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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
packet on all links except the one on which it was received. This guarantees that all routers in the
network will receive a copy of the packet. The only information the router needs to store is whether a
link is a part of the spanning tree (leads to a router) or not.
Multicast spanning trees do not use group membership information when deciding to forward or drop a
given multicast packet.
Reverse Path Broadcasting (RPB)
The Reverse Path Broadcasting (RPB) algorithm is an enhancement of the multicast spanning tree
algorithm. RPB constructs a spanning tree for each multicast source. When the router receives a
multicast packet, it then checks to determine if the packet was received on the shortest path back from
the router to the source. If the packet was received on the shortest path back to the source, the packet
is forwarded on all links except the link on which the packet was received. If the packet was not
received on the shortest link back to the source, the packet is dropped.
If a link-state routing protocol is in use, RPB on a local router can determine if the path from the
source through the local router to an immediately neighboring router. If it is not, the packet will be
dropped at the next router and the packet should not be forwarded.
If a distance-vector routing protocol is in use, a neighboring router can either advertise its previous hop
for the source as part of its routing update messages. This will ‘poison-reverse’ the route (or have the
local router prune the branch from the multicast source to the neighboring router because the
neighboring router has a better route from the source to the next router or subnetwork).
Since multicast packets are forwarded through the shortest route between source and destination, RPB
is fast. A given router also does not need information about the entire spanning tree, nor does it need a
mechanism to stop the forwarding of packets.
RPB does not use multicast group membership information in its forwarding decisions.
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 its 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 users a chance to join a given
multicast group.
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