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RFC 7812

 
 
 

An Architecture for IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-FRR)

Part 2 of 2, p. 23 to 44
Prev RFC Part

 


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10.  Inter-area Forwarding Behavior

   An ABR/LBR has two forwarding roles.  First, it forwards traffic
   within areas.  Second, it forwards traffic from one area into
   another.  These same two roles apply for MRT transit traffic.
   Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
   on MRT-Red or MRT-Blue in that area.  However, it is desirable for
   traffic leaving the area to also exit MRT-Red or MRT-Blue and return
   to shortest path forwarding.

   For unicast MRT-FRR, the need to stay on an MRT forwarding topology
   terminates at the ABR/LBR whose best route is via a different area/
   level.  It is highly desirable to go back to the default forwarding
   topology when leaving an area/level.  There are three basic reasons
   for this.  First, the default topology uses shortest paths; the
   packet will thus take the shortest possible route to the destination.
   Second, this allows a single router failure that manifests itself in
   multiple areas (as would be the case with an ABR/LBR failure) to be
   separately identified and repaired around.  Third, the packet can be
   fast-rerouted again, if necessary, due to a second distinct failure
   in a different area.

   In OSPF, an ABR that receives a packet on MRT-Red or MRT-Blue towards
   destination Z should continue to forward the packet along MRT-Red or
   MRT-Blue only if the best route to Z is in the same OSPF area as the
   interface that the packet was received on.  Otherwise, the packet
   should be removed from MRT-Red or MRT-Blue and forwarded on the
   shortest-path default forwarding topology.

   The above description applies to OSPF.  The same essential behavior
   also applies to IS-IS if one substitutes IS-IS level for OSPF area.
   However, the analogy with OSPF is not exact.  An interface in OSPF
   can only be in one area, whereas an interface in IS-IS can be in both
   Level-1 and Level-2.  Therefore, to avoid confusion and address this
   difference, we explicitly describe the behavior for IS-IS in
   Appendix A.  In the following sections, only the OSPF terminology is
   used.

10.1.  ABR Forwarding Behavior with MRT LDP Label Option 1A

   For LDP forwarding where a single label specifies (MT-ID, FEC), the
   ABR is responsible for advertising the proper label to each neighbor.
   Assume that an ABR has allocated three labels for a particular
   destination: L_primary, L_blue, and L_red.  To those routers in the
   same area as the best route to the destination, the ABR advertises
   the following FEC-label bindings: L_primary for the default topology,
   L_blue for the MRT-Blue MT-ID, and L_red for the MRT-Red MT-ID, as
   expected.  However, to routers in other areas, the ABR advertises the

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   following FEC-label bindings: L_primary for the default topology and
   L_primary for the Rainbow MRT MT-ID.  Associating L_primary with the
   Rainbow MRT MT-ID causes the receiving routers to use L_primary for
   the MRT-Blue MT-ID and for the MRT-Red MT-ID.

   The ABR installs all next hops for the best area: primary next hops
   for L_primary, MRT-Blue next hops for L_blue, and MRT-Red next hops
   for L_red.  Because the ABR advertised (Rainbow MRT MT-ID, FEC) with
   L_primary to neighbors not in the best area, packets from those
   neighbors will arrive at the ABR with a label L_primary and will be
   forwarded into the best area along the default topology.  By
   controlling what labels are advertised, the ABR can thus enforce that
   packets exiting the area do so on the shortest-path default topology.

10.1.1.  Motivation for Creating the Rainbow-FEC

   The desired forwarding behavior could be achieved in the above
   example without using the Rainbow-FEC.  This could be done by having
   the ABR advertise the following FEC-label bindings to neighbors not
   in the best area: L1_primary for the default topology, L1_primary for
   the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID.  Doing this
   would require machinery to spoof the labels used in FEC-label binding
   advertisements on a per-neighbor basis.  Such label-spoofing
   machinery does not currently exist in most LDP implementations and
   doesn't have other obvious uses.

   Many existing LDP implementations do however have the ability to
   filter FEC-label binding advertisements on a per-neighbor basis.  The
   Rainbow-FEC allows us to reuse the existing per-neighbor FEC
   filtering machinery to achieve the desired result.  By introducing
   the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
   advertise the FEC-label binding for the Rainbow-FEC (and filter those
   for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.

   An ABR may choose to either distribute the Rainbow-FEC or distribute
   separate MRT-Blue and MRT-Red advertisements.  This is a local
   choice.  A router that supports the MRT LDP Label Option 1A
   forwarding mechanism MUST be able to receive and correctly interpret
   the Rainbow-FEC.

10.2.  ABR Forwarding Behavior with IP Tunneling (Option 2)

   If IP tunneling is used, then the ABR behavior is dependent upon the
   outermost IP address.  If the outermost IP address is an MRT loopback
   address of the ABR, then the packet is decapsulated and forwarded
   based upon the inner IP address, which should go on the default SPT
   topology.  If the outermost IP address is not an MRT loopback address
   of the ABR, then the packet is simply forwarded along the associated

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   forwarding topology.  A PLR sending traffic to a destination outside
   its local area/level will pick the MRT and use the associated MRT
   loopback address of the selected ABR advertising the lowest cost to
   the external destination.

