IP Multicast
What the IP Multicast domain tests
JNCIP-ENT tests PIM sparse and dense mode, RPF, IGMP, rendezvous points, and SSM. You must know how a receiver join becomes forwarding state and why a multicast packet is dropped.
Core rules
- PIM builds a shared tree (*,G) toward the RP in sparse mode; source-specific (S,G) trees optimize forwarding.
- RPF check uses the unicast route to the source, or to the RP for shared-tree traffic. A packet arriving on any other interface is dropped.
- IGMP is between hosts and the last-hop router; IGMP snooping constrains switch flooding.
- SSM uses 232.0.0.0/8 by default in Junos and does not require an RP for those groups.
- MSDP connects RPs across domains for sparse-mode source discovery.
Worked example
A receiver joins 239.1.1.1. The last-hop router has a route to the RP, but traffic from source 10.1.1.1 fails. The unicast route to 10.1.1.1 points to ge-0/0/1, while multicast arrives on ge-0/0/2. PIM drops the packet on RPF. Correcting the unicast route or the topology so the source is reachable via ge-0/0/2 fixes the (S,G) forwarding.
Traps
Do not configure an RP for SSM groups and expect it to be used. Do not confuse IGMP snooping with PIM; snooping is Layer 2 optimization. Check that all routers have a consistent RP mapping. A missing route to the source breaks RPF even when the join is present.