BFD and VSX support

BFD supports VSX LAG, active gateway, and active forwarding.

For 832x series switches: Several TCAMs (ternary content-addressable memory) are used to avoid decreasing the time-to-live (TTL) to 254 on BFD single-hop packets received/sent on ISL interfaces.

For 8400 series switches: To account for the TTL decrement on active forwarding, the BFD daemon supports packets with TTL equal to 254 on sessions running on ports with this functionality active.