10. Inter-area Forwarding Behavior
An ABR/LBR has two forwarding roles. First, it forwards traffic
within areas. Second, it forwards traffic from one area into
another. These same two roles apply for MRT transit traffic.
Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
on MRT-Red or MRT-Blue in that area. However, it is desirable for
traffic leaving the area to also exit MRT-Red or MRT-Blue and return
to shortest path forwarding.
For unicast MRT-FRR, the need to stay on an MRT forwarding topology
terminates at the ABR/LBR whose best route is via a different area/
level. It is highly desirable to go back to the default forwarding
topology when leaving an area/level. There are three basic reasons
for this. First, the default topology uses shortest paths; the
packet will thus take the shortest possible route to the destination.
Second, this allows a single router failure that manifests itself in
multiple areas (as would be the case with an ABR/LBR failure) to be
separately identified and repaired around. Third, the packet can be
fast-rerouted again, if necessary, due to a second distinct failure
in a different area.
In OSPF, an ABR that receives a packet on MRT-Red or MRT-Blue towards
destination Z should continue to forward the packet along MRT-Red or
MRT-Blue only if the best route to Z is in the same OSPF area as the
interface that the packet was received on. Otherwise, the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology.
The above description applies to OSPF. The same essential behavior
also applies to IS-IS if one substitutes IS-IS level for OSPF area.
However, the analogy with OSPF is not exact. An interface in OSPF
can only be in one area, whereas an interface in IS-IS can be in both
Level-1 and Level-2. Therefore, to avoid confusion and address this
difference, we explicitly describe the behavior for IS-IS in
Appendix A. In the following sections, only the OSPF terminology is
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A
For LDP forwarding where a single label specifies (MT-ID, FEC), the
ABR is responsible for advertising the proper label to each neighbor.
Assume that an ABR has allocated three labels for a particular
destination: L_primary, L_blue, and L_red. To those routers in the
same area as the best route to the destination, the ABR advertises
the following FEC-label bindings: L_primary for the default topology,
L_blue for the MRT-Blue MT-ID, and L_red for the MRT-Red MT-ID, as
expected. However, to routers in other areas, the ABR advertises the
following FEC-label bindings: L_primary for the default topology and
L_primary for the Rainbow MRT MT-ID. Associating L_primary with the
Rainbow MRT MT-ID causes the receiving routers to use L_primary for
the MRT-Blue MT-ID and for the MRT-Red MT-ID.
The ABR installs all next hops for the best area: primary next hops
for L_primary, MRT-Blue next hops for L_blue, and MRT-Red next hops
for L_red. Because the ABR advertised (Rainbow MRT MT-ID, FEC) with
L_primary to neighbors not in the best area, packets from those
neighbors will arrive at the ABR with a label L_primary and will be
forwarded into the best area along the default topology. By
controlling what labels are advertised, the ABR can thus enforce that
packets exiting the area do so on the shortest-path default topology.
10.1.1. Motivation for Creating the Rainbow-FEC
The desired forwarding behavior could be achieved in the above
example without using the Rainbow-FEC. This could be done by having
the ABR advertise the following FEC-label bindings to neighbors not
in the best area: L1_primary for the default topology, L1_primary for
the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing this
would require machinery to spoof the labels used in FEC-label binding
advertisements on a per-neighbor basis. Such label-spoofing
machinery does not currently exist in most LDP implementations and
doesn't have other obvious uses.
Many existing LDP implementations do however have the ability to
filter FEC-label binding advertisements on a per-neighbor basis. The
Rainbow-FEC allows us to reuse the existing per-neighbor FEC
filtering machinery to achieve the desired result. By introducing
the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
advertise the FEC-label binding for the Rainbow-FEC (and filter those
for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.
An ABR may choose to either distribute the Rainbow-FEC or distribute
separate MRT-Blue and MRT-Red advertisements. This is a local
choice. A router that supports the MRT LDP Label Option 1A
forwarding mechanism MUST be able to receive and correctly interpret
10.2. ABR Forwarding Behavior with IP Tunneling (Option 2)
If IP tunneling is used, then the ABR behavior is dependent upon the
outermost IP address. If the outermost IP address is an MRT loopback
address of the ABR, then the packet is decapsulated and forwarded
based upon the inner IP address, which should go on the default SPT
topology. If the outermost IP address is not an MRT loopback address
of the ABR, then the packet is simply forwarded along the associated
forwarding topology. A PLR sending traffic to a destination outside
its local area/level will pick the MRT and use the associated MRT
loopback address of the selected ABR advertising the lowest cost to
the external destination.