   Thus, for these two MRT forwarding mechanisms (MRT LDP Label Option
   1A and IP tunneling Option 2), there is no need for additional
   computation or per-area forwarding state.

10.3.  ABR Forwarding Behavior with MRT LDP Label Option 1B

   The other MRT forwarding mechanism described in Section 6 uses two
   labels: a topology-id label and a FEC-label.  This mechanism would
   require that any router whose MRT-Red or MRT-Blue next hop is an ABR
   would need to determine whether the ABR would forward the packet out
   of the area/level.  If so, then that router should pop off the
   topology-id label before forwarding the packet to the ABR.

   For example, in Figure 3, if node H fails, node E has to put traffic
   towards prefix p onto MRT-Red.  But since node D knows that ABR1 will
   use a best route from another area, it is safe for D to pop the
   topology-id label and just forward the packet to ABR1 along the MRT-
   Red next hop.  ABR1 will use the shortest path in Area 10.

   In all cases for IS-IS and most cases for OSPF, the penultimate
   router can determine what decision the adjacent ABR will make.  The
   one case where it can't be determined is when two ASBRs are in
   different non-backbone areas attached to the same ABR, then the
   ASBR's Area ID may be needed for tie-breaking (prefer the route with
   the largest OSPF area ID), and the Area ID isn't announced as part of
   the ASBR LSA.  In this one case, suboptimal forwarding along the MRT
   in the other area would happen.  If that becomes a realistic
   deployment scenario, protocol extensions could be developed to
   address this issue.

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       +----[C]----     --[D]--[E]                --[D]--[E]
       |           \   /         \               /         \
   p--[A] Area 10 [ABR1]  Area 0 [H]--p   +-[ABR1]  Area 0 [H]-+
       |           /   \         /        |      \         /   |
       +----[B]----     --[F]--[G]        |       --[F]--[G]   |
                                          |                    |
                                          | other              |
                                          +----------[p]-------+
                                            area

         (a) Example topology        (b) Proxy node view in Area 0 nodes


                   +----[C]<---       [D]->[E]
                   V           \             \
                +-[A] Area 10 [ABR1]  Area 0 [H]-+
                |  ^           /             /   |
                |  +----[B]<---       [F]->[G]   V
                |                                |
                +------------->[p]<--------------+

                  (c) rSPT towards destination p



             ->[D]->[E]                         -<[D]<-[E]
            /          \                       /         \
       [ABR1]  Area 0 [H]-+             +-[ABR1]         [H]
                      /   |             |      \
               [F]->[G]   V             V       -<[F]<-[G]
                          |             |
                          |             |
                [p]<------+             +--------->[p]

     (d) MRT-Blue in Area 0           (e) MRT-Red in Area 0

                Figure 3: ABR Forwarding Behavior and MRTs

11.  Prefixes Multiply Attached to the MRT Island

   How a computing router S determines its local MRT Island for each
   supported MRT profile is already discussed in Section 7.

   There are two types of prefixes or FECs that may be multiply attached
   to an MRT Island.  The first type are multihomed prefixes that
   usually connect at a domain or protocol boundary.  The second type
   represent routers that do not support the profile for the MRT Island.

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   The key difference is whether the traffic, once out of the MRT
   Island, might re-enter the MRT Island if a loop-free exit point is
   not selected.

   FRR using LFA has the useful property that it is able to protect
   multihomed prefixes against ABR failure.  For instance, if a prefix
   from the backbone is available via both ABR A and ABR B, if A fails,
   then the traffic should be redirected to B.  This can be accomplished
   with MRT FRR as well.

   If ASBR protection is desired, this has additional complexities if
   the ASBRs are in different areas.  Similarly, protecting labeled BGP
   traffic in the event of an ASBR failure has additional complexities
   due to the per-ASBR label spaces involved.

   As discussed in [RFC5286], a multihomed prefix could be:

   o  An out-of-area prefix announced by more than one ABR,

   o  An AS-External route announced by two or more ASBRs,

   o  A prefix with iBGP multipath to different ASBRs,

   o  etc.

   See Appendix B for a discussion of a general issue with multihomed
   prefixes connected in two different areas.

   There are also two different approaches to protection.  The first is
   tunnel endpoint selection where the PLR picks a router to tunnel to
   where that router is loop-free with respect to the failure-point.
   Conceptually, the set of candidate routers to provide LFAs expands to
   all routers that can be reached via an MRT alternate, attached to the
   prefix.

   The second is to use a proxy-node, which can be named via MPLS label
   or IP address, and pick the appropriate label or IP address to reach
   it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
   point.  A proxy-node can represent a destination prefix that can be
   attached to the MRT Island via at least two routers.  It is termed a
   named proxy-node if there is a way that traffic can be encapsulated
   to reach specifically that proxy-node; this could be because there is
   an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue
   IP addresses are advertised (in an as-yet undefined fashion) for that
   proxy-node.  Traffic to a named proxy-node may take a different path
   than traffic to the attaching router; traffic is also explicitly
   forwarded from the attaching router along a predetermined interface
   towards the relevant prefixes.

