| Routing Area Working Group | A. Atlas |
| Internet-Draft | C. Bowers |
| Intended status: Standards Track | Juniper Networks |
| Expires: June 23, 2016 | G. Enyedi |
| Ericsson | |
| December 21, 2015 |
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-08
This document defines the architecture for IP/LDP Fast-Reroute using Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology that gives link-protection and node-protection with 100% coverage in any network topology that is still connected after the failure.
MRT removes the need to engineer for coverage. MRT is also extremely computationally efficient. For any router in the network, the MRT computation is less than the LFA computation for a node with three or more neighbors.
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."
This Internet-Draft will expire on June 23, 2016.
Copyright (c) 2015 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.
This document describes a solution for IP/LDP fast-reroute [RFC5714]. MRT-FRR creates two alternate trees separate from the primary next-hop forwarding used during stable operation. These two trees are maximally diverse from each other, providing link and node protection for 100% of paths and failures as long as the failure does not cut the network into multiple pieces. This document defines the architecture for IP/LDP fast-reroute with MRT. The associated protocol extensions are defined in [I-D.ietf-ospf-mrt] and [I-D.ietf-mpls-ldp-mrt]. The exact MRT algorithm is defined in [I-D.ietf-rtgwg-mrt-frr-algorithm].
IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse forwarding topologies to provide alternates. A primary next-hop should be on only one of the diverse forwarding topologies; thus, the other can be used to provide an alternate. Once traffic has been moved to one of MRTs, it is not subject to further repair actions. Thus, the traffic will not loop even if a worse failure (e.g. node) occurs when protection was only available for a simpler failure (e.g. link).
In addition to supporting IP and LDP unicast fast-reroute, the diverse forwarding topologies and guarantee of 100% coverage permit fast-reroute technology to be applied to multicast traffic as described in [I-D.atlas-rtgwg-mrt-mc-arch].
Other existing or proposed solutions are partial solutions or have significant issues, as described below.
Summary Comparison of IP/LDP FRR Methods
| Method | Coverage | Alternate Looping? | Computation (in SPFs) |
|---|---|---|---|
| MRT-FRR | 100% Link/Node | None | less than 3 |
| LFA | Partial Link/Node | Possible | per neighbor |
| Remote LFA | Partial Link/Node | Possible | per neighbor (link) or neighbor's neighbor (node) |
| Not-Via | 100% Link/Node | None | per link and node |
| TI-LFA | 100% Link/Node | Possible | per neighbor (link) or neighbor's neighbor (node) |
Fast-reroute is based upon the single failure assumption - that the time between single failures is long enough for a network to reconverge and start forwarding on the new shortest paths. That does not imply that the network will only experience one failure or change.
It is straightforward to analyze a particular network topology for coverage. However, a real network does not always have the same topology. For instance, maintenance events will take links or nodes out of use. Simply costing out a link can have a significant effect on what LFAs are available. Similarly, after a single failure has happened, the topology is changed and its associated coverage. Finally, many networks have new routers or links added and removed; each of those changes can have an effect on the coverage for topology-sensitive methods such as LFA and Remote LFA. If fast-reroute is important for the network services provided, then a method that guarantees 100% coverage is important to accomodate natural network topology changes.
Asymmetric link costs are also a common aspect of networks. There are at least three common causes for them. First, any broadcast interface is represented by a pseudo-node and has asymmetric link costs to and from that pseudo-node. Second, when routers come up or a link with LDP comes up, it is recommended in [RFC5443] and [RFC6987] that the link metric be raised to the maximum cost; this may not be symmetric and for [RFC6987] is not expected to be. Third, techniques such as IGP metric tuning for traffic-engineering can result in asymmetric link costs. A fast-reroute solution needs to handle network topologies with asymmetric link costs.
