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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 LFAs.
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.
The behaviors and limitations of TI-LFA are listed below:
TI-LFA protects unlabeled IPv4 traffic.
TI-LFA does not protect unlabeled IPv6 traffic.
This task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computation to converge traffic flows around link failures.
Ensure that the following topology requirements are met:
Router interfaces are configured as per the topology.
Routers are configured with IS-IS.
Segment routing for IS-IS is configured. See Enabling Segment Routing for IS-IS Protocol.
Enter the following commands in global configuration mode:
Router(config)# ipv4 unnumbered mpls traffic-eng Loopback0
Router(config)# mpls traffic-eng
Router(config-mpls-te)# exit
Router(config)#
| Command or Action | Purpose | |||
|---|---|---|---|---|
| Step 1 |
configure Example: |
Enters mode. |
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| Step 2 |
router isis instance-id Example: |
Enables IS-IS routing for the specified routing instance, and places the router in router configuration mode.
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| Step 3 |
interface type interface-path-id Example: |
Enters interface configuration mode. |
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| Step 4 |
address-family ipv4 [ unicast] Example: |
Specifies the IPv4 address family, and enters router address family configuration mode. |
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| Step 5 |
fast-reroute per-prefix Example: |
Enables per-prefix fast reroute. |
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| Step 6 |
fast-reroute per-prefix ti-lfa Example: |
Enables per-prefix TI-LFA fast reroute link protection. |
TI-LFA has been successfully configured for segment routing.
This task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computation to converge traffic flows around link failures.
 Note |
TI-LFA can be configured on the instance, area, or interface. When configured on the instance or area, all interfaces in the instance or area inherit the configuration. |
Ensure that the following topology requirements are met:
Router interfaces are configured as per the topology.
Routers are configured with OSPF.
Segment routing for OSPF is configured. See Enabling Segment Routing for OSPF Protocol.
Enter the following commands in global configuration mode:
Router(config)# ipv4 unnumbered mpls traffic-eng Loopback0
Router(config)# mpls traffic-eng
Router(config-mpls-te)# exit
Router(config)#
| Command or Action | Purpose | |
|---|---|---|
| Step 1 |
configure Example: |
Enters mode. |
| Step 2 |
router ospf process-name Example: |
Enables OSPF routing for the specified routing process, and places the router in router configuration mode. |
| Step 3 |
area area-id Example: |
Enters area configuration mode. |
| Step 4 |
interface type interface-path-id Example: |
Enters interface configuration mode. |
| Step 5 |
fast-reroute per-prefix Example: |
Enables per-prefix fast reroute. |
| Step 6 |
fast-reroute per-prefix ti-lfa Example: |
Enables per-prefix TI-LFA fast reroute link protection. |
TI-LFA has been successfully configured for segment routing.
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