Routing Area Working Group A. Atlas, Ed.
Internet-Draft R. Kebler
Intended status: Standards Track C. Bowers
Expires: July 23, 2015 Juniper Networks
G. Enyedi
A. Csaszar
J. Tantsura
Ericsson
R. White
VCE
January 19, 2015
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-05
Abstract
With increasing deployment of Loop-Free Alternates (LFA) [RFC5286],
it is clear that a complete solution for IP and LDP Fast-Reroute is
required. This specification provides that solution. IP/LDP Fast-
Reroute with Maximally Redundant Trees (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 all 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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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 July 23, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 5
1.2. Partial Deployment and Backwards Compatibility . . . . . 6
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 8
5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 10
6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 11
6.1. MRT Forwarding Mechanisms . . . . . . . . . . . . . . . . 11
6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 11
6.1.1.1. Topology-scoped FEC encoded using a single label
(Option 1A) . . . . . . . . . . . . . . . . . . . 12
6.1.1.2. Topology and FEC encoded using a two label stack
(Option 1B) . . . . . . . . . . . . . . . . . . . 12
6.1.1.3. Compatibility of Option 1A and 1B . . . . . . . . 13
6.1.1.4. Mandatory support for MRT LDP Label option 1A . . 13
6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 13
6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 14
6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option
1A) . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option
1B) . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2.3. Other considerations for forwarding LDP traffic using
MRT LDP Labels . . . . . . . . . . . . . . . . . . . 15
6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 15
6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 16
6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option
1A) . . . . . . . . . . . . . . . . . . . . . . . 16
6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option
1B) . . . . . . . . . . . . . . . . . . . . . . . 16
6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 17
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6.3.3. Required support . . . . . . . . . . . . . . . . . . 17
7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 17
7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17
7.2. Support for a specific MRT profile . . . . . . . . . . . 18
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. Example algorithm . . . . . . . . . . . . . . . . . . . . 19
8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19
8.2. Router-specific MRT paramaters . . . . . . . . . . . . . 20
8.3. Default MRT profile . . . . . . . . . . . . . . . . . . . 21
9. LDP signaling extensions and considerations . . . . . . . . . 22
10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 22
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23
10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 23
10.2. ABR Forwarding Behavior with IP Tunneling (option 2) . . 24
10.3. ABR Forwarding Behavior with LDP Label option 1B . . . . 24
11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26
11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint
Selection . . . . . . . . . . . . . . . . . . . . . . . 28
11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 29
11.2.1. Computing if an Island Neighbor (IN) is loop-free . 31
11.3. MRT Alternates for Destinations Outside the MRT Island . 32
12. Network Convergence and Preparing for the Next Failure . . . 33
12.1. Micro-forwarding loop prevention and MRTs . . . . . . . 33
12.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . 33
13. Implementation Status . . . . . . . . . . . . . . . . . . . . 34
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
16. Security Considerations . . . . . . . . . . . . . . . . . . . 36
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
17.1. Normative References . . . . . . . . . . . . . . . . . . 36
17.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. General Issues with Area Abstraction . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
This document gives a complete 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
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protocol extensions are defined in [I-D.atlas-ospf-mrt] and
[I-D.atlas-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 | Computation (in SPFs) |
| | | Looping? | |
+---------+-------------+-------------+-----------------------------+
| MRT-FRR | 100% | None | less than 3 |
| | Link/Node | | |
| | | | |
| LFA | Partial | Possible | per neighbor |
| | Link/Node | | |
| | | | |
| Remote | Partial | Possible | per neighbor (link) or |
| LFA | Link/Node | | neighbor's neighbor (node) |
| | | | |
| Not-Via | 100% | None | per link and node |
| | Link/Node | | |
+---------+-------------+-------------+-----------------------------+
Table 1
Loop-Free Alternates (LFA): LFAs [RFC5286] provide limited
topology-dependent coverage for link and node protection.
Restrictions on choice of alternates can be relaxed to improve
coverage, but this can cause forwarding loops if a worse failure
is experienced than protected against. Augmenting a network to
provide better coverage is NP-hard [LFARevisited]. [RFC6571]
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discusses the applicability of LFA to different topologies with a
focus on common PoP architectures.