When a network needs to use a micro-loop prevention mechanism [RFC5715] such as Ordered FIB[RFC6976] or Farside Tunneling[RFC5715], then the whole IGP area needs to have alternates available so that the micro-loop prevention mechanism, which requires slower network convergence, can take the necessary time without adversely impacting traffic. Without complete coverage, traffic to the unprotected destinations will be dropped for significantly longer than with current convergence - where routers individually converge as fast as possible.
MRT-FRR supports partial deployment. As with many new features, the protocols (OSPF, LDP, ISIS) indicate their capability to support MRT. Inside the MRT-capable connected group of routers (referred to as an MRT Island), the MRTs are computed. Alternates to destinations outside the MRT Island are computed and depend upon the existence of a loop-free neighbor of the MRT Island for that destination.
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].
A pair of Maximally Redundant Trees is a pair of directed spanning trees that provides maximally disjoint paths towards their common root. Only links or nodes whose failure would partition the network (i.e. cut-links and cut-vertices) are shared between the trees. The algorithm to compute MRTs is given in [I-D.ietf-rtgwg-mrt-frr-algorithm]. This algorithm can be computed in O(e + n log n); it is less than three SPFs. Modeling results comparing the alternate path lengths obtained with MRT to other approaches are described in [I-D.ietf-rtgwg-mrt-frr-algorithm]. This document describes how the MRTs can be used and not how to compute them.
MRT provides destination-based trees for each destination. Each router stores its normal primary next-hop(s) as well as MRT-Blue next-hop(s) and MRT-Red next-hop(s) toward each destination. The alternate will be selected between the MRT-Blue and MRT-Red.
The most important thing to understand about MRTs is that for each pair of destination-routed MRTs, there is a path from every node X to the destination D on the Blue MRT that is as disjoint as possible from the path on the Red MRT.
For example, in Figure 1, there is a network graph that is 2-connected in (a) and associated MRTs in (b) and (c). One can consider the paths from B to R; on the Blue MRT, the paths are B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R. These are clearly link and node-disjoint. These MRTs are redundant trees because the paths are disjoint.
[E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
| | | | ^ | | |
| | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | |
| | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]<--[B]<--|
(a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R
Figure 1: A 2-connected Network
By contrast, in Figure 2, the network in (a) is not 2-connected. If F, G or the link F<->G failed, then the network would be partitioned. It is clearly impossible to have two link-disjoint or node-disjoint paths from G, I or J to R. The MRTs given in (b) and (c) offer paths that are as disjoint as possible. For instance, the paths from B to R are the same as in Figure 1 and the path from G to R on the Blue MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.
[E]---[D]---|
| | | |----[I]
| | | | |
[R]---[C] [F]---[G] |
| | | | |
| | | |----[J]
[A]---[B]---|
(a)
a non-2-connected graph
[E]<--[D]<--| [E]-->[D]
| ^ | [I] | |----[I]
V | | | V V ^
[R] [C] [F]<--[G] | [R]<--[C] [F]<--[G] |
^ ^ ^ V ^ | |
| | |----[J] | | [J]
[A]-->[B]---| [A]<--[B]<--|
(b) (c)
Blue MRT towards R Red MRT towards R
Figure 2: A non-2-connected network
In normal IGP routing, each router has its shortest-path-tree to all destinations. From the perspective of a particular destination, D, this looks like a reverse SPT (rSPT). To use maximally redundant trees, in addition, each destination D has two MRTs associated with it; by convention these will be called the MRT-Blue and MRT-Red. MRT-FRR is realized by using multi-topology forwarding. There is a MRT-Blue forwarding topology and a MRT-Red forwarding topology.
Any IP/LDP fast-reroute technique beyond LFA requires an additional dataplane procedure, such as an additional forwarding mechanism. The well-known options are multi-topology forwarding (used by MRT-FRR), tunneling (e.g. [RFC6981] or [RFC7490]), and per-interface forwarding (e.g. Loop-Free Failure Insensitive Routing in [EnyediThesis]).
