When a packet of a BGP IPv4 or IPv6 prefix is received, the data path uses the FC that the packet was classified into to look up the forwarding set ID. The data path then performs a modulo operation on the tunnel next-hops of this set ID, to select the one next-hop for forwarding the packet. Therefore, packets matching an FC are only sprayed over the ECMP tunnel-next-hops of the set ID this FC maps to.
Both the BGP or CPM application and IOM use the same algorithm for failover and default class-forwarding set determination, as described in Feature behavior and illustrated in Example configuration and default CBF set election.
If MPLS deletes an LSP from a specified set ID, the IOM handles failover within the same set ID. The IOM reprograms the data path to spray packets of the impacted FCs over the remaining tunnel next-hops of the set ID.
Similarly, the IOM handles failover between class-forwarding sets when MPLS deletes the last LSP in a set ID. The IOM reprograms the data path to spray packets of the impacted FCs over the tunnel next-hops of the failover set ID. In both cases, the failover does not make use of the uniform failover procedure; however, if an LSP activated its FRR backup path, it remains in the set ID and continues to forward traffic of the mapped FCs.
Finally, BGP updates the set IDs, used to reach a BGP next-hop, any time IGP updates the information in the RTM.