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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
118 Switch Management and Operating Concepts
Reverse Path Multicasting (RPM)
Reverse Path Multicasting (RPM) introduces an enhancement to
RPB – an explicit method to prune branches of the spanning
tree that have on active multicast group members for the
source. RPM constructs a tree that spans only subnetworks
with multicast group member and routers along the shortest
path between the source and the destinations.
When a multicast router receives a multicast packet, it is
forwarded using the RPB constructed spanning tree.
Subsequent routers in the tree that have no active path to
another router are referred to as leaf routers. If the multicast
packet if forwarded to a leaf router that has no active multicast
group members for the source, the leaf router will send a prune
message to the previous router. This will remove the leaf
router’s branch from the spanning tree, and no more multicast
packets (from that source) will be forwarded to it. Prune
messages have a TTL equal to one, so they can be sent only one
hop (one router) back toward the source. If the previous router
receives prune messages from all of its branch and leaf routers,
the previous router will then send it’s own prune message back
one router toward the multicast source, and the process will
repeat. In this way, multicast group membership information
can be used to prune the spanning tree between a given
multicast source and the corresponding multicast group.
Since the membership of any given multicast group can change
and the network topology can also change, RPM periodically
removes all of the prune information it has gathered from it’s
memory, and the entire process repeats. This gives all
subsequent routers on the network a chance to receive
multicast packets from all multicast sources on the network. It
also gives all user’s a chance to join a given multicast group.