When there is a link or node failure affecting, but not partitioning, the network, each node will still have at least one path via one of the MRTs to reach the destination D. For example, in Figure 2, C would normally forward traffic to R across the C<->R link. If that C<->R link fails, then C could use the Blue MRT path C->D->E->R.
As is always the case with fast-reroute technologies, forwarding does not change until a local failure is detected. Packets are forwarded along the shortest path. The appropriate alternate to use is pre-computed. [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how to determine whether the MRT-Blue next-hops or the MRT-Red next-hops should be the MRT alternate next-hops for a particular primary next-hop to a particular destination.
MRT alternates are always available to use. It is a local decision whether to use an MRT alternate, a Loop-Free Alternate or some other type of alternate.
As described in [RFC5286], when a worse failure than is anticipated happens, using LFAs that are not downstream neighbors can cause micro-looping. Section 1.1 of [RFC5286] gives an example of link-protecting alternates causing a loop on node failure. Even if a worse failure than anticipated happens, the use of MRT alternates will not cause looping. Therefore, while node-protecting LFAs may be preferred, the certainty that no alternate-induced looping will occur is an advantage of using MRT alternates when the available node-protecting LFA is not a downstream path.
There are three possible types of routers involved in forwarding a packet along an MRT path. At the MRT ingress router, the packet leaves the shortest path to the destination and follows an MRT path to the destination. In a FRR application, the MRT ingress router is the PLR. An MRT transit router takes a packet that arrives already associated with the particular MRT, and forwards it on that same MRT. In some situations (to be discussed later), the packet will need to leave the MRT path and return to the shortest path. This takes place at the MRT egress router. The MRT ingress and egress functionality may depend on the underlying type of packet being forwarded (LDP or IP). The MRT transit functionality is independent of the type of packet being forwarded. We first consider several MRT transit forwarding mechanisms. Then we look at how these forwarding mechanisms can be applied to carrying LDP and IP traffic.
The following options for MRT forwarding mechanisms are considered.
We consider two options for the MRT forwarding mechanisms using MRT LDP labels.
[RFC7307] provides a mechanism to distribute FEC-Label bindings scoped to a given MPLS topology (represented by MPLS MT-ID). To use multi-topology LDP to create MRT forwarding topologies, we associate two MPLS MT-IDs with the MRT-Red and MRT-Blue forwarding topologies, in addition to the default shortest path forwarding topology with MT-ID=0.
With this forwarding mechanism, a single label is distributed for each topology-scoped FEC. For a given FEC in the default topology (call it default-FEC-A), two additional topology-scoped FECs would be created, corresponding to the Red and Blue MRT forwarding topologies (call them red-FEC-A and blue-FEC-A). A router supporting this MRT transit forwarding mechanism advertises a different FEC-label binding for each of the three topology-scoped FECs. When a packet is received with a label corresponding to red-FEC-A (for example), an MRT transit router will determine the next-hop for the MRT-Red forwarding topology for that FEC, swap the incoming label with the outgoing label corresponding to red-FEC-A learned from the MRT-Red next-hop router, and forward the packet.
This forwarding mechanism has the useful property that the FEC associated with the packet is maintained in the labels at each hop along the MRT. We will take advantage of this property when specifying how to carry LDP traffic on MRT paths using multi-topology LDP labels.
This approach is very simple for hardware to support. However, it reduces the label space for other uses, and it increases the memory needed to store the labels and the communication required by LDP to distribute FEC-label bindings.
This forwarding option uses the LDP signaling extensions described in [RFC7307]. The MRT-specific LDP extensions required to support this option are described in [I-D.ietf-mpls-ldp-mrt].
With this forwarding mechanism, a two label stack is used to encode the topology and the FEC of the packet. The top label (topology-id label) identifies the MRT forwarding topology, while the second label (FEC label) identifies the FEC. The top label would be a new FEC type with two values corresponding to MRT Red and Blue topologies.
When an MRT transit router receives a packet with a topology-id label, the router pops the top label and uses that it to guide the next-hop selection in combination with the next label in the stack (the FEC label). The router then swaps the FEC label, using the FEC-label bindings learned through normal LDP mechanisms. The router then pushes the topology-id label for the next-hop.