Thus, for these two MRT forwarding mechanisms (MRT LDP Label Option
1A and IP tunneling Option 2), there is no need for additional
computation or per-area forwarding state.
10.3. ABR Forwarding Behavior with MRT LDP Label Option 1B
The other MRT forwarding mechanism described in Section 6 uses two
labels: a topology-id label and a FEC-label. This mechanism would
require that any router whose MRT-Red or MRT-Blue next hop is an ABR
would need to determine whether the ABR would forward the packet out
of the area/level. If so, then that router should pop off the
topology-id label before forwarding the packet to the ABR.
For example, in Figure 3, if node H fails, node E has to put traffic
towards prefix p onto MRT-Red. But since node D knows that ABR1 will
use a best route from another area, it is safe for D to pop the
topology-id label and just forward the packet to ABR1 along the MRT-
Red next hop. ABR1 will use the shortest path in Area 10.
In all cases for IS-IS and most cases for OSPF, the penultimate
router can determine what decision the adjacent ABR will make. The
one case where it can't be determined is when two ASBRs are in
different non-backbone areas attached to the same ABR, then the
ASBR's Area ID may be needed for tie-breaking (prefer the route with
the largest OSPF area ID), and the Area ID isn't announced as part of
the ASBR LSA. In this one case, suboptimal forwarding along the MRT
in the other area would happen. If that becomes a realistic
deployment scenario, protocol extensions could be developed to
address this issue.
+----[C]---- --[D]--[E] --[D]--[E]
| \ / \ / \
p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
| / \ / | \ / |
+----[B]---- --[F]--[G] | --[F]--[G] |
| other |
(a) Example topology (b) Proxy node view in Area 0 nodes
V \ \
+-[A] Area 10 [ABR1] Area 0 [H]-+
| ^ / / |
| +----[B]<--- [F]->[G] V
(c) rSPT towards destination p
/ \ / \
[ABR1] Area 0 [H]-+ +-[ABR1] [H]
/ | | \
[F]->[G] V V -<[F]<-[G]
(d) MRT-Blue in Area 0 (e) MRT-Red in Area 0
Figure 3: ABR Forwarding Behavior and MRTs11. Prefixes Multiply Attached to the MRT Island
How a computing router S determines its local MRT Island for each
supported MRT profile is already discussed in Section 7.
There are two types of prefixes or FECs that may be multiply attached
to an MRT Island. The first type are multihomed prefixes that
usually connect at a domain or protocol boundary. The second type
represent routers that do not support the profile for the MRT Island.
The key difference is whether the traffic, once out of the MRT
Island, might re-enter the MRT Island if a loop-free exit point is
FRR using LFA has the useful property that it is able to protect
multihomed prefixes against ABR failure. For instance, if a prefix
from the backbone is available via both ABR A and ABR B, if A fails,
then the traffic should be redirected to B. This can be accomplished
with MRT FRR as well.
If ASBR protection is desired, this has additional complexities if
the ASBRs are in different areas. Similarly, protecting labeled BGP
traffic in the event of an ASBR failure has additional complexities
due to the per-ASBR label spaces involved.
As discussed in [RFC5286], a multihomed prefix could be:
o An out-of-area prefix announced by more than one ABR,
o An AS-External route announced by two or more ASBRs,
o A prefix with iBGP multipath to different ASBRs,
See Appendix B for a discussion of a general issue with multihomed
prefixes connected in two different areas.
There are also two different approaches to protection. The first is
tunnel endpoint selection where the PLR picks a router to tunnel to
where that router is loop-free with respect to the failure-point.
