D-Link DGS-3308TG User Manual page 57

8-port Gigabit Ethernet Switch User's Guide
Reverse Path Broadcasting (RPB)
The RPB algorithm is a modification of the spanning tree algorithm. Instead of building a network-wide spanning tree, a
virtual spanning tree is constructed for each multicast source.
When a router receives a multicast packet from a source, the router will check to see if the link on which the packet was
received is the shortest path to the source. If it is, the packet is forwarded to all ports except the one on which it was
received. If it is not, the packet is dropped.
If a link-state routing protocol is in use (such as OSPF), the router can determine if it is on the shortest path between itself
and a neighboring router. If it is not, then the packet would be discarded at the next router. A link-state routing protocol
would provide this information, so the first router could discard the packet.
If a distance-vector routing protocol (such as RIP) is being used, the neighboring router can advertise its previous hop for
the source as part of its routing table update messages or it can 'poision-reverse' the route.
RPB does not use multicast membership information in the construction of multicast distribution trees.
Truncated Reverse Path Broadcasting (TRPB)
The TRPB algorithm is a modification of the RPB algorithm. It uses IGMP to determine if members of a multicast group
are present on the router's subnetwork. If the subnetwork has no multicast members and it is a leaf router (the only router
on the subnetwork), TRPB will truncate the distribution tree. If the router is not a leaf router, the tree is not changed.
TRPB does use multicast group membership information in the construction of distribution trees.
Reverse Path Multicasting (RPM)
The RPM algorithm is an enhancement of the RPB and TRPB algorithms. RPM constructs delivery trees that span only
subnetworks with group members or subnetworks along the shortest path to routers attached to subnetworks that have
group members.
The RPM tree is then 'pruned' so that multicast packets are forwarded only along paths that lead to group members.
The first multicast packet received by the router is forwarded according to the RPB algorithm. Leaf routers that receive a
multicast packet for which they have no group members will send a 'prune' message back to the router from which the
message was received.
Prune messages indicate that multicast packets for a given membership group should not be forwarded on the link as
there are no group members. Prune messages have a TTL of one, so they are only sent back one hop from the router that
sends them.
The router one hop closer to the multicast source records the prune information in its memory. If the closer router has no
group members on its subnetwork, it will send its own prune message to the next router on the path back to the multicast
source, and so on. This is continued until multicast packets from a given source are only forwarded on paths that lead to
multicast group members for that source.
The group membership and the topology of the network and the multicast distribution trees can change dynamically. To
accommodate this, the RPM algorithm periodically removes all the prune information from the router's memory. The next
multicast packet received by the router gives new multicast group members on its subnet a chance to join the multicast
group and leaf routers with new members on their subnetworks also get a chance to join.
RPM requires a relatively large amount of router memory space to maintain all the information for the multicast source
and group members.
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