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   For IP traffic, multihomed prefixes can use tunnel endpoint
   selection.  For IP traffic that is destined to a router outside the
   MRT Island, if that router is the egress for a FEC advertised into
   the MRT Island, then the named proxy-node approach can be used.

   For LDP traffic, there is always a FEC advertised into the MRT
   Island.  The named proxy-node approach should be used, unless the
   computing router S knows the label for the FEC at the selected tunnel
   endpoint.

   If a FEC is advertised from outside the MRT Island into the MRT
   Island and the forwarding mechanism specified in the profile includes
   LDP Label Option 1A, then the routers learning that FEC MUST also
   advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors
   inside the MRT Island.  Any router receiving a FEC corresponding to a
   router outside the MRT Island or to a multihomed prefix MUST compute
   and install the transit MRT-Blue and MRT-Red next hops for that FEC.
   The FEC-label bindings for the topology-scoped FECs ((MT-ID 0, FEC),
   (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
   neighbors inside the MRT Island.

11.1.  Protecting Multihomed Prefixes Using Tunnel Endpoint Selection

   Tunnel endpoint selection is a local matter for a router in the MRT
   Island since it pertains to selecting and using an alternate and does
   not affect the transit MRT-Red and MRT-Blue forwarding topologies.

   Let the computing router be S and the next hop F be the node whose
   failure is to be avoided.  Let the destination be prefix p.  Have A
   be the router to which the prefix p is attached for S's shortest path
   to p.

   The candidates for tunnel endpoint selection are those to which the
   destination prefix is attached in the area/level.  For a particular
   candidate B, it is necessary to determine if B is loop-free to reach
   p with respect to S and F for node-protection or at least with
   respect to S and the link (S, F) for link-protection.  If B will
   always prefer to send traffic to p via a different area/level, then
   this is definitional.  Otherwise, distance-based computations are
   necessary and an SPF from B's perspective may be necessary.  The
   following equations give the checks needed; the rationale is similar
   to that given in [RFC5286].  In the inequalities below, D_opt(X,Y)
   means the shortest distance from node X to node Y, and D_opt(X,p)
   means the shortest distance from node X to prefix p.

   Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)

   Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)

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   The latter is equivalent to the following, which avoids the need to
   compute the shortest path from F to p.

  Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)

   Finally, the rules for Endpoint selection are given below.  The basic
   idea is to repair to the prefix-advertising router selected for the
   shortest-path and only to select and tunnel to a different endpoint
   if necessary (e.g., A=F or F is a cut-vertex or the link (S,F) is a
   cut-link).

   1.  Does S have a node-protecting alternate to A?  If so, select
       that.  Tunnel the packet to A along that alternate.  For example,
       if LDP is the forwarding mechanism, then push the label (MRT-Red,
       A) or (MRT-Blue, A) onto the packet.

   2.  If not, then is there a router B that is loop-free to reach p
       while avoiding both F and S?  If so, select B as the endpoint.
       Determine the MRT alternate to reach B while avoiding F.  Tunnel
       the packet to B along that alternate.  For example, with LDP,
       push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.

   3.  If not, then does S have a link-protecting alternate to A?  If
       so, select that.

   4.  If not, then is there a router B that is loop-free to reach p
       while avoiding S and the link from S to F?  If so, select B as
       the endpoint and the MRT alternate for reaching B from S that
       avoid the link (S,F).

   The tunnel endpoint selected will receive a packet destined to itself
   and, being the egress, will pop that MPLS label (or have signaled
   Implicit Null) and forward based on what is underneath.  This
   suffices for IP traffic since the tunnel endpoint can use the IP
   header of the original packet to continue forwarding the packet.
   However, tunneling of LDP traffic requires targeted LDP sessions for
   learning the FEC-label binding at the tunnel endpoint.

11.2.  Protecting Multihomed Prefixes Using Named Proxy-Nodes

   Instead, the named proxy-node method works with LDP traffic without
   the need for targeted LDP sessions.  It also has a clear advantage
   over tunnel endpoint selection, in that it is possible to explicitly
   forward from the MRT Island along an interface to a loop-free island
   neighbor when that interface may not be a primary next hop.

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   A named proxy-node represents one or more destinations and, for LDP
   forwarding, has a FEC associated with it that is signaled into the
   MRT Island.  Therefore, it is possible to explicitly label packets to
   go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
   Island, the label will swap to meaning (MT-ID 0, FEC).  It would be
   possible to have named proxy-nodes for IP forwarding, but this would
   require extensions to signal two IP addresses to be associated with
   MRT-Red and MRT-Blue for the proxy-node.  A named proxy-node can be
   uniquely represented by the two routers in the MRT Island to which it
   is connected.  The extensions to signal such IP addresses will be
   defined elsewhere.  The details of what label-bindings must be
   originated will be described in another document.

