Sunday, May 31, 2020

Segment Routing - Topology Independent LFA

Combination of segment routing and Loop Free Alternate is known as Topology Independent LFA or Ti-LFA. The main reason for configuring LFA is to provide the protection against the destination prefixes. In case of failure of primary link, source node calculates the backup the path as described in the "Loop Free Alternate: IP Fast Reroute (FRR)". The biggest disadvantage of the existing RSVP-TE-Fast reroute, Loop Free Alternate (LFA) and remote LFA, which has seen wide adoption can't guarantee 100% coverage for all failure scenarios. As per the RFC 6571, section 4.3, simulation results proves that LFA provides average 89% coverage across various topologies.

Below are the various types of issues which can be seen with Loop Free Alternate(LFA) and Remote Loop Free Alternate(rLFA)

1. LFA has issue with topology more than 3 nodes. With 3 nodes topology it perfectly works good. If the topology has more than 3 nodes, micro loop can occur as per shown in the image.

2. Micro-Loop avoidance can be taken care by finding PQ node as per RFC 7490. XR-1 will create T-LDP session with XR-3, XR-3 has the best path to XR-5 via XR-4.
3. Now increase the metric to 100 between XR-3 and XR-4. In this case, if XR-1 creates T-LDP session XR-3, XR-3 will again finds XR-1 as best and forward the traffic back to XR-3. This will be same as of micro loop.
Implementing Segment Routing with Topology Independent LFA (TI-LFA) provides 100% loop-free guaranteed coverage against any link, SRLG and node failure. Ti-LFA protects both labelled as well as unlabelled traffic. Stay tuned for the upcoming post (Segment Routing - Ti-LFA - Adjacency Sid Protection) how Ti-LFA solves the above issues by leveraging Segment Routing.

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