Segment Routing TI-LFA Double Segment Example

TI-LFA can add one or more segment when switching traffic to backup and loop free alternate path. According to below topology let’s assume we want to protect not only the link between Node-A and Node-B but we also want to protect Node-B. Without TI-LFA, we can’t protect Node-B. According to formula it seems it isn’t possible to add backup next-hop. For TI-LFA to work properly we need to have Adj-SIDs and thanks to SR, it is already distributed.

Remember that Adj-SIDs are absolute values and allocated dynamically by IGP. Adj-SID values are distributed by the Segment Routing Control Plane but it is only locally significant. Every will assign different Adj-SID for every neighbor.

If Total Cost of Alternate Next-Hop to Destination = 11 > (Total cost of Source to Alternate Next-Hop = 1) + (Total Cost of Source to Destination = 2)

TI-LFA will solve this solution because it can calculate the backup path with a label stack and label stack may include Prefix=SID and Adj-SID.

Incoming Active Segment: NULL

Operation: PUSH 16005 (Prefix-SID of Node-E)

Egress Interface: The interface towards to Node-B

TI-LFA Backup: PUSH (16005,145,16004) and forward Interface towards to Node-C

  1. After Node-B fails, Node-A Pushes the label stack (16005,145,16004) and forwards packet to Node-C
  2. Node-C Pops the outer label (PHP is enabled) and forwards packet to Node-D
  3. Node-D pops the outer label (Adj-SID) and forwards packet to Node-E which is the final destination.

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