   Computing the MRT next hops to a named proxy-node and the MRT
   alternate for the computing router S to avoid a particular failure
   node F is straightforward.  The details of the simple constant-time
   functions, Select_Proxy_Node_NHs() and
   Select_Alternates_Proxy_Node(), are given in [RFC7811].  A key point
   is that computing these MRT next hops and alternates can be done as
   new named proxy-nodes are added or removed without requiring a new
   MRT computation or impacting other existing MRT paths.  This maps
   very well to, for example, how OSPFv2 (see [RFC2328], Section 16.5)
   does incremental updates for new summary-LSAs.

   The remaining question is how to attach the named proxy-node to the
   MRT Island; all the routers in the MRT Island MUST do this
   consistently.  No more than two routers in the MRT Island can be
   selected; one should only be selected if there are no others that
   meet the necessary criteria.  The named proxy-node is logically part
   of the area/level.

   There are two sources for candidate routers in the MRT Island to
   connect to the named proxy-node.  The first set is made up of those
   routers in the MRT Island that are advertising the prefix; the named-
   proxy-cost assigned to each prefix-advertising router is the
   announced cost to the prefix.  The second set is made up of those
   routers in the MRT Island that are connected to routers not in the
   MRT Island but in the same area/level; such routers will be defined
   as Island Border Routers (IBRs).  The routers connected to the IBRs
   that are not in the MRT Island and are in the same area/level as the
   MRT Island are Island Neighbors (INs).

   Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
   may come from any router inside the MRT Island, it is necessary that
   whatever router to which an IBR forwards the packet be loop-free with
   respect to the whole MRT Island for the destination.  Thus, an IBR is
   a candidate router only if it possesses at least one IN whose
   shortest path to the prefix does not enter the MRT Island.  A method

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   for identifying Loop-Free Island Neighbors (LFINs) is given in
   [RFC7811].  The named-proxy-cost assigned to each (IBR, IN) pair is
   cost(IBR, IN) + D_opt(IN, prefix).

   From the set of prefix-advertising routers and the set of IBRs with
   at least one LFIN, the two routers with the lowest named-proxy-cost
   are selected.  Ties are broken based upon the lowest Router ID.  For
   ease of discussion, the two selected routers will be referred to as
   proxy-node attachment routers.

   A proxy-node attachment router has a special forwarding role.  When a
   packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
   prefix), if the proxy-node attachment router is an IBR, it MUST swap
   to the shortest path forwarding topology (e.g., swap to the label for
   (MT-ID 0, prefix) or remove the outer IP encapsulation) and forward
   the packet to the IN whose cost was used in the selection.  If the
   proxy-node attachment router is not an IBR, then the packet MUST be
   removed from the MRT forwarding topology and sent along the
   interface(s) that caused the router to advertise the prefix; this
   interface might be out of the area/level/AS.

11.3.  MRT Alternates for Destinations outside the MRT Island

   A natural concern with new functionality is how to have it be useful
   when it is not deployed across an entire IGP area.  In the case of
   MRT FRR, where it provides alternates when appropriate LFAs aren't
   available, there are also deployment scenarios where it may make
   sense to only enable some routers in an area with MRT FRR.  A simple
   example of such a scenario would be a ring of six or more routers
   that is connected via two routers to the rest of the area.

   Destinations inside the local island can obviously use MRT
   alternates.  Destinations outside the local island can be treated
   like a multihomed prefix and either endpoint selection or Named
   Proxy-Nodes can be used.  Named proxy-nodes MUST be supported when
   LDP forwarding is supported and a label-binding for the destination
   is sent to an IBR.

   Naturally, there are more-complicated options to improve coverage,
   such as connecting multiple MRT Islands across tunnels, but the need
   for the additional complexity has not been justified.

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12.  Network Convergence and Preparing for the Next Failure

   After a failure, MRT detours ensure that packets reach their intended
   destination while the IGP has not reconverged onto the new topology.
   As link-state updates reach the routers, the IGP process calculates
   the new shortest paths.  Two things need attention: micro-loop
   prevention and MRT recalculation.

12.1.  Micro-loop Prevention and MRTs

   A micro-loop is a transient packet-forwarding loop among two or more
   routers that can occur during convergence of IGP forwarding state.
   [RFC5715] discusses several techniques for preventing micro-loops.
   This section discusses how MRT-FRR relates to two of the micro-loop
   prevention techniques discussed in [RFC5715]: Nearside and Farside
   Tunneling.

   In Nearside Tunneling, a router (PLR) adjacent to a failure performs
   local repair and informs remote routers of the failure.  The remote
   routers initially tunnel affected traffic to the nearest PLR, using
   tunnels that are unaffected by the failure.  Once the forwarding
   state for normal shortest path routing has converged, the remote
   routers return the traffic to shortest path forwarding.  MRT-FRR is
   relevant for Nearside Tunneling for the following reason.  The
   process of tunneling traffic to the PLRs and waiting a sufficient
   amount of time for IGP forwarding state convergence with Nearside
   Tunneling means that traffic will generally rely on the local repair
   at the PLR for longer than it would in the absence of Nearside
   Tunneling.  Since MRT-FRR provides 100% coverage for single link and
   node failure, it may be an attractive option to provide the local
   repair paths when Nearside Tunneling is deployed.