Remote LFA: Remote LFAs [I-D.ietf-rtgwg-remote-lfa] improve
coverage over LFAs for link protection but still cannot guarantee
complete coverage. The trade-off of looping traffic to improve
coverage is still made. Remote LFAs can provide node-protection
[I-D.psarkar-rtgwg-rlfa-node-protection] but not guaranteed
coverage and the computation required is quite high (an SPF for
each PQ-node evaluated). [I-D.bryant-ipfrr-tunnels] describes
additional mechanisms to further improve coverage, at the cost of
added complexity.
Not-Via: Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is the
only other solution that provides 100% coverage for link and node
failures and does not have potential looping. However, the
computation is very high (an SPF per failure point) and academic
implementations [LightweightNotVia] have found the address
management complexity to be high.
1.1. Importance of 100% Coverage
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
[RFC3137] that the link metric be raised to the maximum cost; this
may not be symmetric and for [RFC3137] is not expected to be. Third,
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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[I-D.ietf-rtgwg-ordered-fib] 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.
1.2. Partial Deployment and Backwards Compatibility
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.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]
3. Terminology
network graph: A graph that reflects the network topology where all
links connect exactly two nodes and broadcast links have been
transformed into the standard pseudo-node representation.
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.
These can 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.
Any RT is an MRT but many MRTs are not RTs.
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MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and 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 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 MT-ID that refers to the
multiple MRT topologies and to the default topology. This is
referred to as the Rainbow MRT 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 and both routers 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.
cut-link: A link whose removal partitions the network. A cut-link
by definition must be connected between two cut-vertices. If
there are multiple parallel links, then they are referred to as
cut-links in this document if removing the set of parallel links
would partition the network graph.
cut-vertex: A vertex whose removal partitions the network graph.
2-connected: A graph that has no cut-vertices. This is a graph
that requires two nodes to be removed before the network is
partitioned.
2-connected cluster: A maximal set of nodes that are 2-connected.
2-edge-connected: A network graph where at least two links must be
removed to partition the network.
block: Either a 2-connected cluster, a cut-edge, or an isolated
vertex.
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DAG: Directed Acyclic Graph - a graph where all links are directed
and there are no cycles in it.
ADAG: Almost Directed Acyclic Graph - a graph that, if all links
incoming to the root were removed, would be a DAG.
GADAG: Generalized ADAG - a graph that is the combination of the
ADAGs of all blocks.
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 for the associated prefix or because MRT-Red and MRT-
Blue IP addresses are advertised in an undefined fashion for that
proxy-node.
4. Maximally Redundant Trees (MRT)
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.
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[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
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5. Maximally Redundant Trees (MRT) and Fast-Reroute
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. [I-D.ietf-rtgwg-ipfrr-notvia-addresses] or
[I-D.ietf-rtgwg-remote-lfa]), 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.
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6. Unicast Forwarding with MRT Fast-Reroute
As mentioned before, MRT FRR needs multi-topology forwarding.
Unfortunately, neither IP nor LDP provides extra bits for a packet to
indicate its topology. Once the MRTs are computed, the two sets of
MRTs can be used as two additional forwarding topologies. The same
considerations apply for forwarding along the MRTs as for handling
multiple topologies.
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.
6.1. MRT Forwarding Mechanisms
The following options for MRT forwarding mechanisms are considered.
1. MRT LDP Labels
A. Topology-scoped FEC encoded using a single label
B. Topology and FEC encoded using a two label stack
2. MRT IP Tunnels
A. MRT IPv4 Tunnels
B. MRT IPv6 Tunnels
6.1.1. MRT LDP labels
We consider two options for the MRT forwarding mechanisms using MRT
LDP labels.
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6.1.1.1. Topology-scoped FEC encoded using a single label (Option 1A)
[I-D.ietf-mpls-ldp-multi-topology] provides a mechanism to distribute
FEC-Label bindings scoped to a given topology (represented by MT-ID).
To use multi-topology LDP to create MRT forwarding topologies, we
associate two 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
[I-D.ietf-mpls-ldp-multi-topology]. The MRT-specific LDP extensions
required to support this option are described in
[I-D.atlas-mpls-ldp-mrt].