As with Option 1A, this forwarding mechanism also has the useful property that the FEC associated with the packet is maintained in the labels at each hop along the MRT.
This forwarding mechanism has minimal usage of additional labels, memory and LDP communication. It does increase the size of packets and the complexity of the required label operations and look-ups.
This forwarding option is consistent with context-specific label spaces, as described in [RFC5331]. However, the precise LDP behavior required to support this option for MRT has not been specified.
In principle, MRT transit forwarding mechanisms 1A and 1B can coexist in the same network, with a packet being forwarding along a single MRT path using the single label of option 1A for some hops and the two label stack of option 1B for other hops.
If a router supports a profile that includes the MRT LDP Label option for MRT transit forwarding mechanism, then it MUST support option 1A, which encodes topology-scoped FECs using a single label.
IP tunneling can also be used as an MRT transit forwarding mechanism. Each router supporting this MRT transit forwarding mechanism announces two additional loopback addresses and their associated MRT color. Those addresses are used as destination addresses for MRT-blue and MRT-red IP tunnels respectively. The special loopback addresses allow the transit nodes to identify the traffic as being forwarded along either the MRT-blue or MRT-red topology to reach the tunnel destination. For example, an MRT ingress router can cause a packet to be tunneled along the MRT-red path to router X by encapsulating the packet using the MRT-red loopback address advertised by router X. Upon receiving the packet, router X would remove the encapsulation header and forward the packet based on the original destination address.
Announcements of these two additional loopback addresses per router with their MRT color requires IGP extensions, which have not been defined.
Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the tunneling mechanism.
Note that the two forwarding mechanisms using LDP Label options do not require additional loopbacks per router, as is required by the IP tunneling mechanism. This is because LDP labels are used on a hop-by-hop basis to identify MRT-blue and MRT-red forwarding topologies.
In the previous section, we examined several options for providing MRT transit forwarding functionality, which is independent of the type of traffic being carried. We now look at the MRT ingress functionality, which will depend on the type of traffic being carried (IP or LDP). We start by considering LDP traffic.
We also simplify the initial discussion by assuming that the network consists of a single IGP area, and that all routers in the network participate in MRT. Other deployment scenarios that require MRT egress functionality are considered later in this document.
In principle, it is possible to carry LDP traffic in MRT IP tunnels. However, for LDP traffic, it is desirable to avoid tunneling. Tunneling LDP traffic to a remote node requires knowledge of remote FEC-label bindings so that the LDP traffic can continue to be forwarded properly when it leaves the tunnel. This requires targeted LDP sessions which can add management complexity. As described below, the two MRT forwarding mechanisms that use LDP labels do not require targeted LDP sessions.
The MRT LDP Label option 1A forwarding mechanism uses topology-scoped FECs encoded using a single label as described in section Section 6.1.1.1. When a PLR receives an LDP packet that needs to be forwarded on the Red MRT (for example), it does a label swap operation, replacing the usual LDP label for the FEC with the Red MRT label for that FEC received from the next-hop router in the Red MRT computed by the PLR. When the next-hop router in the Red MRT receives the packet with the Red MRT label for the FEC, the MRT transit forwarding functionality continues as described in Section 6.1.1.1. In this way the original FEC associated with the packet is maintained at each hop along the MRT.
The MRT LDP Label option 1B forwarding mechanism encodes the topology and the FEC using a two label stack as described in Section 6.1.1.2. When a PLR receives an LDP packet that needs to be forwarded on the Red MRT, it first does a normal LDP label swap operation, replacing the incoming normal LDP label associated with a given FEC with the outgoing normal LDP label for that FEC learned from the next-hop on the Red MRT. In addition, the PLR pushes the topology-identification label associated with the Red MRT, and forward the packet to the appropriate next-hop on the Red MRT. When the next-hop router in the Red MRT receives the packet with the Red MRT label for the FEC, the MRT transit forwarding functionality continues as described in Section 6.1.1.2. As with option 1A, the original FEC associated with the packet is maintained at each hop along the MRT.