   MRT-FRR is also relevant for the Farside Tunneling micro-loop
   prevention technique.  In Farside Tunneling, remote routers tunnel
   traffic affected by a failure to a node downstream of the failure
   with respect to traffic destination.  This node can be viewed as
   being on the farside of the failure with respect to the node
   initiating the tunnel.  Note that the discussion of Farside Tunneling
   in [RFC5715] focuses on the case where the farside node is
   immediately adjacent to a failed link or node.  However, the farside
   node may be any node downstream of the failure with respect to
   traffic destination, including the destination itself.  The tunneling
   mechanism used to reach the farside node must be unaffected by the
   failure.  The alternative forwarding paths created by MRT-FRR have
   the potential to be used to forward traffic from the remote routers
   upstream of the failure all the way to the destination.  In the event
   of failure, either the MRT-Red or MRT-Blue path from the remote
   upstream router to the destination is guaranteed to avoid a link

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   failure or inferred node failure.  The MRT forwarding paths are also
   guaranteed to not be subject to micro-loops because they are locked
   to the topology before the failure.

   We note that the computations in [RFC7811] address the case of a PLR
   adjacent to a failure determining which choice of MRT-Red or MRT-Blue
   will avoid a failed link or node.  More computation may be required
   for an arbitrary remote upstream router to determine whether to
   choose MRT-Red or MRT-Blue for a given destination and failure.

12.2.  MRT Recalculation for the Default MRT Profile

   This section describes how the MRT recalculation SHOULD be performed
   for the Default MRT Profile.  This is intended to support FRR
   applications.  Other approaches are possible, but they are not
   specified in this document.

   When a failure event happens, traffic is put by the PLRs onto the MRT
   topologies.  After that, each router recomputes its SPT and moves
   traffic over to that.  Only after all the PLRs have switched to using
   their SPTs and traffic has drained from the MRT topologies should
   each router install the recomputed MRTs into the FIBs.

   At each router, therefore, the sequence is as follows:

   1.  Receive failure notification

   2.  Recompute SPT.

   3.  Install the new SPT in the FIB.

   4.  If the network was stable before the failure occurred, wait a
       configured (or advertised) period for all routers to be using
       their SPTs and traffic to drain from the MRTs.

   5.  Recompute MRTs.

   6.  Install new MRTs in the FIB.

   While the recomputed MRTs are not installed in the FIB, protection
   coverage is lowered.  Therefore, it is important to recalculate the
   MRTs and install them quickly.

   New protocol extensions for advertising the time needed to recompute
   shortest path routes and install them in the FIB will be defined
   elsewhere.

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13.  Operational Considerations

   The following aspects of MRT-FRR are useful to consider when
   deploying the technology in different operational environments and
   network topologies.

13.1.  Verifying Forwarding on MRT Paths

   The forwarding paths created by MRT-FRR are not used by normal (non-
   FRR) traffic.  They are only used to carry FRR traffic for a short
   period of time after a failure has been detected.  It is RECOMMENDED
   that an operator proactively monitor the MRT forwarding paths in
   order to be certain that the paths will be able to carry FRR traffic
   when needed.  Therefore, an implementation SHOULD provide an operator
   with the ability to test MRT paths with Operations, Administration,
   and Maintenance (OAM) traffic.  For example, when MRT paths are
   realized using LDP labels distributed for topology-scoped FECs, an
   implementation can use the MPLS ping and traceroute as defined in
   [RFC4379] and extended in [RFC7307] for topology-scoped FECs.

13.2.  Traffic Capacity on Backup Paths

   During a fast-reroute event initiated by a PLR in response to a
   network failure, the flow of traffic in the network will generally
   not be identical to the flow of traffic after the IGP forwarding
   state has converged, taking the failure into account.  Therefore,
   even if a network has been engineered to have enough capacity on the
   appropriate links to carry all traffic after the IGP has converged
   after the failure, the network may still not have enough capacity on
   the appropriate links to carry the flow of traffic during a fast-
   reroute event.  This can result in more traffic loss during the fast-
   reroute event than might otherwise be expected.

   Note that there are two somewhat distinct aspects to this phenomenon.
   The first is that the path from the PLR to the destination during the
   fast-reroute event may be different from the path after the IGP
   converges.  In this case, any traffic for the destination that
   reaches the PLR during the fast-reroute event will follow a different
   path from the PLR to the destination than will be followed after IGP
   convergence.

   The second aspect is that the amount of traffic arriving at the PLR
   for affected destinations during the fast-reroute event may be larger
   than the amount of traffic arriving at the PLR for affected
   destinations after IGP convergence.  Immediately after a failure, any
   non-PLR routers that were sending traffic to the PLR before the
   failure will continue sending traffic to the PLR, and that traffic
   will be carried over backup paths from the PLR to the destinations.

Top      Up      ToC       Page 35 
   After IGP convergence, upstream non-PLR routers may direct some
   traffic away from the PLR.