Conceptually, the set of candidate routers to provide LFAs expands to
all routers that can be reached via an MRT alternate, attached to the
The second is to use a proxy-node, which can be named via MPLS label
or IP address, and pick the appropriate label or IP address to reach
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
point. A proxy-node can represent a destination prefix that can be
attached to the MRT Island via at least two routers. It is termed a
named proxy-node if there is a way that traffic can be encapsulated
to reach specifically that proxy-node; this could be because there is
an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue
IP addresses are advertised (in an as-yet undefined fashion) for that
proxy-node. Traffic to a named proxy-node may take a different path
than traffic to the attaching router; traffic is also explicitly
forwarded from the attaching router along a predetermined interface
towards the relevant prefixes.
For IP traffic, multihomed prefixes can use tunnel endpoint
selection. For IP traffic that is destined to a router outside the
MRT Island, if that router is the egress for a FEC advertised into
the MRT Island, then the named proxy-node approach can be used.
For LDP traffic, there is always a FEC advertised into the MRT
Island. The named proxy-node approach should be used, unless the
computing router S knows the label for the FEC at the selected tunnel
If a FEC is advertised from outside the MRT Island into the MRT
Island and the forwarding mechanism specified in the profile includes
LDP Label Option 1A, then the routers learning that FEC MUST also
advertise labels for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors
inside the MRT Island. Any router receiving a FEC corresponding to a
router outside the MRT Island or to a multihomed prefix MUST compute
and install the transit MRT-Blue and MRT-Red next hops for that FEC.
The FEC-label bindings for the topology-scoped FECs ((MT-ID 0, FEC),
(MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be provided via LDP to
neighbors inside the MRT Island.
11.1. Protecting Multihomed Prefixes Using Tunnel Endpoint Selection
Tunnel endpoint selection is a local matter for a router in the MRT
Island since it pertains to selecting and using an alternate and does
not affect the transit MRT-Red and MRT-Blue forwarding topologies.
Let the computing router be S and the next hop F be the node whose
failure is to be avoided. Let the destination be prefix p. Have A
be the router to which the prefix p is attached for S's shortest path
The candidates for tunnel endpoint selection are those to which the
destination prefix is attached in the area/level. For a particular
candidate B, it is necessary to determine if B is loop-free to reach
p with respect to S and F for node-protection or at least with
respect to S and the link (S, F) for link-protection. If B will
always prefer to send traffic to p via a different area/level, then
this is definitional. Otherwise, distance-based computations are
necessary and an SPF from B's perspective may be necessary. The
following equations give the checks needed; the rationale is similar
to that given in [RFC5286]. In the inequalities below, D_opt(X,Y)
means the shortest distance from node X to node Y, and D_opt(X,p)
means the shortest distance from node X to prefix p.
Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)
The latter is equivalent to the following, which avoids the need to
compute the shortest path from F to p.
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)
Finally, the rules for Endpoint selection are given below. The basic
idea is to repair to the prefix-advertising router selected for the
shortest-path and only to select and tunnel to a different endpoint
if necessary (e.g., A=F or F is a cut-vertex or the link (S,F) is a
1. Does S have a node-protecting alternate to A? If so, select
that. Tunnel the packet to A along that alternate. For example,
if LDP is the forwarding mechanism, then push the label (MRT-Red,
A) or (MRT-Blue, A) onto the packet.
2. If not, then is there a router B that is loop-free to reach p
while avoiding both F and S? If so, select B as the endpoint.
Determine the MRT alternate to reach B while avoiding F. Tunnel
the packet to B along that alternate. For example, with LDP,
push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.
3. If not, then does S have a link-protecting alternate to A? If
so, select that.
4. If not, then is there a router B that is loop-free to reach p
while avoiding S and the link from S to F? If so, select B as
the endpoint and the MRT alternate for reaching B from S that
avoid the link (S,F).
The tunnel endpoint selected will receive a packet destined to itself
and, being the egress, will pop that MPLS label (or have signaled
Implicit Null) and forward based on what is underneath. This
suffices for IP traffic since the tunnel endpoint can use the IP
header of the original packet to continue forwarding the packet.
However, tunneling of LDP traffic requires targeted LDP sessions for
learning the FEC-label binding at the tunnel endpoint.