Note that forwarding LDP traffic using MRT LDP Labels can be done without the use of targeted LDP sessions when an MRT path to the destination FEC is used. The alternates selected in [I-D.ietf-rtgwg-mrt-frr-algorithm] use the MRT path to the destination FEC, so targeted LDP sessions are not needed. If instead one found it desirable to have the PLR use an MRT to reach the primary next-next-hop for the FEC, and then continue forwarding the LDP packet along the shortest path tree from the primary next-next-hop, this would require tunneling to the primary next-next-hop and a targeted LDP session for the PLR to learn the FEC-label binding for primary next-next-hop to correctly forward the packet.
For greatest hardware compatibility, routers implementing MRT fast-reroute of LDP traffic MUST support Option 1A of encoding the MT-ID in the labels (See Section 9).
For IPv4 traffic, there is no currently practical alternative except tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red forwarding topology. For IPv6 traffic, in principle one could define bits in the IPv6 options header to indicate the MRT-Blue or MRT-Red forwarding topology. However, in this document, we have chosen not to define a solution that would work for IPv6 traffic but not for IPv4 traffic.
The choice of tunnel egress MAY be flexible since any router closer to the destination than the next-hop can work. This architecture assumes that the original destination in the area is selected (see Section 11 for handling of multi-homed prefixes); another possible choice is the next-next-hop towards the destination. As discussed in the previous section, for LDP traffic, using the MRT to the original destination simplifies MRT-FRR by avoiding the need for targeted LDP sessions to the next-next-hop. For IP, that consideration doesn't apply. However, consistency with LDP is RECOMMENDED.
Some situations require tunneling IP traffic along an MRT to a tunnel endpoint that is not the destination of the IP traffic. These situations will be discussed in detail later. We note here that an IP packet with a destination in a different IGP area/level from the PLR should be tunneled on the MRT to the ABR/LBR on the shortest path to the destination. For a destination outside of the PLR's MRT Island, the packet should be tunneled on the MRT to a non-proxy-node immediately before the named proxy-node on that particular color MRT.
An IP packet can be tunneled along an MRT path by pushing the appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed to IP headers, has the the advantage that more installed routers can do line-rate encapsulation and decapsulation using LDP than using IP. Also, no additional IP addresses would need to be allocated or signaled.
The MRT LDP Label option 1A forwarding mechanism uses topology-scoped FECs encoded using a single label as described in section Section 6.1.1.1. When a PLR receives an IP packet that needs to be forwarded on the Red MRT to a particular tunnel endpoint, it does a label push operation. The label pushed is the Red MRT label for a FEC originated by the tunnel endpoint, learned from the next-hop on the Red MRT.
The MRT LDP Label option 1B forwarding mechanism encodes the topology and the FEC using a two label stack as described in Section 6.1.1.2. When a PLR receives an IP packet that needs to be forwarded on the Red MRT to a particular tunnel endpoint, the PLR pushes two labels on the IP packet. The first (inner) label is the normal LDP label learned from the next-hop on the Red MRT, associated with a FEC originated by the tunnel endpoint. The second (outer) label is the topology-identification label associated with the Red MRT.
For completeness, we note here a potential variation that uses a single label as opposed to two labels. In order to tunnel an IP packet over an MRT to the destination of the IP packet (as opposed to an arbitrary tunnel endpoint), then we could just push a topology-identification label directly onto the packet. An MRT transit router would need to pop the topology-id label, do an IP route lookup in the context of that topology-id , and push the topology-id label.
In order to tunnel over the MRT to a particular tunnel endpoint, the PLR encapsulates the original IP packet with an additional IP header using the MRT-Blue or MRT-Red loopack address of the tunnel endpoint.
