Handling of Multicast Packets

This feature supports multicast Reverse-Path Check (RPF) in the presence of IGP shortcuts. When the multicast source for a packet is reachable via an IGP shortcut, the RPF check fails because PIM requires a bidirectional path to the source but IGP shortcuts are unidirectional.

The implementation of the IGP shortcut feature provides IGP with the capability to populate the multicast RTM with the prefix IP next-hop when both the igp-shortcut option and the multicast-import option are enabled in IGP.

This change is made possible with the enhancement introduced by which SPF keeps track of both the direct first hop and the tunneled first hop of a node that is added to the Dijkstra tree.

Note that IGP does not pass LFA next-hop information to the mcast RTM in this case. Only ECMP next-hops are passed. As a consequence, features such as PIM Multicast-Only FRR (MoFRR) only work with ECMP next-hops when IGP shortcuts are enabled.

Finally, note that the concurrent enabling of the advertise-tunnel-link option and the multicast-import option results a multicast RTM that is a copy of the unicast RTM and is populated with mix of IP and tunnel NHs. RPF succeeds for a prefix resolved to a IP NH, but fails for a prefix resolved to a tunnel NH. Table 1 summarizes the interaction of the igp-shortcut and advertise-tunnel-link options with unicast and multicast RTMs.

Table 1. Impact of IGP Shortcut and Forwarding Adjacency on Unicast and Multicast RTM

Unicast RTM (Primary SPF)

Multicast RTM (Primary SPF)

Unicast RTM (LFA SPF)

Multicast RTM (LFA SPF)

OSPF

igp-shortcut

 ✓ ✓1 ✓ X2

advertise-tunnel-link

 ✓ ✓3 ✓ ✓4

IS-IS

igp-shortcut

 ✓ ✓1 ✓ X2

advertise-tunnel-link

 ✓ ✓3 ✓ ✓4
Parent topic: IGP Shortcut and Forwarding Adjacency
1 Multicast RTM is different from unicast RTM as it is populated with IP NHs only, including ECMP IP NHs. RPF check can be performed for all prefixes.
2 LFA NH is not computed for the IP primary next-hop of a prefix passed to multicast RTM even if the same IP primary next-hop ends up being installed in the unicast RTM. The LFA next-hop, however, is computed and installed in the unicast RTM for a primary IP next-hop of a prefix.
3 Multicast RTM is a copy of the unicast RTM and, so, is populated with mix of IP and tunnel NHs. RPF succeeds for a prefix resolved to a IP NH but fails for a prefix resolved to a tunnel NH.
4 Multicast RTM is a copy of the unicast RTM and, so, is populated with mix of IP and tunnel LFA NHs. RPF succeeds for a prefix resolved to a primary or LFA IP NH but fails for a prefix resolved to a primary or LFA tunnel NH.