Configure Topology-Independent Loop-Free Alternate (TI-LFA) Topology-Independent Loop-Free Alternate (TI-LFA) uses segment routing to provide link protection in topologies where other fast reroute techniques cannot provide protection. 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. TI-LFA supports IPv4 only. Note TI-LFA provides link protection. The link is excluded during the post convergence backup path calculation. • Configuring TI-LFA for IS-IS, on page 1 • Configuring TI-LFA for OSPF, on page 3 • Configuring and Verifying TI-LFA: Example, on page 4 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. • Routers are configured with IS-IS. • Segment routing LSPs are configured. SUMMARY STEPS 1. configure 2. router isis instance-id Configure Topology-Independent Loop-Free Alternate (TI-LFA) 1
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Topology-Independent Loop-Free Alternate (TI-LFA) uses segment routing to provide link protection intopologies where other fast reroute techniques cannot provide protection. The goal of TI-LFA is to reducethe packet loss that results while routers converge after a topology change due to a link failure. Rapid failurerepair (< 50 msec) is achieved through the use of pre-calculated backup paths that are loop-free and safe touse until the distributed network convergence process is completed.
TI-LFA supports IPv4 only.Note
TI-LFA provides link protection. The link is excluded during the post convergence backup path calculation.
• Configuring TI-LFA for IS-IS, on page 1• Configuring TI-LFA for OSPF, on page 3• Configuring and Verifying TI-LFA: Example, on page 4
Configuring TI-LFA for IS-ISThis task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computationto 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.
Configure Topology-Independent Loop-Free Alternate (TI-LFA)Configuring TI-LFA for IS-IS
Configuring TI-LFA for OSPFThis task describes how to enable per-prefix Topology Independent Loop-Free Alternate (TI-LFA) computationto converge traffic flows around link failures.
TI-LFA can be configured on the instance, area, or interface. When configured on the instance or area, allinterfaces in the instance or area inherit the configuration.
Note
Before you begin
Ensure that the following topology requirements are met:
• Router interfaces are configured as per the topology.
• Routers are configured with OSPF.
• Segment routing LSPs are configured.
SUMMARY STEPS
1. configure2. router ospf process-name3. area area-id4. interface type interface-path-id5. fast-reroute per-prefix6. fast-reroute per-prefix ti-lfa
DETAILED STEPS
PurposeCommand or Action
configureStep 1
Enables OSPF routing for the specified routing process,and places the router in router configuration mode.
router ospf process-name
Example:
Step 2
RP/0/RP0/CPU0:router(config)# router ospf 1
Enters area configuration mode.area area-id
Example:
Step 3
RP/0/RP0/CPU0:router(config-ospf)# area 1
Enters interface configuration mode.interface type interface-path-id
TI-LFA has been successfully configured for segment routing.
Configuring and Verifying TI-LFA: ExampleIn this example, we configure TI-LFA for segment routing TE tunnels using single or disjoint PQ nodes.
The following figures show the two topologies used in this example:
• Topology 1 uses a single PQ Node, and therefore has two SIDs; from the source router, R1, through thePQ Node, to the destination router, R5.
Figure 1: Topology 1: Single PQ Node
• Topology 2 uses disjoint PQ Nodes, and therefore consists of three SIDs; from the source router, R1,through the P Node and the Q Node to the destination router, R5.
For this example, we configured TI-LFA on the specific interface. TI-LFA can be configured on theinstance or area; all interfaces in the instance or area would inherit the configuration.
Note
Step 2 Configure autoroute announce on the segment routing tunnel interface connecting R1 to R5.
If we configure segment routing to be used with prefix SIDs, the segment routing labels are assigned from the definedglobal block. In this example, we configure the source router to use adjacency SIDs dynamically, and therefore, the labelassignment does not use the SRGB.
Step 4 Commit your configuration.
Example:
RP/0/RSP0/CPU0:R1# commit
Step 5 Verify the IP FRR protection on the path between the source and destination router.
The following output is for a single PQ node topology.
Example:
RP/0/RSP0/CPU0:R1# show mpls traffic-eng forwarding tunnels 1 detailTunnel Outgoing Outgoing Next Hop BytesName Label Interface Switched------------- ----------- ------------ --------------- ------------te1 (SR) Pop Te0/1/1/1.100 10.15.1.2 31340256
The output for a single PQ node confirms that the primary path from R1 to R5 (label is popped as there are no transitrouters) is protected by the backup path with the labels 50103 (for the path from router R1 to the PQ Node, router R3)and 50105 (for the path from router R3 to the destination router R5).
The output for disjoint PQ nodes confirms that the primary path from R1 to R5 (label is popped as there are no transitrouters) is protected by the backup path with the labels 50103 (for the path from router R1 to the P Node, router R3),50104 (for the path from PNode (router R3) to QNode, router R4) and 50105 (for the path from router R4 to the destinationrouter R5).
When dynamic segment routing is configured on a router, adjacency SIDs are used and the labels are not pickedfrom the SRGB (as shown, in this example, by the local label of 24002).
Note
TI-LFA has been successfully configured for segment routing.