PIM Sparse Mode (PIM-SM) and PIM Source Specific Multicast (PIM-SSM) are supported without synchronizing PIM-SM or PIM-SSM states across the multichassis tunnel (MCT) cluster.
The PIM snooping mechanism snoops the Join and Prune messages that are exchanged in the MCT VLANs and learns the interested VLAN member interfaces. Optimal traffic forwarding is achieved with PIM snooping states synchronized between the MCT peers, using the BGP EVPN Join Sync method.
The Reverse Path Forwarding (RPF) check and L3 multicast route lookup are based on the unicast routing protocol running in the MCT cluster.
PIM-SM and PIM-SSM can be enabled on VLAN router interfaces that are extended over MCT. These VE interfaces act as normal PIM routers, and converge just like any PIM router in an L3 domain. PIM-enabled VE interfaces on these MCT VLANs see each other as peers. They can exchange Join and Prune messages natively to form the L3 multicast forwarding tree over the interchassis link (ICL).
PIM-SM and PIM-SSM control traffic travels over the ICL link of the cluster, encapsulated with the tunnel encapsulation method used by MCT. For example MPLS/VxLAN.
For PIM-SM, any MCT node can be the rendezvous point (RP) and root of the shared multicast tree. This RP can be a PIM router on a VE interface for any extended VLAN, or it can be any other PIM-enabled L3 interface on the MCT node. Generally a PIM-enabled loopback interface performs the RP functionalities. All other functions of PIM are supported natively, such as FHR source registration, LHR thresholding and SPT switch-over, BSR protocol, and Anycast RP functionality.
The PIM Hello packet over ICL is snooped for multicast router (mrouter) detection. With this information, traffic from a directly connected multicast source in the MCT VLAN, which has a non-designated router (DR) PIM router interface, can forward on the ICL port that was learned as mrouter. The PIM-DR on the other MCT node can see the source traffic and perform the FHR functionality, which registers the multicast source with the RP.