Internet Engineering Task Force (IETF) A. Atlas Request for Comments: 7812 C. Bowers Category: Standards Track Juniper Networks ISSN: 2070-1721 G. Enyedi Ericsson June 2016 An Architecture for IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-FRR)
AbstractThis document defines the architecture for IP and 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. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7812. Copyright Notice Copyright (c) 2016 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.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4 1.2. Partial Deployment and Backwards Compatibility . . . . . 5 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7 5. MRT and Fast Reroute . . . . . . . . . . . . . . . . . . . . 9 6. Unicast Forwarding with MRT Fast Reroute . . . . . . . . . . 9 6.1. Introduction to MRT Forwarding Options . . . . . . . . . 10 6.1.1. MRT LDP Labels . . . . . . . . . . . . . . . . . . . 10 18.104.22.168. Topology-Scoped FEC Encoded Using a Single Label (Option 1A) . . . . . . . . . . . . . . . . . . . 10 22.214.171.124. Topology and FEC Encoded Using a Two-Label Stack (Option 1B) . . . . . . . . . . . . . . . . . . . 11 126.96.36.199. Compatibility of MRT LDP Label Options 1A and 1B 12 188.8.131.52. Required Support for MRT LDP Label Options . . . 12 6.1.2. MRT IP Tunnels (Options 2A and 2B) . . . . . . . . . 12 6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 13 6.2.1. Forwarding LDP Traffic Using MRT LDP Label Option 1A 13 6.2.2. Forwarding LDP Traffic Using MRT LDP Label Option 1B 14 6.2.3. Other Considerations for Forwarding LDP Traffic Using MRT LDP Labels . . . . . . . . . . . . . . . . . . . 14 6.2.4. Required Support for LDP Traffic . . . . . . . . . . 14 6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 14 6.3.1. Tunneling IP Traffic Using MRT LDP Labels . . . . . . 15 184.108.40.206. Tunneling IP Traffic Using MRT LDP Label Option 1A . . . . . . . . . . . . . . . . . . . . . . . 15 220.127.116.11. Tunneling IP Traffic Using MRT LDP Label Option 1B . . . . . . . . . . . . . . . . . . . . . . . 15 6.3.2. Tunneling IP Traffic Using MRT IP Tunnels . . . . . . 16 6.3.3. Required Support for IP Traffic . . . . . . . . . . . 16 7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 16 7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17 7.2. Support for a Specific MRT Profile . . . . . . . . . . . 17 7.3. Excluding Additional Routers and Interfaces from the MRT Island . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.3.1. Existing IGP Exclusion Mechanisms . . . . . . . . . . 18 7.3.2. MRT-Specific Exclusion Mechanism . . . . . . . . . . 19 7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19 7.5. Algorithm for MRT Island Identification . . . . . . . . . 19 8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19 8.2. Router-Specific MRT Parameters . . . . . . . . . . . . . 21 8.3. Default MRT Profile . . . . . . . . . . . . . . . . . . . 21 9. LDP Signaling Extensions and Considerations . . . . . . . . . 22
10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 23 10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23 10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 24 10.2. ABR Forwarding Behavior with IP Tunneling (Option 2) . . 24 10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B . . 25 11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26 11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint Selection . . . . . . . . . . . . . . . . . . . . . . . 28 11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes . 29 11.3. MRT Alternates for Destinations outside the MRT Island . 31 12. Network Convergence and Preparing for the Next Failure . . . 32 12.1. Micro-loop Prevention and MRTs . . . . . . . . . . . . . 32 12.2. MRT Recalculation for the Default MRT Profile . . . . . 33 13. Operational Considerations . . . . . . . . . . . . . . . . . 34 13.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 34 13.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 34 13.3. MRT IP Tunnel Loopback Address Management . . . . . . . 36 13.4. MRT-FRR in a Network with Degraded Connectivity . . . . 36 13.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 37 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 15. Security Considerations . . . . . . . . . . . . . . . . . . . 38 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 38 16.1. Normative References . . . . . . . . . . . . . . . . . . 38 16.2. Informative References . . . . . . . . . . . . . . . . . 39 Appendix A. Inter-level Forwarding Behavior for IS-IS . . . . . 41 Appendix B. General Issues with Area Abstraction . . . . . . . . 