11.2. Protecting Multihomed Prefixes Using Named Proxy-Nodes
Instead, the named proxy-node method works with LDP traffic without
the need for targeted LDP sessions. It also has a clear advantage
over tunnel endpoint selection, in that it is possible to explicitly
forward from the MRT Island along an interface to a loop-free island
neighbor when that interface may not be a primary next hop.
A named proxy-node represents one or more destinations and, for LDP
forwarding, has a FEC associated with it that is signaled into the
MRT Island. Therefore, it is possible to explicitly label packets to
go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
Island, the label will swap to meaning (MT-ID 0, FEC). It would be
possible to have named proxy-nodes for IP forwarding, but this would
require extensions to signal two IP addresses to be associated with
MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be
uniquely represented by the two routers in the MRT Island to which it
is connected. The extensions to signal such IP addresses will be
defined elsewhere. The details of what label-bindings must be
originated will be described in another document.
Computing the MRT next hops to a named proxy-node and the MRT
alternate for the computing router S to avoid a particular failure
node F is straightforward. The details of the simple constant-time
functions, Select_Proxy_Node_NHs() and
Select_Alternates_Proxy_Node(), are given in [RFC7811]. A key point
is that computing these MRT next hops and alternates can be done as
new named proxy-nodes are added or removed without requiring a new
MRT computation or impacting other existing MRT paths. This maps
very well to, for example, how OSPFv2 (see [RFC2328], Section 16.5)
does incremental updates for new summary-LSAs.
The remaining question is how to attach the named proxy-node to the
MRT Island; all the routers in the MRT Island MUST do this
consistently. No more than two routers in the MRT Island can be
selected; one should only be selected if there are no others that
meet the necessary criteria. The named proxy-node is logically part
of the area/level.
There are two sources for candidate routers in the MRT Island to
connect to the named proxy-node. The first set is made up of those
routers in the MRT Island that are advertising the prefix; the named-
proxy-cost assigned to each prefix-advertising router is the
announced cost to the prefix. The second set is made up of those
routers in the MRT Island that are connected to routers not in the
MRT Island but in the same area/level; such routers will be defined
as Island Border Routers (IBRs). The routers connected to the IBRs
that are not in the MRT Island and are in the same area/level as the
MRT Island are Island Neighbors (INs).
Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
may come from any router inside the MRT Island, it is necessary that
whatever router to which an IBR forwards the packet be loop-free with
respect to the whole MRT Island for the destination. Thus, an IBR is
a candidate router only if it possesses at least one IN whose
shortest path to the prefix does not enter the MRT Island. A method
for identifying Loop-Free Island Neighbors (LFINs) is given in
[RFC7811]. The named-proxy-cost assigned to each (IBR, IN) pair is
cost(IBR, IN) + D_opt(IN, prefix).
From the set of prefix-advertising routers and the set of IBRs with
at least one LFIN, the two routers with the lowest named-proxy-cost
are selected. Ties are broken based upon the lowest Router ID. For
ease of discussion, the two selected routers will be referred to as
proxy-node attachment routers.
A proxy-node attachment router has a special forwarding role. When a
packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
prefix), if the proxy-node attachment router is an IBR, it MUST swap
to the shortest path forwarding topology (e.g., swap to the label for
(MT-ID 0, prefix) or remove the outer IP encapsulation) and forward
the packet to the IN whose cost was used in the selection. If the
proxy-node attachment router is not an IBR, then the packet MUST be
removed from the MRT forwarding topology and sent along the
interface(s) that caused the router to advertise the prefix; this
interface might be out of the area/level/AS.
11.3. MRT Alternates for Destinations outside the MRT Island
A natural concern with new functionality is how to have it be useful
when it is not deployed across an entire IGP area. In the case of
MRT FRR, where it provides alternates when appropriate LFAs aren't
available, there are also deployment scenarios where it may make
sense to only enable some routers in an area with MRT FRR. A simple
example of such a scenario would be a ring of six or more routers
that is connected via two routers to the rest of the area.
Destinations inside the local island can obviously use MRT
alternates. Destinations outside the local island can be treated
like a multihomed prefix and either endpoint selection or Named
Proxy-Nodes can be used. Named proxy-nodes MUST be supported when
LDP forwarding is supported and a label-binding for the destination
is sent to an IBR.
