Specifications
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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
OL-21227-02
Chapter 1 Cisco MWR 2941 Router Overview
Features
Multicast Forwarding
Forwarding of multicast traffic is accomplished by multicast-capable routers. These routers create
distribution trees that control the path that IP multicast traffic takes through the network in order to
deliver traffic to all receivers.
Multicast traffic flows from the source to the multicast group over a distribution tree that connects all of
the sources to all of the receivers in the group. This tree may be shared by all sources (a shared tree) or
a separate distribution tree can be built for each source (a source tree). The shared tree may be one-way
or bidirectional.
Before describing the structure of source and shared trees, it is helpful to explain the notations that are
used in multicast routing tables. These notations include the following:
• (S,G) = (unicast source for the multicast group G, multicast group G)
• (*,G) = (any source for the multicast group G, multicast group G)
The notation of (S,G), pronounced “S comma G,” enumerates a shortest path tree where S is the IP
address of the source and G is the multicast group address.
Shared trees are (*,G) and the source trees are (S,G) and always routed at the sources.
Multicast Distribution Source Tree (Shortest Path Tree)
The simplest form of a multicast distribution tree is a source tree. A source tree has its root at the source
host and has branches forming a spanning tree through the network to the receivers. Because this tree
uses the shortest path through the network, it is also referred to as a shortest path tree (SPT).
Figure 7 shows an example of an SPT for group 224.1.1.1 rooted at the source, Host A, and connecting
two receivers, Hosts B and C.
Figure 7 Source Tree
Using standard notation, the SPT for the example shown in Figure 7 would be (192.168.1.1, 224.1.1.1).
The (S,G) notation implies that a separate SPT exists for each individual source sending to each
group—which is correct.
Host A
Host B Host C
Notation: (S, G)
S = source
G=group
AB
CE
DF
192.168.1.1
192.168.2.2 192.168.3.3
224.1.1.1 traffic
Source
Receiver Receiver
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