   In order to reduce or eliminate the potential for transient traffic
   loss due to inadequate capacity during fast-reroute events, an
   operator can model the amount of traffic taking different paths
   during a fast-reroute event.  If it is determined that there is not
   enough capacity to support a given fast-reroute event, the operator
   can address the issue either by augmenting capacity on certain links
   or modifying the backup paths themselves.

   The MRT Lowpoint algorithm produces a pair of diverse paths to each
   destination.  These paths are generated by following the directed
   links on a common GADAG.  The decision process for constructing the
   GADAG in the MRT Lowpoint algorithm takes into account individual IGP
   link metrics.  At any given node, links are explored in order from
   lowest IGP metric to highest IGP metric.  Additionally, the process
   for constructing the MRT-Red and Blue trees uses SPF traversals of
   the GADAG.  Therefore, the IGP link metric values affect the computed
   backup paths.  However, adjusting the IGP link metrics is not a
   generally applicable tool for modifying the MRT backup paths.
   Achieving a desired set of MRT backup paths by adjusting IGP metrics
   while at the same time maintaining the desired flow of traffic along
   the shortest paths is not possible in general.

   MRT-FRR allows an operator to exclude a link from the MRT Island, and
   thus the GADAG, by advertising it as MRT-Ineligible.  Such a link
   will not be used on the MRT forwarding path for any destination.
   Advertising links as MRT-Ineligible is the main tool provided by MRT-
   FRR for keeping backup traffic off of lower bandwidth links during
   fast-reroute events.

   Note that all of the backup paths produced by the MRT Lowpoint
   algorithm are closely tied to the common GADAG computed as part of
   that algorithm.  Therefore, it is generally not possible to modify a
   subset of paths without affecting other paths.  This precludes more
   fine-grained modification of individual backup paths when using only
   paths computed by the MRT Lowpoint algorithm.

   However, it may be desirable to allow an operator to use MRT-FRR
   alternates together with alternates provided by other FRR
   technologies.  A policy-based alternate selection process can allow
   an operator to select the best alternate from those provided by MRT
   and other FRR technologies.  As a concrete example, it may be
   desirable to implement a policy where a downstream LFA (if it exists
   for a given failure mode and destination) is preferred over a given
   MRT alternate.  This combination gives the operator the ability to
   affect where traffic flows during a fast-reroute event, while still

Top      Up      ToC       Page 36 
   producing backup paths that use no additional labels for LDP traffic
   and will not loop under multiple failures.  This and other choices of
   alternate selection policy can be evaluated in the context of their
   effect on fast-reroute traffic flow and available capacity, as well
   as other deployment considerations.

   Note that future documents may define MRT profiles in addition to the
   default profile defined here.  Different MRT profiles will generally
   produce alternate paths with different properties.  An implementation
   may allow an operator to use different MRT profiles instead of or in
   addition to the default profile.

13.3.  MRT IP Tunnel Loopback Address Management

   As described in Section 6.1.2, if an implementation uses IP tunneling
   as the mechanism to realize MRT forwarding paths, each node must
   advertise an MRT-Red and an MRT-Blue loopback address.  These IP
   addresses must be unique within the routing domain to the extent that
   they do not overlap with each other or with any other routing table
   entries.  It is expected that operators will use existing tools and
   processes for managing infrastructure IP addresses to manage these
   additional MRT-related loopback addresses.

13.4.  MRT-FRR in a Network with Degraded Connectivity

   Ideally, routers in a service provider network using MRT-FRR will be
   initially deployed in a 2-connected topology, allowing MRT-FRR to
   find completely diverse paths to all destinations.  However, a
   network can differ from an ideal 2-connected topology for many
   possible reasons, including network failures and planned maintenance
   events.

   MRT-FRR is designed to continue to function properly when network
   connectivity is degraded.  When a network contains cut-vertices or
   cut-links dividing the network into different 2-connected blocks,
   MRT-FRR will continue to provide completely diverse paths for
   destinations within the same block as the PLR.  For a destination in
   a different block from the PLR, the redundant paths created by MRT-
   FRR will be link and node diverse within each block, and the paths
   will only share links and nodes that are cut-links or cut-vertices in
   the topology.

   If a network becomes partitioned with one set of routers having no
   connectivity to another set of routers, MRT-FRR will function
   independently in each set of connected routers, providing redundant
   paths to destinations in same set of connected routers as a given
   PLR.

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13.5.  Partial Deployment of MRT-FRR in a Network

   A network operator may choose to deploy MRT-FRR only on a subset of
   routers in an IGP area.  MRT-FRR is designed to accommodate this
   partial deployment scenario.  Only routers that advertise support for
   a given MRT profile will be included in a given MRT Island.  For a
   PLR within the MRT Island, MRT-FRR will create redundant forwarding
   paths to all destinations with the MRT Island using maximally
   redundant trees all the way to those destinations.  For destinations
   outside of the MRT Island, MRT-FRR creates paths to the destination
   that use forwarding state created by MRT-FRR within the MRT Island
   and shortest path forwarding state outside of the MRT Island.  The
   paths created by MRT-FRR to non-Island destinations are guaranteed to
   be diverse within the MRT Island (if topologically possible).
   However, the part of the paths outside of the MRT Island may not be
   diverse.

