Liveness of SR Policy using Seamless BFD

Performing liveness checks, such as enhanced fault detection, has several benefits. Once a failing path in a network is detected, operators can implement immediate corrective measures to avoid any traffic disruption.
This year's testing also covered Seamless Bidirectional Forwarding Detection (S-BFD), among other focus areas. The given topology comprised two PE routers, each attached to a pair of dual spines. The configurations established S-BFD sessions from an initiator node to a reflector node, transmitting continuous probes along the primary path.
To artificially induce failure without physically breaking the links, we applied an ACL on one of the spine links that dropped any MPLS packets (including S‑BFD) but not IGP packets, thus allowing IGP to continue attracting traffic to the failed link. Because routers encapsulated S‑BFD in MPLS (the same as the protected SR-TE), the packets got dropped, too. We didn't bring down the link; thus, we could see how fast the network could detect partial failure by the S‐BFD probes that were not arriving.

Once the S‐BFD detected the absence of returning probes from any instance on the primary path, it triggered the SR‐TE policy to seamlessly switch traffic over to the backup path within the configured failure threshold and delay. Therefore, network operations were not interrupted, which was helpful in the effective functioning of S‐BFD, as it enabled preventive measures in mitigating path failures in an SR‐MPLS environment.