For greatest hardware compatibility and ease in removing the MRT-topology marking at area/level boundaries, routers that support MPLS and implement IP MRT fast-reroute MUST support tunneling of IP traffic using MRT LDP Labels Option 1A (topology-scoped FEC encoded using a single label).
The purpose of communicating support for MRT in the IGP is to indicate that the MRT-Blue and MRT-Red forwarding topologies are created for transit traffic. The MRT architecture allows for different, potentially incompatible options. In order to create constistent MRT forwarding topologies, the routers participating in a particular MRT Island need to use the same set of options. These options are grouped into MRT profiles. In addition, the routers in an MRT Island all need to use the same set of nodes and links within the Island when computing the MRT forwarding topologies. This section describes the information used by a router to determine the nodes and links to include in a particular MRT Island. Some of this information is shared among routers using the newly-defined IGP signaling extensions for MRT described in [I-D.ietf-ospf-mrt] and [I-D.ietf-isis-mrt]. Other information already exists in the IGPs and can be used by MRT in Island formation, subject to the interpretation defined here.
Deployment scenarios using multi-topology OSPF or IS-IS, or running both ISIS and OSPF on the same routers is out of scope for this specification. As with LFA, it is expected that OSPF Virtual Links will not be supported.
All links in an MRT Island MUST be bidirectional and belong to the same IGP area or level. For ISIS, a link belonging to both level 1 and level 2 would qualify to be in multiple MRT Islands. A given ABR or LBR can belong to multiple MRT Islands, corresponding to the areas or levels in which it participates. Inter-area forwarding behavior is discussed in Section 10.
All routers in an MRT Island MUST support the same MRT profile. A router advertises support for a given MRT profile using the IGP extensions defined in [I-D.ietf-ospf-mrt] and [I-D.ietf-isis-mrt] using an 8-bit Profile ID value. A given router can support multiple MRT profiles and participate in multiple MRT Islands. The options that make up an MRT profile, as well as the default MRT profile, are defined in Section 8.
MRT takes into account existing IGP mechanisms for discouraging traffic from using particular links and routers, and it introduces an MRT-specific exclusion mechanism for links.
Mechanisms for discouraging traffic from using particular links already exist in ISIS and OSPF. In ISIS, an interface configured with a metric of 2^24-2 (0xFFFFFE) will only be used as a last resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF) will not be advertised into the topology.) In OSPF, an interface configured with a metric of 2^16-1 (0xFFFF) will only be used as a last resort. These metrics can be configured manually to enforce administrative policy, or they can be set in an automated manner as with LDP IGP synchronization [RFC5443].
Mechanisms also exist in ISIS and OSPF to prevent transit traffic from using a particular router. In ISIS, the overload bit is used for this purpose. In OSPF, [RFC6987] specifies setting all outgoing interface metrics to 0xFFFF to accomplish this.
The following rules for MRT Island formation ensure that MRT FRR protection traffic does not use a link or router that is discouraged from carrying traffic by existing IGP mechanisms.
This architecture also defines a means of excluding an otherwise usable link from MRT Islands. [I-D.ietf-ospf-mrt] and [I-D.ietf-isis-mrt] define the IGP extensions for OSPF and ISIS used to advertise that a link is MRT-Ineligible. A link with either interface advertised as MRT-Ineligible MUST be excluded from an MRT Island. Note that an interface advertised as MRT-Ineligigle by a router is ineligible with respect to all profiles advertised by that router.
All of the routers in an MRT Island MUST be connected by bidirectional links with other routers in the MRT Island. Disconnected MRT Islands will operate independently of one another.
An algorithm that allows a computing router to identify the routers and links in the local MRT Island satisfying the above rules is given in section 5.2 of [I-D.ietf-rtgwg-mrt-frr-algorithm].
An MRT Profile is a set of values and options related to MRT behavior. The complete set of options is designated by the corresponding 8-bit Profile ID value.
This document specifies the values and options that correspond to the Default MRT Profile (Profile ID = 0). Future documents may define other MRT Profiles by specifying the MRT Profile Options below.
