D-Link DES-3326S manual Reverse Path Multicasting RPM, 118

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide

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.

118 Switch Management and Operating Concepts