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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
The transition states a host will go through to join or leave a multicast group are shown in the diagram
below.
Figure 5-19. IGMP State Transitions
Multicast Routing Algorithms
An algorithm is not a program. An algorithm is a statement of how a problem can be solved. A program is
written to implement an algorithm.
Multicast packets are delivered by constructing multicast trees where the multicast router is the trunk,
the branches are the various subnetworks that may be present, and the leaves are end recipients of the
multicast packets. Several algorithms have been developed to construct these trees and to prune
branches that have no active mulitcast group members.
Flooding
The simplest algorithm for the delivery of multicast packets is for the multicast router to forward a
multicast packet to all interfaces. This is referred to as flooding. An equally simple 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 interfaces, 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
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