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Routing Multicast Traffic

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F I G U R E 9 . 1 2

Source tree forwarding

Also notice in the drawing that the shortest path to the receivers was cho-

sen. This is known as choosing the shortest path tree (SPT). You can see from
the preceding output that there are three sources for the same group of
224.2.127.254. This indicates that there are three SPT groups shown here:
(195.134.100.102, 224.2.127.254), (207.98.103.221, 224.2.127.254), and
(128.39.2.23, 224.2.127.254). Each of these sources has its own shortest
path tree to the receivers.

Shared Tree

There are two types of shared tree distribution:

Unidirectional

Bidirectional

They both work a little differently than source tree distribution. Shared

tree architecture lies in the characteristic that there may be multiple sources
for one multicast group. Instead of each individual source creating its own
SPT and distributing the data apart from the other sources, a shared root is
designated. Multiple sources for a multicast group forward their data to a

Corporate

Intranet

HostA

HostB

HostD

HostC

Router1

Router3

Router2

Router4

Switch 1

Switch 2

Switch 3

Switch 4

Source 172.16.1.51

for group 225.3.1.100

Member 224.2.127.254

Member 225.3.1.100

225.3.1.100 data

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