The following diagram illustrates how the PIM-SSM architecture requires routers to perform the following actions:
support IGMPv3 source-specific host membership reports and queries at the edge routers
initiate PIM-SSM (S,G) joins directly and immediately after receiving an IGMPv3 join report from the designated router
restrict forwarding to SPTs within the SSM address range by all PIM-SSM routers
The following rules apply to Layer 3 devices with SSM enabled:
Receive IGMPv3 membership join reports in the SSM range and, if no entry (S,G) exists in the SSM channel table, create one.
Receive IGMPv2 membership join reports, but only for groups that already use a static (S,G) entry in the SSM channel table.
Send periodic join messages to maintain a steady SSM tree state.
Use standard PIM-SM SPT procedures for unicast routing changes, but ignore rules associated with the SPT for the (S,G) route entry.
Receive prune messages and use standard PIM-SM procedures to remove interfaces from the source tree.
Forward data packets to interfaces from the downstream neighbors that sent an SSM join, or to interfaces with locally attached SSM group members.
Drop data packets that do not use an exact-match lookup (S,G) in their forwarding database for S and G.