6.1.1.2. Topology and FEC encoded using a two label stack (Option 1B)
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
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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 [RFC 5331]. However, the precise LDP
behavior required to support this option for MRT has not been
specified.
6.1.1.3. Compatibility of Option 1A and 1B
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.
6.1.1.4. Mandatory support for MRT LDP Label option 1A
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.
6.1.2. MRT IP tunnels (Options 2A and 2B)
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. 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.
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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.
6.2. Forwarding LDP Unicast Traffic over MRT Paths
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 very 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. The two MRT LDP
Label forwarding mechanisms have the useful property that the FEC
associated with the packet is maintained in the labels at each hop
along the MRT, as long as an MRT to the originator of the FEC is
used. The MRT IP tunneling mechanism does not have this useful
property. Therefore, this document only considers the two MRT LDP
Label forwarding mechanisms for protecting LDP traffic with MRT fast-
reroute.
6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option 1A)
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.
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6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option 1B)
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.
6.2.3. Other considerations for forwarding LDP traffic using MRT LDP
Labels
Note that forwarding LDP traffic using MRT LDP Labels requires that
an MRT to the originator of the FEC be used. For example, one might
find 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.
However, 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).
6.3. Forwarding IP Unicast Traffic over MRT Paths
For IP traffic, there is no currently practical alternative except
tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
forwarding topology. 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.
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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.
6.3.1. Tunneling IP traffic using MRT LDP Labels
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.
6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option 1A)
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.
6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option 1B)
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 optimization. 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.
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6.3.2. Tunneling IP traffic using MRT IP Tunnels
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.
6.3.3. Required support
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).
7. MRT Island Formation
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.atlas-ospf-mrt] and
[I-D.li-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.
7.1. IGP Area or Level
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.
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7.2. Support for a specific MRT profile
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.atlas-ospf-mrt] and [I-D.li-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.
7.3. Excluding additional routers and interfaces from the MRT Island
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.
7.3.1. Existing IGP exclusion mechanisms
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, [RFC3137] 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.
1. A bidirectional link MUST be excluded from an MRT Island if
either the forward or reverse cost on the link is 0xFFFFFE (for
ISIS) or 0xFFFF for OSPF.
2. A router MUST be excluded from an MRT Island if it is advertised
with the overload bit set (for ISIS), or it is advertised with
metric values of 0xFFFF on all of its outgoing interfaces (for
OSPF).
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7.3.2. MRT-specific exclusion mechanism
This architecture also defines a means of excluding an otherwise
usable link from MRT Islands. [I-D.atlas-ospf-mrt] and
[I-D.li-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.
7.4. Connectivity
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.
7.5. Example algorithm
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 5.1 of [I-D.ietf-rtgwg-mrt-frr-algorithm].
8. MRT Profile
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.
8.1. MRT Profile Options
Below is a description of the values and options that define an MRT
Profile.
MRT Algorithm: This identifies the particular MRT algorithm used by
the router for this profile. Algorithm transitions can be managed
by advertising multiple MRT profiles.
MRT-Red MT-ID: This specifies the MT-ID to be associated with the
MRT-Red forwarding topology. It is needed for use in LDP
signaling. All routers in the MRT Island MUST agree on a value.
MRT-Blue MT-ID: This specifies the MT-ID to be associated with the
MRT-Blue forwarding topology. It is needed for use in LDP
signaling. All routers in the MRT Island MUST agree on a value.
GADAG Root Selection Policy: This specifes the manner in which the
GADAG root is selected. All routers in the MRT island need to use
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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 in the IGP 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 which supports a specific MRT forwarding mechanism must
program appropriate next-hops into the forwarding plane. The
current options are MRT LDP Labels, IPv4 Tunneling, IPv6
Tunneling, and None. If the MRT LDP Labels option is supported,
then option 1A and the appropriate signaling extensions MUST be
supported. 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. The None option in may be useful for
multicast global protection.
Recalculation: As part of what process and timing should the new
MRTs be computed on a modified topology? Section 12.2 describes
the minimum behavior required to support fast-reroute.
