Multicast Spanning Trees; Reverse Path Broadcasting (Rpb) - Enterasys VH-2402-L3 Management Manual

refinement of flooding is to have the router check to
determine if a given multicast packet has been received
before (in a certain amount of time). If it has, then the
packet does not need to be forwarded at all and can be
dropped. If the packet is being received for the first time, it
should be flooded to all interface, except the interface on
which it was received. This will ensure that all routers on the
network will receive at least one copy of the multicast
packet.
There are some obvious disadvantages to this simple
algorithm. Flooding duplicates a lot of packets and uses a
lot of network bandwidth. A multicast router must also keep
a record of the multicast packets it has received (for a period
of time) to determine if a given packet has been previously
received. So flooding uses a lot of router memory.

Multicast Spanning Trees

A multicast delivery tree that spans the entire network with a
single active link between routers (or subnetwork) is called a
multicast spanning tree. Links (or branches) are chosen
such that there is only one active path between any two
routers. When a router receives a multicast packet, it
forwards the 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
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