non
   100% coverage, coverage informations is also required.

   Hence an implementation :

   o  MUST be able to display, for every prefixes, the primary nexthop
      as well as the alternate nexthop information.

   o  MUST provide coverage information per activation domain of LFA
      (area, level, topology, instance, virtual router, address family
      ...).

   o  MUST provide number of protected prefixes as well as non protected
      prefixes globally.

   o  SHOULD provide number of protected prefixes as well as non
      protected prefixes per link.

   o  MAY provide number of protected prefixes as well as non protected
      prefixes per priority if implementation supports prefix-priority
      insertion in RIB/FIB.

   o  SHOULD provide a reason for chosing an alternate (policy and
      criteria) and for excluding an alternate.

   o  SHOULD provide the list of non protected prefixes and the reason
      why they are not protected (no protection required or no alternate
      available).

4.4.  Coverage monitoring

   It is pretty easy to evaluate the coverage of a network in a nominal
   situation, but topology changes may change the coverage.  In some
   situations, the network may no longer be able to provide the required




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   level of protection.  Hence, it becomes very important for service
   providers to get alerted about changes of coverage.

   An implementation SHOULD :

   o  provide an alert system if total coverage (for a node) is below a
      defined threshold or comes back to a normal situation.

   o  provide an alert system if coverage of a specific link is below a
      defined threshold or comes back to a normal situation.

   An implementation MAY :

   o  provide an alert system if a specific destination is not protected
      anymore or when protection comes back up for this destination

   Although the procedures for providing alerts are beyond the scope of
   this document, we recommend that implementations consider standard
   and well used mechanisms like syslog or SNMP traps.

4.5.  Deterministic and documents LFA selection behavior

   The operator may choose to run simulations in order to ensure full
   coverage of a certain type for the whole network or a given subset of
   the network.  This is particularly likely if he operates the network
   in the sense of the third backbone profiles described in [RFC6571],
   that is, he seeks to design and engineer the network topology in a
   way that a certain coverage is always achieved.  Obviously a complete
   and exact simulation of the IP FRR coverage can only be achieved, if
   the behavior is deterministic and if the algorithm used is available
   to the simulation tool.  Thus, an implementation SHOULD:

   o  Behave deterministic in its selection LFA process.  I.e. in the
      same topology and with the same policy configuration, the
      implementation MUST always choose the same alternate for a given
      prefix.

   o  Document its behavior.  The implementation SHOULD provide enough
      documentation of its behavior that allows an implementer of a
      simulation tool, to foresee the exact choice of the LFA
      implementation for every prefix in a given topology.  This SHOULD
      take into account all possible policy configuration options.  One
      possible way to document this behavior is to disclose the
      algorithm used to choose alternates.







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5.  Security Considerations

   This document does not introduce any change in security consideration
   compared to [RFC5286].

6.  Contributors

   Significant contributions were made by Pierre Francois, Hannes
   Gredler, Chris Bowers, Jeff Tantsura and Mustapha Aissaoui which the
   authors would like to acknowledge.

7.  Acknowledgements

8.  IANA Considerations

   This document has no action for IANA.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4203]  Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
              of Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 4203, October 2005.

   [RFC4205]  Kompella, K. and Y. Rekhter, "Intermediate System to
              Intermediate System (IS-IS) Extensions in Support of
              Generalized Multi-Protocol Label Switching (GMPLS)", RFC
              4205, October 2005.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

9.2.  Informative References

   [I-D.ietf-rtgwg-remote-lfa]
              Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
              Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-04
              (work in progress), November 2013.

   [I-D.litkowski-rtgwg-lfa-rsvpte-cooperation]
              Litkowski, S., Decraene, B., Filsfils, C., and K. Raza,
              "Interactions between LFA and RSVP-TE", draft-litkowski-
              rtgwg-lfa-rsvpte-cooperation-02 (work in progress), August
              2013.



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   [I-D.psarkar-isis-node-admin-tag]
              psarkar@juniper.net, p., Gredler, H., Hegde, S.,
              Raghuveer, H., Litkowski, S., and B. Decraene,
              "Advertising Per-node Admin Tags in IS-IS", draft-psarkar-
              isis-node-admin-tag-00 (work in progress), October 2013.

   [I-D.psarkar-rtgwg-rlfa-node-protection]
              psarkar@juniper.net, p., Gredler, H., Hegde, S.,
              Raghuveer, H., cbowers@juniper.net, c., and S. Litkowski,
              "Remote-LFA Node Protection and Manageability", draft-
              psarkar-rtgwg-rlfa-node-protection-03 (work in progress),
              December 2013.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630, September
              2003.

   [RFC3906]  Shen, N. and H. Smit, "Calculating Interior Gateway
              Protocol (IGP) Routes Over Traffic Engineering Tunnels",
              RFC 3906, October 2004.

   [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
              2005.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
              5714, January 2010.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010.

   [RFC6571]  Filsfils, C., Francois, P., Shand, M., Decraene, B.,
              Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, June 2012.

Authors' Addresses

   Stephane Litkowski
   Orange

   Email: stephane.litkowski@orange.com






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   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com


   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Kamran Raza
   Cisco Systems

   Email: skraza@cisco.com


   Martin Horneffer
   Deutsche Telekom

   Email: Martin.Horneffer@telekom.de


   Pushpasis Sarkar
   Juniper Networks

   Email: psarkar@juniper.net























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