Below is a description of the values and options that define an MRT Profile.
If a router advertises support for multiple MRT profiles, then it MUST create the transit forwarding topologies for each of those, unless the profile specifies the None option for MRT Forwarding Mechanism. A router MUST NOT advertise multiple MRT profiles that overlap in their MRT-Red MT-ID or MRT-Blue MT-ID.
For some profiles, additional router-specific MRT parameters may need to be distributed via the IGP. While the set of options indicated by the MRT Profile ID must be identical for all routers in an MRT Island, these router-specific MRT parameters may differ between routers in the same MRT island. Several such parameters are described below.
The extensions to OSPF and ISIS for advertising a router's GADAG Root Selection Priority value are defined in [I-D.ietf-ospf-mrt] and [I-D.ietf-isis-mrt]. IGP extensions for the advertising a router's MRT-Red and MRT-Blue Loopback Addresses have not been defined.
The following set of options defines the default MRT Profile. The default MRT profile is indicated by the MRT Profile ID value of 0.
The protocol extensions for LDP are defined in [I-D.ietf-mpls-ldp-mrt]. A router must indicate that it has the ability to support MRT; having this explicit allows the use of MRT-specific processing, such as special handling of FECs sent with the Rainbow MRT MT-ID.
A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles. The FEC-label bindings for the default shortest-path based MT-ID 0 MUST still be sent (even though it could be inferred from the Rainbow FEC-label bindings) to ensure continuous operation of normal LDP forwarding. The Rainbow MRT MT-ID is defined to provide an easy way to handle the special signaling that is needed at ABRs or LBRs. It avoids the problem of needing to signal different MPLS labels to different LDP neighbors for the same FEC. Because the Rainbow MRT MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not MRT profile specific.
[I-D.ietf-mpls-ldp-mrt] contains the IANA request for the Rainbow MRT MPLS MT-ID.
Unless otherwise specified, in this section we will use the terms area and ABR to indicate either an OSPF area and OSPF ABR or ISIS level and ISIS LBR.
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 failures that might appear in multiple areas (e.g. ABR/LBR failures) to be separately identified and repaired around. Third, the packet can be fast-rerouted again, if necessary, due to a failure in a different area.
An ABR/LBR 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 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.
To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the ABR/LBR needs to arrange that packets destined to a different area arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue.
For LDP forwarding where a single label specifies (MT-ID, FEC), the ABR/LBR is responsible for advertising the proper label to each neighbor. Assume that an ABR/LBR has allocated three labels for a particular destination; those labels are L_primary, L_blue, and L_red. To those routers in the same area as the best route to the destination, the ABR/LBR 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/LBR advertises the 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/LBR 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/LBR 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/LBR 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/LBR can thus enforce that packets exiting the area do so on the shortest-path default topology.
The desired forwarding behavior could be achieved in the above example without using the Rainbow-FEC. This could be done by having the ABR/LBR 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 implmentations and doesn't have other obvious uses.
Many existing LDP implmentations do however have the ability to filter FEC-label binding advertisements on a per-neighbor basis. The Rainbow-FEC allows us to re-use 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.
The use of the Rainbow-FEC by the ABR for non-best-area advertisements is RECOMMENDED. An ABR MAY advertise the label for the default topology in separate MRT-Blue and MRT-Red advertisements. However, a router that supports the LDP Label MRT Forwarding Mechanism MUST be able to receive and correctly interpret the Rainbow-FEC.
If IP tunneling is used, then the ABR/LBR behavior is dependent upon the outermost IP address. If the outermost IP address is an MRT loopback address of the ABR/LBR, 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/LBR, then the packet is simply forwarded along the associated 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/LBR 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.
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/LBR would need to determine whether the ABR/LBR would forward the packet out of the area/level. If so, then that router should pop off the topology-identification label before forwarding the packet to the ABR/LBR.