14.  IANA Considerations

   IANA has created the "MRT Profile Identifier Registry".  The range is
   0 to 255.  The Default MRT Profile defined in this document has value
   0.  Values 1-200 are allocated by Standards Action.  Values 201-220
   are for Experimental Use.  Values 221-254 are for Private Use.  Value
   255 is reserved for future registry extension.  (The allocation and
   use policies are described in [RFC5226].)

   The initial registry is shown below.

      Value    Description                               Reference
      -------  ----------------------------------------  ------------
      0        Default MRT Profile                       RFC 7812
      1-200    Unassigned
      201-220  Experimental Use
      221-254  Private Use
      255      Reserved (for future registry extension)

   The "MRT Profile Identifier Registry" is a new registry in the IANA
   Matrix.  Following existing conventions, http://www.iana.org/
   protocols displays a new header: "Maximally Redundant Tree (MRT)
   Parameters".  Under that header, there is an entry for "MRT Profile
   Identifier Registry", which links to the registry itself at
   http://www.iana.org/assignments/mrt-parameters.

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15.  Security Considerations

   In general, MRT forwarding paths do not follow shortest paths.  The
   transit forwarding state corresponding to the MRT paths is created
   during normal operations (before a failure occurs).  Therefore, a
   malicious packet with an appropriate header injected into the network
   from a compromised location would be forwarded to a destination along
   a non-shortest path.  When this technology is deployed, a network
   security design should not rely on assumptions about potentially
   malicious traffic only following shortest paths.

   It should be noted that the creation of non-shortest forwarding paths
   is not unique to MRT.

   MRT-FRR requires that routers advertise information used in the
   formation of MRT backup paths.  While this document does not specify
   the protocol extensions used to advertise this information, we
   discuss security considerations related to the information itself.
   Injecting false MRT-related information could be used to direct some
   MRT backup paths over compromised transmission links.  Combined with
   the ability to generate network failures, this could be used to send
   traffic over compromised transmission links during a fast-reroute
   event.  In order to prevent this potential exploit, a receiving
   router needs to be able to authenticate MRT-related information that
   claims to have been advertised by another router.

16.  References

16.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC7307]  Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
              King, "LDP Extensions for Multi-Topology", RFC 7307,
              DOI 10.17487/RFC7307, July 2014,
              <http://www.rfc-editor.org/info/rfc7307>.

Top      Up      ToC       Page 39 
   [RFC7811]  Enyedi, G., Ed., Csaszar, A., Atlas, A., Ed., Bowers, C.,
              and A. Gopalan, "An Algorithm for Computing IP/LDP Fast
              Reroute Using Maximally Redundant Trees (MRT-FRR)",
              RFC 7811, DOI 10.17487/RFC7811, June 2016,
              <http://www.rfc-editor.org/info/rfc7811>.

16.2.  Informative References

   [EnyediThesis]
              Enyedi, G., "Novel Algorithms for IP Fast Reroute",
              Department of Telecommunications and Media Informatics,
              Budapest University of Technology and Economics Ph.D.
              Thesis, February 2011,
              <https://repozitorium.omikk.bme.hu/bitstream/
              handle/10890/1040/ertekezes.pdf>.

   [LDP-MRT]  Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and
              IJ. Wijnands, "LDP Extensions to Support Maximally
              Redundant Trees", Work in Progress, draft-ietf-mpls-ldp-
              mrt-03, May 2016.

   [MRT-ARCH]
              Atlas, A., Kebler, R., Wijnands, IJ., Csaszar, A., and G.
              Enyedi, "An Architecture for Multicast Protection Using
              Maximally Redundant Trees", Work in Progress, draft-atlas-
              rtgwg-mrt-mc-arch-02, July 2013.

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

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.

   [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,
              <http://www.rfc-editor.org/info/rfc5286>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <http://www.rfc-editor.org/info/rfc5331>.

Top      Up      ToC       Page 40 
   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <http://www.rfc-editor.org/info/rfc5340>.

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
              2009, <http://www.rfc-editor.org/info/rfc5443>.

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

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, DOI 10.17487/RFC5715, January
              2010, <http://www.rfc-editor.org/info/rfc5715>.

   [RFC6976]  Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
              Francois, P., and O. Bonaventure, "Framework for Loop-Free
              Convergence Using the Ordered Forwarding Information Base
              (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
              2013, <http://www.rfc-editor.org/info/rfc6976>.

   [RFC6981]  Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
              and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
              DOI 10.17487/RFC6981, August 2013,
              <http://www.rfc-editor.org/info/rfc6981>.

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

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <http://www.rfc-editor.org/info/rfc7490>.