42 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 43 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44 RFC5714]. MRT-FRR creates two alternate forwarding trees that are distinct 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. [RFC7811] describes how to compute maximally redundant trees using a specific algorithm: the MRT Lowpoint algorithm. The MRT Lowpoint algorithm is used by a router that supports the Default MRT Profile, as specified in this document. IP/LDP Fast Reroute using Maximally Redundant Trees (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 the MRTs by one Point of Local Repair (PLR), that traffic is not subject to further repair actions by another PLR, even in the event of multiple simultaneous failures. Therefore, traffic repaired by MRT-FRR will not loop between different PLRs responding to different simultaneous failures. While MRT provides 100% protection for a single link or node failure, it may not protect traffic in the event of multiple simultaneous failures, nor does it take into account Shared Risk Link Groups (SRLGs). Also, while the MRT Lowpoint algorithm is computationally efficient, it is also new. In order for MRT-FRR to function properly, all of the other nodes in the network that support MRT must correctly compute next hops based on the same algorithm and install the corresponding forwarding state. This is in contrast to other FRR methods where the calculation of backup paths generally involves repeated application of the simpler and widely deployed Shortest Path First (SPF) algorithm, and backup paths themselves reuse the forwarding state used for shortest path forwarding of normal traffic. Section 13 provides operational guidance related to verification of MRT forwarding paths. 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 [MRT-ARCH]. However, the current document does not address the multicast applications of MRTs.
When a network needs to use Ordered FIB [RFC6976] or Nearside Tunneling [RFC5715] as a micro-loop prevention mechanism [RFC5715], then the whole IGP area needs to have alternates available. This allows the micro-loop prevention mechanism, which requires slower network convergence, to 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. See Section 12.1 for more discussion of micro- loop prevention and MRTs. Section 7, and partial deployment is discussed in more detail in Section 13.5. RFC2119].
Redundant Trees (RT): A pair of trees where the path from any node X to the root R along the first tree is node-disjoint with the path from the same node X to the root along the second tree. Redundant trees can always be computed in 2-connected graphs. Maximally Redundant Trees (MRT): A pair of trees where the path from any node X to the root R along the first tree and the path from the same node X to the root along the second tree share the minimum number of nodes and the minimum number of links. Each such shared node is a cut-vertex. Any shared links are cut-links. In graphs that are not 2-connected, it is not possible to compute RTs. However, it is possible to compute MRTs. MRTs are maximally redundant in the sense that they are as redundant as possible given the constraints of the network graph. Directed Acyclic Graph (DAG): A graph where all links are directed and there are no cycles in it. Almost Directed Acyclic Graph (ADAG): A graph with one node designated as the root. The graph has the property that if all links incoming to the root were removed, then the resulting graph would be a DAG. Generalized ADAG (GADAG): A graph that is the combination of the ADAGs of all blocks. MRT-Red: MRT-Red is used to describe one of the two MRTs; it is used to describe the associated forwarding topology and MPLS Multi-Topology IDentifier (MT-ID). Specifically, MRT-Red is the decreasing MRT where links in the GADAG are taken in the direction from a higher topologically ordered node to a lower one. MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is used to described the associated forwarding topology and MPLS MT-ID. Specifically, MRT-Blue is the increasing MRT where links in the GADAG are taken in the direction from a lower topologically ordered node to a higher one. Rainbow MRT: It is useful to have an MPLS MT-ID that refers to the multiple MRT forwarding topologies and to the default forwarding topology. This is referred to as the Rainbow MRT MPLS MT-ID and is used by LDP to reduce signaling and permit the same label to always be advertised to all peers for the same (MT-ID, Prefix). MRT Island: The set of routers that support a particular MRT profile and the links connecting them that support MRT.