Area/Level Border Behavior: Should inter-area traffic on the MRT-
Blue or MRT-Red be put back onto the shortest path tree? Should
it be swapped from MRT-Blue or MRT-Red in one area/level to MRT-
Red or MRT-Blue in the next area/level to avoid the potential
failure of an ABR? (See [I-D.atlas-rtgwg-mrt-mc-arch] for use-
case details.
Other Profile-Specific Behavior: Depending upon the use-case for
the profile, there may be additional profile-specific behavior.
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.
8.2. Router-specific MRT paramaters
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.
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GADAG Root Selection Priority: A GADAG Root Selection Policy MAY
rely on the GADAG Root Selection Priority values advertised by
each router in the MRT island. A GADAG Root Selection Policy may
use the GADAG Root Selection Priority to allow network operators
to configure a parameter to ensure that the GADAG root is selected
from a particular subset of routers. An example of this use of
the GADAG Root Selection Priority value by the GADAG Root
Selection Policy is given in the Default MRT profile below.
MRT-Red Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Red forwarding topology.
It can be specified for either IPv4 and IPv6.
MRT-Blue Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Blue forwarding topology.
It can be specified for either IPv4 and IPv6.
The extensions to OSPF and ISIS for advertising a router's GADAG Root
Selection Priority value are defined in [I-D.atlas-ospf-mrt] and
[I-D.li-isis-mrt]. IGP extensions for the advertising a router's
MRT-Red and MRT-Blue Loopback Addresses have not been defined.
8.3. Default MRT profile
The following set of options defines the default MRT Profile. The
default MRT profile is indicated by the MRT Profile ID value of 0.
MRT Algorithm: MRT Lowpoint algorithm defined in
[I-D.ietf-rtgwg-mrt-frr-algorithm].
MRT-Red MT-ID: TBA-MRT-ARCH-1, final value assigned by IANA
allocated from the LDP MT-ID space (prototype experiments have
used 3997)
MRT-Blue MT-ID: TBA-MRT-ARCH-2, final value assigned by IANA
allocated from the LDP MT-ID space (prototype experiments have
used 3998)
GADAG Root Selection Policy: Among the routers in the MRT Island
and with the highest priority advertised, an implementation MUST
pick the router with the highest Router ID to be the GADAG root.
Forwarding Mechanisms: MRT LDP Labels
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.
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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.
9. LDP signaling extensions and considerations
The protocol extensions for LDP are defined in
[I-D.atlas-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 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.atlas-mpls-ldp-mrt] contains the IANA request for the Rainbow
MRT MT-ID.
10. Inter-area Forwarding Behavior
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
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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.
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A
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.
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/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
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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.
10.2. ABR Forwarding Behavior with IP Tunneling (option 2)
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.
10.3. ABR Forwarding Behavior with 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/
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
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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.atlas-ospf-mrt].
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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) Blue MRT in Area 0 (e) Red MRT 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 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.
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The key difference is whether the traffic, once out of the MRT
Island, remains in the same area/level and might reenter 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:
o An out-of-area prefix announced by more than one ABR,
o An AS-External route announced by 2 or more ASBRs,
o A prefix with iBGP multipath to different ASBRs,
o etc.
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
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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.
11.1. Protecting Multi-Homed 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].
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.
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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 end-point.
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 will not work for LDP traffic without targeted LDP
sesssions for learning the FEC-label binding at the tunnel endpoint.
11.2. Protecting Multi-Homed 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.
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
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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.atlas-ospf-mrt]. The details of what label-bindings
must be originated are described in [I-D.atlas-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 [[RFC2328] Section 16.5] does incremental updates
for new summary-LSAs.
The key 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
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
regard 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 discussed below.
The named-proxy-cost assigned to each (IBR, IN) pair is cost(IBR, IN)
+ D_opt(IN, prefix).
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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 default 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.
11.2.1. Computing if an Island Neighbor (IN) is loop-free
As discussed, the Island Neighbor needs to be loop-free with regard
to the whole MRT Island for the destination. Conceptually, the cost
of transiting the MRT Island should be regarded as 0. This can be
done by collapsing the MRT Island into a single node, as seen in
Figure 4, and then computing SPFs from each Island Neighbor and from
the MRT Island itself.
