The operation of BFD over a 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. This tracking is required as LDP can now 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. The failure of either or both of the BFD session tracking the FEC next hop and the one tracking the Hello adjacency cause the LFA backup NHLFE for the FEC to be activated, or the FEC to be re-resolved if there is no FRR backup.
The following CLI command provides the option to track only with an IPv4 BFD session, only with an IPv6 BFD session, or both:
config>router>ldp>if-params>if>bfd-enable [ipv4] [ipv6]
This command provides the flexibility required in case the user does not need to track both the Hello adjacency and next hops of FECs. For example, if the user configures bfd-enable ipv6 only to save on the number of BFD sessions, 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 but the IPv6 Hello adjacency is unaffected 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, all IPv4 and IPv6 prefix FECs are updated.
The following behavior applies to the tracking of an mLDP FEC:
IPv4 and IPv6 mLDP FECs are only tracked with the Hello adjacency because they do not have the concept of downstream next hop.
The upstream LSR peer for an mLDP FEC supports the multicast upstream FRR procedures, and the upstream peer is tracked using the Hello adjacency on each link or the IPv6 transport address if there is a T-LDP session.
The tracking of a targeted LDP peer with BFD does not change with the support of IPv6 peers. BFD tracks the transport address conveyed by the Hello adjacency that bootstrapped the LDP IPv6 session.