Island Border Router (IBR): A router in the MRT Island that is connected to a router not in the MRT Island, both of which are in a common area or level. Island Neighbor (IN): A router that is not in the MRT Island but is adjacent to an IBR and in the same area/level as the IBR. named proxy-node: A proxy-node can represent a destination prefix that can be attached to the MRT Island via at least two routers. It is named 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 (Forwarding Equivalence Class) for the associated prefix or because MRT-Red and MRT-Blue IP addresses are advertised in an undefined fashion for that proxy-node. RFC7811]. This algorithm can be computed in O(e + n log n); it is less than three SPFs. 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 C, G, or the link C<->G failed, then the network would be partitioned. It is clearly impossible to have two link-disjoint or node-disjoint paths from G, J, or H 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->C->D->E->R and on the Red MRT is G->C->B->A->R. [E]---[D]---| |---[J] | | | | | | | | | | [R] [F] [C]---[G] | | | | | | | | | | | [A]---[B]---| |---[H] (a) a graph that is not 2-connected [E]<--[D]<--| [J] [E]-->[D]---| |---[J] | ^ | | | | | ^ V | | | V V V | [R] [F] [C]<--[G] | [R] [F] [C]<--[G] | ^ ^ ^ | ^ | | | | | | V | V | | [A]-->[B]---| |---[H] [A]<--[B]<--| [H] (b) Blue MRT towards R (c) Red MRT towards R Figure 2: A Network That Is Not 2-Connected
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, B would normally forward traffic to R across the path B->A->R. If the B<->A link fails, then B could use the MRT-Blue path B->F->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. [RFC7811] 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, an LFA, 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 looping among alternates. 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.
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. 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. In general, this approach will also increase the time needed to install the FRR entries in the Forwarding Information Base (FIB) and, hence, the time needed before the next failure can be protected. This forwarding option uses the LDP signaling extensions described in [RFC7307]. The MRT-specific LDP extensions required to support this option will be described elsewhere.
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.
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. 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. Section 18.104.22.168. When a PLR receives an LDP packet that needs to be forwarded on the MRT-Red (for example), it does a label swap operation, replacing the usual LDP label for the FEC with the MRT-Red label for that FEC received from the next-hop router in the MRT-Red computed by the PLR. When the next-hop router in the MRT-Red receives the packet with the
MRT-Red label for the FEC, the MRT transit forwarding functionality continues as described in Section 22.214.171.124. In this way, the original FEC associated with the packet is maintained at each hop along the MRT. Section 126.96.36.199. When a PLR receives an LDP packet that needs to be forwarded on the MRT-Red, 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 MRT-Red. In addition, the PLR pushes the topology-id label associated with the MRT-Red, and forward the packet to the appropriate next hop on the MRT-Red. When the next-hop router in the MRT-Red receives the packet with the MRT-Red label for the FEC, the MRT transit forwarding functionality continues as described in Section 188.8.131.52. As with Option 1A, the original FEC associated with the packet is maintained at each hop along the MRT. RFC7811] 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 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. Section 9).
chosen not to define a solution that would work for IPv6 traffic but not for IPv4 traffic. The choice of tunnel egress is 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 multihomed 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. 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 Area Border Router (ABR) or Level Border Router (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. Section 184.108.40.206. When a PLR receives an IP packet that needs to be forwarded on the MRT-Red to a particular tunnel endpoint, it does a label push operation. The label pushed is the MRT-Red label for a FEC originated by the tunnel endpoint, learned from the next hop on the MRT-Red. Section 220.127.116.11. When a PLR receives an IP packet that needs to be forwarded on the MRT-Red 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 MRT-Red, associated with a FEC originated by the tunnel endpoint. The second (outer) label is the topology-id label associated with the MRT-Red. 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, one could just push a topology-id 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 label, and push the topology-id label.
defining extensions to existing IGPs to carry this information makes sense. These new protocol extensions will be defined elsewhere. Deployment scenarios using multi-topology OSPF or IS-IS, or running both IS-IS and OSPF on the same routers is out of scope for this specification. As with LFA, MRT-FRR does not support OSPF Virtual Links. At a high level, an MRT Island is defined as the set of routers supporting the same MRT profile, in the same IGP area/level and with bidirectional links interconnecting those routers. More detailed descriptions of these criteria are given below. Section 10. Section 8. The process of MRT Island formation takes place independently for each MRT profile advertised by a given router. For example, consider a network with 40 connected routers in the same area advertising support for MRT Profile A and MRT Profile B. Two distinct MRT Islands will be formed corresponding to Profile A and Profile B, with each island containing all 40 routers. A complete set of maximally redundant trees will be computed for each island following the rules defined for each profile. If we add a third MRT Profile to this example, with Profile C being advertised by a connected subset of 30 routers, there will be a third MRT Island formed corresponding to those 30 routers, and a third set of maximally redundant trees will be computed. In this example, 40 routers would compute and install two sets of MRT transit forwarding entries corresponding to Profiles A and B, while 30 routers would compute and install three sets of MRT transit forwarding entries corresponding to Profiles A, B, and C.