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[G]---[E]---(V)---(U)---(T)
| \ | | |
| \ | | |
| \ | | |
[H]---[F]---(R)---(S)----|
(1) Network Graph with Partial Deployment
[E],[F],[G],[H] : No support for MRT
(R),(S),(T),(U),(V): MRT Island - supports MRT
[G]---[E]----| |---(V)---(U)---(T)
| \ | | | | |
| \ | ( MRT Island ) [ proxy ] | |
| \ | | | | |
[H]---[F]----| |---(R)---(S)----|
(2) Graph for determining (3) Graph for MRT computation
loop-free neighbors
Figure 4: Computing alternates to destinations outside the MRT Island
The simple way to do this without manipulating the topology is to
compute the SPFs from each IN and a node in the MRT Island (e.g. the
GADAG root), but use a link metric of 0 for all links between routers
in the MRT Island. The distances computed via SPF this way will be
refered to as Dist_mrt0.
An IN is loop-free with respect to a destination D if: Dist_mrt0(IN,
D) < Dist_mrt0(IN, MRT Island Router) + Dist_mrt0(MRT Island Router,
D). Any router in the MRT Island can be used since the cost of
transiting between MRT Island routers is 0. The GADAG Root is
recommended for consistency.
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 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
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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.
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 re-calculation.
12.1. Micro-forwarding loop prevention and MRTs
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 [I-D.ietf-rtgwg-ordered-fib].
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.
12.2. MRT Recalculation
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:
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1. Receive failure notification
2. Recompute SPT
3. Install new SPT
4. If the network was stable before the failure occured, 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.
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.
13. Implementation Status
[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
o Organization responsible for the implementation: Juniper Networks
o Implementation name: MRT-FRR algorithm
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o Implementation description: The MRT-FRR algorithm using OSPF as
the IGP has been implemented and verified.
o The implementation's level of maturity: prototype
o Protocol coverage: This implementation of the MRT algorithm
includes Island identification, GADAG root selection, Lowpoint
algorithm, augmentation of GADAG with additional links, and
calculation of MRT transit next-hops alternate next-hops based on
draft "draft-ietf-rtgwg-mrt-frr-algorithm-00". This
implementation also includes the M-bit in OSPF based on "draft-
atlas-ospf-mrt-01" as well as LDP MRT Capability based on "draft-
atlas-mpls-ldp-mrt-00".
o Licensing: proprietary
o Implementation experience: Implementation was useful for verifying
functionality and lack of gaps. It has also been useful for
improving aspects of the algorithm.
o Contact information: akatlas@juniper.net, shraddha@juniper.net,
kishoret@juniper.net
Huawei Technology Implementation
o Organization responsible for the implementation: Huawei Technology
Co., Ltd.
o Implementation name: MRT-FRR algorithm and IS-IS extensions for
MRT.
o Implementation description: The MRT-FRR algorithm, IS-IS
extensions for MRT and LDP multi-topology have been implemented
and verified.
o The implementation's level of maturity: prototype
o Protocol coverage: This implementation of the MRT algorithm
includes Island identification, GADAG root selection, Lowpoint
algorithm, augmentation of GADAG with additional links, and
calculation of MRT transit next-hops alternate next-hops based on
draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03". This
implementation also includes IS-IS extension for MRT based on
"draft-li-mrt-00".
o Licensing: proprietary
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o Implementation experience: It is important produce a second
implementation to verify the algorithm is implemented correctly
without looping. It is important to verify the ISIS extensions
work for MRT-FRR.
o Contact information: lizhenbin@huawei.com, eric.wu@huawei.com
14. 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, and Raveendra Torvi for their suggestions
and review.
15. IANA Considerations
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.
Please allocate values from the LDP Multi-Topology (MT) ID Name Space
[I-D.ietf-mpls-ldp-multi-topology] for the following: Default Profile
MRT-Red MT-ID (TBA-MRT-ARCH-1) and Default Profile MRT-Blue MT-ID
(TBA-MRT-ARCH-2). Please allocate MT-ID values less than 4096 so
that they can also be signalled in PIM.
16. Security Considerations
This architecture is not currently believed to introduce new security
concerns.