Naturally, there are more-complicated options to improve coverage,
such as connecting multiple MRT Islands across tunnels, but the need
for the additional complexity has not been justified.
12. Network Convergence and Preparing for the Next Failure
After a failure, MRT detours ensure that packets reach their intended
destination while the IGP has not reconverged onto the new topology.
As link-state updates reach the routers, the IGP process calculates
the new shortest paths. Two things need attention: micro-loop
prevention and MRT recalculation.
12.1. Micro-loop Prevention and MRTs
A micro-loop is a transient packet-forwarding loop among two or more
routers that can occur during convergence of IGP forwarding state.
[RFC5715] discusses several techniques for preventing micro-loops.
This section discusses how MRT-FRR relates to two of the micro-loop
prevention techniques discussed in [RFC5715]: Nearside and Farside
In Nearside Tunneling, a router (PLR) adjacent to a failure performs
local repair and informs remote routers of the failure. The remote
routers initially tunnel affected traffic to the nearest PLR, using
tunnels that are unaffected by the failure. Once the forwarding
state for normal shortest path routing has converged, the remote
routers return the traffic to shortest path forwarding. MRT-FRR is
relevant for Nearside Tunneling for the following reason. The
process of tunneling traffic to the PLRs and waiting a sufficient
amount of time for IGP forwarding state convergence with Nearside
Tunneling means that traffic will generally rely on the local repair
at the PLR for longer than it would in the absence of Nearside
Tunneling. Since MRT-FRR provides 100% coverage for single link and
node failure, it may be an attractive option to provide the local
repair paths when Nearside Tunneling is deployed.
MRT-FRR is also relevant for the Farside Tunneling micro-loop
prevention technique. In Farside Tunneling, remote routers tunnel
traffic affected by a failure to a node downstream of the failure
with respect to traffic destination. This node can be viewed as
being on the farside of the failure with respect to the node
initiating the tunnel. Note that the discussion of Farside Tunneling
in [RFC5715] focuses on the case where the farside node is
immediately adjacent to a failed link or node. However, the farside
node may be any node downstream of the failure with respect to
traffic destination, including the destination itself. The tunneling
mechanism used to reach the farside node must be unaffected by the
failure. The alternative forwarding paths created by MRT-FRR have
the potential to be used to forward traffic from the remote routers
upstream of the failure all the way to the destination. In the event
of failure, either the MRT-Red or MRT-Blue path from the remote
upstream router to the destination is guaranteed to avoid a link
failure or inferred node failure. The MRT forwarding paths are also
guaranteed to not be subject to micro-loops because they are locked
to the topology before the failure.
We note that the computations in [RFC7811] address the case of a PLR
adjacent to a failure determining which choice of MRT-Red or MRT-Blue
will avoid a failed link or node. More computation may be required
for an arbitrary remote upstream router to determine whether to
choose MRT-Red or MRT-Blue for a given destination and failure.
12.2. MRT Recalculation for the Default MRT Profile
This section describes how the MRT recalculation SHOULD be performed
for the Default MRT Profile. This is intended to support FRR
applications. Other approaches are possible, but they are not
specified in this document.
When a failure event happens, traffic is put by the PLRs onto the MRT
topologies. After that, each router recomputes its SPT and moves
traffic over to that. Only after all the PLRs have switched to using
their SPTs and traffic has drained from the MRT topologies should
each router install the recomputed MRTs into the FIBs.
At each router, therefore, the sequence is as follows:
1. Receive failure notification
2. Recompute SPT.
3. Install the new SPT in the FIB.
4. If the network was stable before the failure occurred, wait a
configured (or advertised) period for all routers to be using
their SPTs and traffic to drain from the MRTs.
5. Recompute MRTs.
6. Install new MRTs in the FIB.
While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them quickly.
