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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
116 Switch Management and Operating Concepts
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
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.