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Appendix A.  Inter-level Forwarding Behavior for IS-IS

   In the description below, we use the terms "Level-1-only interface",
   "Level-2-only interface", and "Level-1-and-Level-2 interface" to mean
   an interface that has formed only a Level-1 adjacency, only a Level-2
   adjacency, or both Level-1 and Level-2 adjacencies.  Note that IS-IS
   also defines the concept of areas.  A router is configured with an
   IS-IS area identifier, and a given router may be configured with
   multiple IS-IS area identifiers.  For an IS-IS Level-1 adjacency to
   form between two routers, at least one IS-IS area identifier must
   match.  IS-IS Level-2 adjacencies do not require any area identifiers
   to match.  The behavior described below does not explicitly refer to
   IS-IS area identifiers.  However, IS-IS area identifiers will
   indirectly affect the behavior by affecting the formation of Level-1
   adjacencies.

   First, consider a packet destined to Z on MRT-Red or MRT-Blue
   received on a Level-1-only interface.  If the best shortest path
   route to Z was learned from a Level-1 advertisement, then the packet
   should continue to be forwarded along MRT-Red or MRT-Blue.  If,
   instead, the best route was learned from a Level-2 advertisement,
   then the packet should be removed from MRT-Red or MRT-Blue and
   forwarded on the shortest-path default forwarding topology.

   Now consider a packet destined to Z on MRT-Red or MRT-Blue received
   on a Level-2-only interface.  If the best route to Z was learned from
   a Level-2 advertisement, then the packet should continue to be
   forwarded along MRT-Red or MRT-Blue.  If, instead, the best route was
   learned from a Level-1 advertisement, then the packet should be
   removed from MRT-Red or MRT-Blue and forwarded on the shortest-path
   default forwarding topology.

   Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
   received on a Level-1-and-Level-2 interface.  This packet should
   continue to be forwarded along MRT-Red or MRT-Blue, regardless of
   which level the route was learned from.

   An implementation may simplify the decision-making process above by
   using the interface of the next hop for the route to Z to determine
   the level from which the best route to Z was learned.  If the next
   hop points out a Level-1-only interface, then the route was learned
   from a Level-1 advertisement.  If the next hop points out a Level-
   2-only interface, then the route was learned from a Level-2
   advertisement.  A next hop that points out a Level-1-and-Level-2
   interface does not provide enough information to determine the source
   of the best route.  With this simplification, an implementation would
   need to continue forwarding along MRT-Red or MRT-Blue when the next-
   hop points out a Level-1-and-Level-2 interface.  Therefore, a packet

Top      Up      ToC       Page 42 
   on MRT-Red or MRT-Blue going from Level-1 to Level-2 (or vice versa)
   that traverses a Level-1-and-Level-2 interface in the process will
   remain on MRT-Red or MRT-Blue.  This simplification may not always
   produce the optimal forwarding behavior, but it does not introduce
   interoperability problems.  The packet will stay on an MRT backup
   path longer than necessary, but it will still reach its destination.

Appendix B.  General Issues with Area Abstraction

   When a multihomed prefix is connected in two different areas, it may
   be impractical to protect them without adding the complexity of
   explicit tunneling.  This is also a problem for LFA and Remote-LFA.

          50
        |----[ASBR Y]---[B]---[ABR 2]---[C]      Backbone Area 0:
        |                                |           ABR 1, ABR 2, C, D
        |                                |
        |                                |       Area 20:  A, ASBR X
        |                                |
        p ---[ASBR X]---[A]---[ABR 1]---[D]      Area 10: B, ASBR Y
           5                                  p is a Type 1 AS-external


             Figure 4: AS External Prefixes in Different Areas

   Consider the network in Figure 4 and assume there is a richer
   connective topology that isn't shown, where the same prefix is
   announced by ASBR X and ASBR Y, which are in different non-backbone
   areas.  If the link from A to ASBR X fails, then an MRT alternate
   could forward the packet to ABR 1 and ABR 1 could forward it to D,
   but then D would find the shortest route is back via ABR 1 to Area
   20.  This problem occurs because the routers, including the ABR, in
   one area are not yet aware of the failure in a different area.

   The only way to get it from A to ASBR Y is to explicitly tunnel it to
   ASBR Y.  If the traffic is unlabeled or the appropriate MPLS labels
   are known, then explicit tunneling MAY be used as long as the
   shortest path of the tunnel avoids the failure point.  In that case,
   A must determine that it should use an explicit tunnel instead of an
   MRT alternate.

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Acknowledgements

   The authors would like to thank Mike Shand for his valuable review
   and contributions.

   The authors would like to thank Joel Halpern, Hannes Gredler, Ted
   Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
   Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
   Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
   Alvaro Retana for their suggestions and review.

Contributors

   Robert Kebler
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States
   Email: rkebler@juniper.net

   Andras Csaszar
   Ericsson
   Konyves Kalman krt 11
   Budapest  1097
   Hungary
   Email: Andras.Csaszar@ericsson.com

   Jeff Tantsura
   Ericsson
   300 Holger Way
   San Jose, CA  95134
   United States
   Email: jeff.tantsura@ericsson.com

   Russ White
   VCE
   Email: russw@riw.us

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Authors' Addresses

   Alia Atlas
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States

   Email: akatlas@juniper.net


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

   Email: cbowers@juniper.net


   Gabor Sandor Enyedi
   Ericsson
   Konyves Kalman krt 11.
   Budapest  1097
   Hungary

   Email: Gabor.Sandor.Enyedi@ericsson.com