New protocol extensions for advertising the time needed to recompute
shortest path routes and install them in the FIB will be defined
13. Operational Considerations
The following aspects of MRT-FRR are useful to consider when
deploying the technology in different operational environments and
13.1. Verifying Forwarding on MRT Paths
The forwarding paths created by MRT-FRR are not used by normal (non-
FRR) traffic. They are only used to carry FRR traffic for a short
period of time after a failure has been detected. It is RECOMMENDED
that an operator proactively monitor the MRT forwarding paths in
order to be certain that the paths will be able to carry FRR traffic
when needed. Therefore, an implementation SHOULD provide an operator
with the ability to test MRT paths with Operations, Administration,
and Maintenance (OAM) traffic. For example, when MRT paths are
realized using LDP labels distributed for topology-scoped FECs, an
implementation can use the MPLS ping and traceroute as defined in
[RFC4379] and extended in [RFC7307] for topology-scoped FECs.
13.2. Traffic Capacity on Backup Paths
During a fast-reroute event initiated by a PLR in response to a
network failure, the flow of traffic in the network will generally
not be identical to the flow of traffic after the IGP forwarding
state has converged, taking the failure into account. Therefore,
even if a network has been engineered to have enough capacity on the
appropriate links to carry all traffic after the IGP has converged
after the failure, the network may still not have enough capacity on
the appropriate links to carry the flow of traffic during a fast-
reroute event. This can result in more traffic loss during the fast-
reroute event than might otherwise be expected.
Note that there are two somewhat distinct aspects to this phenomenon.
The first is that the path from the PLR to the destination during the
fast-reroute event may be different from the path after the IGP
converges. In this case, any traffic for the destination that
reaches the PLR during the fast-reroute event will follow a different
path from the PLR to the destination than will be followed after IGP
The second aspect is that the amount of traffic arriving at the PLR
for affected destinations during the fast-reroute event may be larger
than the amount of traffic arriving at the PLR for affected
destinations after IGP convergence. Immediately after a failure, any
non-PLR routers that were sending traffic to the PLR before the
failure will continue sending traffic to the PLR, and that traffic
will be carried over backup paths from the PLR to the destinations.
After IGP convergence, upstream non-PLR routers may direct some
traffic away from the PLR.
In order to reduce or eliminate the potential for transient traffic
loss due to inadequate capacity during fast-reroute events, an
operator can model the amount of traffic taking different paths
during a fast-reroute event. If it is determined that there is not
enough capacity to support a given fast-reroute event, the operator
can address the issue either by augmenting capacity on certain links
or modifying the backup paths themselves.
The MRT Lowpoint algorithm produces a pair of diverse paths to each
destination. These paths are generated by following the directed
links on a common GADAG. The decision process for constructing the
GADAG in the MRT Lowpoint algorithm takes into account individual IGP
link metrics. At any given node, links are explored in order from
lowest IGP metric to highest IGP metric. Additionally, the process
for constructing the MRT-Red and Blue trees uses SPF traversals of
the GADAG. Therefore, the IGP link metric values affect the computed
backup paths. However, adjusting the IGP link metrics is not a
generally applicable tool for modifying the MRT backup paths.
Achieving a desired set of MRT backup paths by adjusting IGP metrics
while at the same time maintaining the desired flow of traffic along
the shortest paths is not possible in general.
MRT-FRR allows an operator to exclude a link from the MRT Island, and
thus the GADAG, by advertising it as MRT-Ineligible. Such a link
will not be used on the MRT forwarding path for any destination.
Advertising links as MRT-Ineligible is the main tool provided by MRT-
FRR for keeping backup traffic off of lower bandwidth links during
Note that all of the backup paths produced by the MRT Lowpoint
algorithm are closely tied to the common GADAG computed as part of
that algorithm. Therefore, it is generally not possible to modify a
subset of paths without affecting other paths. This precludes more
fine-grained modification of individual backup paths when using only
paths computed by the MRT Lowpoint algorithm.
However, it may be desirable to allow an operator to use MRT-FRR
alternates together with alternates provided by other FRR
technologies. A policy-based alternate selection process can allow
an operator to select the best alternate from those provided by MRT
and other FRR technologies. As a concrete example, it may be
desirable to implement a policy where a downstream LFA (if it exists
for a given failure mode and destination) is preferred over a given
MRT alternate. This combination gives the operator the ability to
affect where traffic flows during a fast-reroute event, while still
producing backup paths that use no additional labels for LDP traffic
and will not loop under multiple failures. This and other choices of
alternate selection policy can be evaluated in the context of their
effect on fast-reroute traffic flow and available capacity, as well
as other deployment considerations.
