Segment Routing Configuration Guide for Cisco NCS 5000 Series Routers, IOS XR Release 6.3.x
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Topology-Independent Loop-Free Alternate (TI-LFA) uses segment routing to provide link protection in topologies where other
fast reroute techniques cannot provide protection.
Classic Loop-Free Alternate (LFA) is topology dependent, and therefore cannot protect all destinations in all networks. A
limitation of LFA is that, even if one or more LFAs exist, the optimal LFA may not always be provided.
Remote LFA (RLFA) extends the coverage to 90-95% of the destinations, but it also does not always provide the most desired
repair path. RLFA also adds more operational complexity by requiring a targeted LDP session to the RLFAs to protect LDP traffic.
TI-LFA provides a solution to these limitations while maintaining the simplicity of the IPFRR solution.
The goal of TI-LFA is to reduce the packet loss that results while routers converge after a topology change due to a link
failure. Rapid failure repair (< 50 msec) is achieved through the use of pre-calculated backup paths that are loop-free and
safe to use until the distributed network convergence process is completed.
The optimal repair path is the path that the traffic will eventually follow after the IGP has converged. This is called the
post-convergence path. This path is preferred for the following reasons:
Optimal for capacity planning — During the capacity-planning phase of the network, the capacity of a link is provisioned while
taking into consideration that such link with be used when other links fail.
Simple to operate — There is no need to perform a case-by-case adjustments to select the best LFA among multiple candidate
Fewer traffic transitions — Since the repair path is equal to the post-convergence path, the traffic switches paths only once.
The following topology illustrates the optimal and automatic selection of the TI-LFA repair path.
Node 2 protects traffic to destination Node 5.
With classic LFA, traffic would be steered to Node 4 after a failure of the protected link. This path is not optimal, since
traffic is routed over edge node Node 4 that is connected to lower capacity links.
TI-LFA calculates a post-convergence path and derives the segment list required to steer packets along the post-convergence
path without looping back.
In this example, if the protected link fails, the shortest path from Node2 to Node5 would be:
Node2 → Node6 → Node7 → Node3 → Node5
Node7 is the PQ-node for destination Node5. TI-LFA encodes a single segment (prefix SID of Node7) in the header of the packets
on the repair path.
Behaviors and Limitations of TI-LFA
The behaviors and limitations of TI-LFA are listed below:
TI-LFA protects unlabeled IPv4 traffic.
TI-LFA does not protect unlabeled IPv6 traffic.
Configuring TI-LFA for IS-IS
This task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computation to converge traffic
flows around link failures.
Before you begin
Ensure that the following topology requirements are met:
Router interfaces are configured as per the topology.