The operation of BFD over an LDP interface tracks the next hop of prefix IPv4 and prefix IPv6 in addition to tracking of the LDP peer address of the Hello adjacency over that link. Tracking is required because LDP can resolve both IPv4 and IPv6 prefix FECs over a single IPv4 or IPv6 LDP session and therefore, the next hop of a prefix does not necessarily match the LDP peer source address of the Hello adjacency. If any of the BFD tracking sessions fail, the LFA backup NHLFE for the FEC is activated, or, if there is no FRR backup, the FEC is re-resolved.
The user can configure tracking with only an IPv4 BFD session, only an IPv6 BFD session, or with both using the config>router>ldp>if-params>if>bfd-enable [ipv4] [ipv6] command.
This command provides flexibility in case the user does not need to track both Hello adjacency and the next hops of FECs.
For example, if the user configures bfd-enable ipv6 only to save on the number of BFD sessions, then LDP tracks the IPv6 Hello adjacency and the next hops of IPv6 prefix FECs. LDP does not track next hops of IPv4 prefix FECs resolved over the same LDP IPv6 adjacency. If the IPv4 data plane encounters errors and the IPv6 Hello adjacency is not affected and remains up, traffic for the IPv4 prefix FECs resolved over that IPv6 adjacency is blackholed. If the BFD tracking the IPv6 Hello adjacency times out, then all IPv4 and IPv6 prefix FECs are updated.