Note that future documents may define MRT profiles in addition to the
default profile defined here. Different MRT profiles will generally
produce alternate paths with different properties. An implementation
may allow an operator to use different MRT profiles instead of or in
addition to the default profile.
13.3. MRT IP Tunnel Loopback Address Management
As described in Section 6.1.2, if an implementation uses IP tunneling
as the mechanism to realize MRT forwarding paths, each node must
advertise an MRT-Red and an MRT-Blue loopback address. These IP
addresses must be unique within the routing domain to the extent that
they do not overlap with each other or with any other routing table
entries. It is expected that operators will use existing tools and
processes for managing infrastructure IP addresses to manage these
additional MRT-related loopback addresses.
13.4. MRT-FRR in a Network with Degraded Connectivity
Ideally, routers in a service provider network using MRT-FRR will be
initially deployed in a 2-connected topology, allowing MRT-FRR to
find completely diverse paths to all destinations. However, a
network can differ from an ideal 2-connected topology for many
possible reasons, including network failures and planned maintenance
MRT-FRR is designed to continue to function properly when network
connectivity is degraded. When a network contains cut-vertices or
cut-links dividing the network into different 2-connected blocks,
MRT-FRR will continue to provide completely diverse paths for
destinations within the same block as the PLR. For a destination in
a different block from the PLR, the redundant paths created by MRT-
FRR will be link and node diverse within each block, and the paths
will only share links and nodes that are cut-links or cut-vertices in
If a network becomes partitioned with one set of routers having no
connectivity to another set of routers, MRT-FRR will function
independently in each set of connected routers, providing redundant
paths to destinations in same set of connected routers as a given
13.5. Partial Deployment of MRT-FRR in a Network
A network operator may choose to deploy MRT-FRR only on a subset of
routers in an IGP area. MRT-FRR is designed to accommodate this
partial deployment scenario. Only routers that advertise support for
a given MRT profile will be included in a given MRT Island. For a
PLR within the MRT Island, MRT-FRR will create redundant forwarding
paths to all destinations with the MRT Island using maximally
redundant trees all the way to those destinations. For destinations
outside of the MRT Island, MRT-FRR creates paths to the destination
that use forwarding state created by MRT-FRR within the MRT Island
and shortest path forwarding state outside of the MRT Island. The
paths created by MRT-FRR to non-Island destinations are guaranteed to
be diverse within the MRT Island (if topologically possible).
However, the part of the paths outside of the MRT Island may not be
14. IANA Considerations
IANA has created the "MRT Profile Identifier Registry". The range is
0 to 255. The Default MRT Profile defined in this document has value
0. Values 1-200 are allocated by Standards Action. Values 201-220
are for Experimental Use. Values 221-254 are for Private Use. Value
255 is reserved for future registry extension. (The allocation and
use policies are described in [RFC5226].)
The initial registry is shown below.
Value Description Reference
------- ---------------------------------------- ------------
0 Default MRT Profile RFC 7812
201-220 Experimental Use
221-254 Private Use
255 Reserved (for future registry extension)
The "MRT Profile Identifier Registry" is a new registry in the IANA
Matrix. Following existing conventions, http://www.iana.org/
protocols displays a new header: "Maximally Redundant Tree (MRT)
Parameters". Under that header, there is an entry for "MRT Profile
Identifier Registry", which links to the registry itself at
15. Security Considerations
In general, MRT forwarding paths do not follow shortest paths. The
transit forwarding state corresponding to the MRT paths is created
during normal operations (before a failure occurs). Therefore, a
malicious packet with an appropriate header injected into the network
from a compromised location would be forwarded to a destination along
a non-shortest path. When this technology is deployed, a network
security design should not rely on assumptions about potentially
malicious traffic only following shortest paths.
It should be noted that the creation of non-shortest forwarding paths
is not unique to MRT.
MRT-FRR requires that routers advertise information used in the
formation of MRT backup paths. While this document does not specify
the protocol extensions used to advertise this information, we
discuss security considerations related to the information itself.