17. References
17.1. Normative References
[I-D.ietf-rtgwg-mrt-frr-algorithm]
Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast-Reroute", draft-rtgwg-mrt-frr-
algorithm-01 (work in progress), July 2014.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.
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[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
17.2. 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,
<http://timon.tmit.bme.hu/theses/thesis_book.pdf>.
[I-D.atlas-mpls-ldp-mrt]
Atlas, A., Tiruveedhula, K., Tantsura, J., and IJ.
Wijnands, "LDP Extensions to Support Maximally Redundant
Trees", draft-atlas-mpls-ldp-mrt-01 (work in progress),
July 2014.
[I-D.atlas-ospf-mrt]
Atlas, A., Hegde, S., Bowers, C., and J. Tantsura, "OSPF
Extensions to Support Maximally Redundant Trees", draft-
atlas-ospf-mrt-02 (work in progress), July 2014.
[I-D.atlas-rtgwg-mrt-mc-arch]
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
Envedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
arch-02 (work in progress), July 2013.
[I-D.bryant-ipfrr-tunnels]
Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
(work in progress), November 2007.
[I-D.ietf-mpls-ldp-multi-topology]
Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi Topology", draft-ietf-
mpls-ldp-multi-topology-12 (work in progress), April 2014.
[I-D.ietf-rtgwg-ipfrr-notvia-addresses]
Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
and MPLS Fast Reroute Using Not-via Addresses", draft-
ietf-rtgwg-ipfrr-notvia-addresses-11 (work in progress),
May 2013.
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[I-D.ietf-rtgwg-ordered-fib]
Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-free
convergence using oFIB", draft-ietf-rtgwg-ordered-fib-12
(work in progress), May 2013.
[I-D.ietf-rtgwg-remote-lfa]
Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate Fast Re-Route", draft-
ietf-rtgwg-remote-lfa-10 (work in progress), January 2015.
[I-D.li-isis-mrt]
Li, Z., Wu, N., Zhao, Q., Atlas, A., Bowers, C., and J.
Tantsura, "Intermediate System to Intermediate System (IS-
IS) Extensions for Maximally Redundant Trees(MRT)", draft-
li-isis-mrt-01 (work in progress), July 2014.
[I-D.psarkar-rtgwg-rlfa-node-protection]
psarkar@juniper.net, p., Gredler, H., Hegde, S.,
Raghuveer, H., Bowers, C., and S. Litkowski, "Remote-LFA
Node Protection and Manageability", draft-psarkar-rtgwg-
rlfa-node-protection-04 (work in progress), April 2014.
[LFARevisited]
Retvari, G., Tapolcai, J., Enyedi, G., and A. Csaszar, "IP
Fast ReRoute: Loop Free Alternates Revisited", Proceedings
of IEEE INFOCOM , 2011,
<http://opti.tmit.bme.hu/~tapolcai/papers/
retvari2011lfa_infocom.pdf>.
[LightweightNotVia]
Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
Fast ReRoute: Lightweight Not-Via without Additional
Addresses", Proceedings of IEEE INFOCOM , 2009,
<http://mycite.omikk.bme.hu/doc/71691.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137,
June 2001.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, March 2009.
Atlas, et al. Expires July 23, 2015 [Page 38]
Internet-Draft MRT Unicast FRR Architecture January 2015
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
[RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B.,
Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, June 2012.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, July
2013.
Appendix A. General Issues with Area Abstraction
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 5: AS external prefixes in different areas
Consider the network in Figure 5 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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Authors' Addresses
Alia Atlas (editor)
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: akatlas@juniper.net
Robert Kebler
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: rkebler@juniper.net
Chris Bowers
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: cbowers@juniper.net
Gabor Sandor Enyedi
Ericsson
Konyves Kalman krt 11.
Budapest 1097
Hungary
Email: Gabor.Sandor.Enyedi@ericsson.com
Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary
Email: Andras.Csaszar@ericsson.com
Atlas, et al. Expires July 23, 2015 [Page 40]
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Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
USA
Email: jeff.tantsura@ericsson.com
Russ White
VCE
Email: russw@riw.us
Atlas, et al. Expires July 23, 2015 [Page 41]