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-Identification 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 ISIS 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 OPSF area ID) and the Area ID isn't announced as part of the ASBR link-state advertisement (LSA). In this one case, suboptimal forwarding along the MRT in the other area would happen. If that becomes a realistic deployment scenario, OSPF extensions could be considered. This is not covered in [I-D.ietf-ospf-mrt].
+----[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) Blue MRT in Area 0 (e) Red MRT in Area 0
Figure 3: ABR Forwarding Behavior and MRTs
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 multi-homed 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. The key difference is whether the traffic, once out of the MRT Island, remains in the same area/level and 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 multi-homed 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 multi-homed prefix could be:
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, that 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.
For IP traffic, multi-homed 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, 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 multi-homed 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.
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].
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)
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).
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, tunnelling of LDP traffic requires targeted LDP sessions for learning the FEC-label binding at the tunnel endpoint.
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.
A named proxy-node represents one or more destinations and, for LDP forwarding, has a FEC associated with it that is signalled 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 are not defined in [I-D.ietf-ospf-mrt]. The details of what label-bindings must be originated are described in [I-D.ietf-mpls-ldp-mrt].
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 [I-D.ietf-rtgwg-mrt-frr-algorithm]. 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 2 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 are 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 are 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 for identifying loop-free Island Neighbors(LFINs) is given in [I-D.ietf-rtgwg-mrt-frr-algorithm]. 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.
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 6 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 multi-homed 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.
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 re-calculation.
As is well known[RFC5715], micro-loops can occur during IGP convergence; such loops can be local to the failure or remote from the failure. Managing micro-loops is an orthogonal issue to having alternates for local repair, such as MRT fast-reroute provides.
There are two possible micro-loop prevention mechanisms discussed in [RFC5715]. The first is Ordered FIB [RFC6976]. The second is Farside Tunneling which requires tunnels or an alternate topology to reach routers on the farside of the failure.
Since MRTs provide an alternate topology through which traffic can be sent and which can be manipulated separately from the SPT, it is possible that MRTs could be used to support Farside Tunneling. Details of how to do so are outside the scope of this document.
Micro-loop mitigation mechanisms can also work when combined with MRT.
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 shortest path tree (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:
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.
[RFC Editor: please remove this section prior to publication.]
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC6982]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.
According to [RFC6982], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".
Juniper Networks Implementation
Huawei Technology Implementation
An implementation SHOULD provide an operator with the ability to test MRT paths with 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.
For a given topology, GADAG root, and profile, MRT will provide a node-protecting alternate path from each PLR to each destination for any single link or node failure, if such a path exists. Therefore, an implementation may choose to only use the alternates determined by MRT to provide 100% FRR coverage.
However, it may be desirable to allow an operator to use MRT 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 an example, it may be desirable to implement a policy where a node-protecting LFA (if it exists for a given failure mode and destination) is preferred over a given MRT alternate. [I-D.ietf-rtgwg-lfa-manageability] discusses many of the potential criteria that one might take into account when evaluating different alternates for selection.
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.
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, and Janos Farkas for their suggestions and review.
Please create an MRT Profile registry for the MRT Profile TLV. The range is 0 to 255. The default MRT Profile has value 0. Values 1-200 are by Standards Action. Values 201-220 are for experimentation. Values 221-255 are for vendor private use.
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.
Robert Kebler Juniper Networks 10 Technology Park Drive Westford, MA 01886 USA 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 USA Email: jeff.tantsura@ericsson.com Russ White VCE Email: russw@riw.us
| [I-D.ietf-rtgwg-mrt-frr-algorithm] | Envedi, G., Csaszar, A., Atlas, A., Bowers, C. and A. Gopalan, "Algorithms for computing Maximally Redundant Trees for IP/LDP Fast- Reroute", Internet-Draft draft-ietf-rtgwg-mrt-frr-algorithm-06, October 2015. |
| [RFC5286] | Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, DOI 10.17487/RFC5286, September 2008. |
When a multi-homed 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.