Injecting false MRT-related information could be used to direct some
MRT backup paths over compromised transmission links. Combined with
the ability to generate network failures, this could be used to send
traffic over compromised transmission links during a fast-reroute
event. In order to prevent this potential exploit, a receiving
router needs to be able to authenticate MRT-related information that
claims to have been advertised by another router.
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
[RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307,
DOI 10.17487/RFC7307, July 2014,
Appendix A. Inter-level Forwarding Behavior for IS-IS
In the description below, we use the terms "Level-1-only interface",
"Level-2-only interface", and "Level-1-and-Level-2 interface" to mean
an interface that has formed only a Level-1 adjacency, only a Level-2
adjacency, or both Level-1 and Level-2 adjacencies. Note that IS-IS
also defines the concept of areas. A router is configured with an
IS-IS area identifier, and a given router may be configured with
multiple IS-IS area identifiers. For an IS-IS Level-1 adjacency to
form between two routers, at least one IS-IS area identifier must
match. IS-IS Level-2 adjacencies do not require any area identifiers
to match. The behavior described below does not explicitly refer to
IS-IS area identifiers. However, IS-IS area identifiers will
indirectly affect the behavior by affecting the formation of Level-1
First, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-only interface. If the best shortest path
route to Z was learned from a Level-1 advertisement, then the packet
should continue to be forwarded along MRT-Red or MRT-Blue. If,
instead, the best route was learned from a Level-2 advertisement,
then the packet should be removed from MRT-Red or MRT-Blue and
forwarded on the shortest-path default forwarding topology.
Now consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-2-only interface. If the best route to Z was learned from
a Level-2 advertisement, then the packet should continue to be
forwarded along MRT-Red or MRT-Blue. If, instead, the best route was
learned from a Level-1 advertisement, then the packet should be
removed from MRT-Red or MRT-Blue and forwarded on the shortest-path
default forwarding topology.
Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-and-Level-2 interface. This packet should
continue to be forwarded along MRT-Red or MRT-Blue, regardless of
which level the route was learned from.
An implementation may simplify the decision-making process above by
using the interface of the next hop for the route to Z to determine
the level from which the best route to Z was learned. If the next
hop points out a Level-1-only interface, then the route was learned
from a Level-1 advertisement. If the next hop points out a Level-
2-only interface, then the route was learned from a Level-2
advertisement. A next hop that points out a Level-1-and-Level-2
interface does not provide enough information to determine the source
of the best route. With this simplification, an implementation would
need to continue forwarding along MRT-Red or MRT-Blue when the next-
hop points out a Level-1-and-Level-2 interface. Therefore, a packet
on MRT-Red or MRT-Blue going from Level-1 to Level-2 (or vice versa)
that traverses a Level-1-and-Level-2 interface in the process will
remain on MRT-Red or MRT-Blue. This simplification may not always
produce the optimal forwarding behavior, but it does not introduce
interoperability problems. The packet will stay on an MRT backup
path longer than necessary, but it will still reach its destination.
Appendix B. General Issues with Area Abstraction
When a multihomed prefix is connected in two different areas, it may
be impractical to protect them without adding the complexity of
explicit tunneling. This is also a problem for LFA and Remote-LFA.
|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
| | ABR 1, ABR 2, C, D
| | Area 20: A, ASBR X
p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y
5 p is a Type 1 AS-external
Figure 4: AS External Prefixes in Different Areas
Consider the network in Figure 4 and assume there is a richer
connective topology that isn't shown, where the same prefix is
announced by ASBR X and ASBR Y, which are in different non-backbone
areas. If the link from A to ASBR X fails, then an MRT alternate
could forward the packet to ABR 1 and ABR 1 could forward it to D,
but then D would find the shortest route is back via ABR 1 to Area
20. This problem occurs because the routers, including the ABR, in
one area are not yet aware of the failure in a different area.
The only way to get it from A to ASBR Y is to explicitly tunnel it to
ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels
are known, then explicit tunneling MAY be used as long as the
shortest path of the tunnel avoids the failure point. In that case,
A must determine that it should use an explicit tunnel instead of an
The authors would like to thank Mike Shand for his valuable review
The authors would like to thank Joel Halpern, Hannes Gredler, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
Alvaro Retana for their suggestions and review.
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Konyves Kalman krt 11
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