RFC5443]. Mechanisms also already exist in IS-IS and OSPF to discourage or prevent transit traffic from using a particular router. In IS-IS, the overload bit is prevents transit traffic from using a router. For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing interface metrics to 0xFFFF to discourage transit traffic from using a router. ([RFC6987] defines the metric value 0xFFFF as MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3, [RFC5340] specifies that a router be excluded from the intra-area SPT computation if the V6-bit or R-bit of the Link State Advertisement (LSA) options is not set in the Router LSA. The following rules for MRT Island formation ensure that MRT FRR protection traffic does not use a link or router that is discouraged or prevented from carrying traffic by existing IGP mechanisms. 1. A bidirectional link MUST be excluded from an MRT Island if either the forward or reverse cost on the link is 0xFFFFFE (for IS-IS) or 0xFFFF for OSPF. 2. A router MUST be excluded from an MRT Island if it is advertised with the overload bit set (for IS-IS), or it is advertised with metric values of 0xFFFF on all of its outgoing interfaces (for OSPFv2 and OSPFv3). 3. A router MUST be excluded from an MRT Island if it is advertised with either the V6-bit or R-bit of the LSA options not set in the Router LSA.
Section 5.2 of [RFC7811].
GADAG Root Selection Policy: This specifies the manner in which the GADAG root is selected. All routers in the MRT Island need to use the same GADAG root in the calculations used construct the MRTs. A valid GADAG Root Selection Policy MUST be such that each router in the MRT Island chooses the same GADAG root based on information available to all routers in the MRT Island. GADAG Root Selection Priority values, advertised as router-specific MRT parameters, MAY be used in a GADAG Root Selection Policy. MRT Forwarding Mechanism: This specifies which forwarding mechanism the router uses to carry transit traffic along MRT paths. A router that supports a specific MRT forwarding mechanism must program appropriate next hops into the forwarding plane. The current options are MRT LDP Label Option 1A, MRT LDP Label Option 1B, IPv4 Tunneling, IPv6 Tunneling, and None. If IPv4 is supported, then both MRT-Red and MRT-Blue IPv4 loopback addresses SHOULD be specified. If IPv6 is supported, both MRT-Red and MRT- Blue IPv6 loopback addresses SHOULD be specified. Recalculation: Recalculation specifies the process and timing by which new MRTs are computed after the topology has been modified. Area/Level Border Behavior: This specifies how traffic traveling on the MRT-Blue or MRT-Red in one area should be treated when it passes into another area. Other Profile-Specific Behavior: Depending upon the use-case for the profile, there may be additional profile-specific behavior. When a new MRT Profile is defined, new and unique values should be allocated from the "MPLS Multi-Topology Identifiers Registry", corresponding to the MRT-Red and MRT-Blue MT-ID values for the new 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 the MRT Forwarding Mechanism. The ability of MRT-FRR to support transit forwarding entries for multiple profiles can be used to facilitate a smooth transition from an existing deployed MRT Profile to a new MRT Profile. The new profile can be activated in parallel with the existing profile, installing the transit forwarding entries for the new profile without affecting the transit forwarding entries for the existing profile. Once the new transit forwarding state has been verified, the router can be configured to use the alternates computed by the new profile in the event of a failure.
RFC7811]. MRT-Red MPLS MT-ID: This temporary registration has been allocated from the "MPLS Multi-Topology Identifiers" registry. The registration request appears in [LDP-MRT]. MRT-Blue MPLS MT-ID: This temporary registration has been allocated from the "MPLS Multi-Topology Identifiers" registry. The registration request appears in [LDP-MRT].
GADAG Root Selection Policy: Among the routers in the MRT Island with the lowest numerical value advertised for GADAG Root Selection Priority, an implementation MUST pick the router with the highest Router ID to be the GADAG root. Note that a lower numerical value for GADAG Root Selection Priority indicates a higher preference for selection. Forwarding Mechanisms: MRT LDP Label Option 1A Recalculation: Recalculation of MRTs SHOULD occur as described in Section 12.2. This allows the MRT forwarding topologies to support IP/LDP fast-reroute traffic. Area/Level Border Behavior: As described in Section 10, ABRs/LBRs SHOULD ensure that traffic leaving the area also exits the MRT-Red or MRT-Blue forwarding topology. LDP-MRT].