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 Distribution Shared Tree

Unlike source trees that have their root at the source, shared trees use a single common root placed at

some chosen point in the network. This shared root is called a rendezvous point (RP).

Figure 8 shows a shared tree for the group 224.2.2.2 with the root located at Router D. This shared tree

is unidirectional. Source traffic is sent towards the RP on a source tree. The traffic is then forwarded

down the shared tree from the RP to reach all of the receivers (unless the receiver is located between the

source and the RP, in which case it will be serviced directly).

Figure 8 Shared Distribution Tree

In this example, multicast traffic from the sources, Hosts A and D, travels to the root (Router D) and

then down the shared tree to the two receivers, Hosts B and C. Because all sources in the multicast group

use a common shared tree, a wildcard notation written as (*, G), pronounced “star comma G,” represents

the tree. In this case, * means all sources, and G represents the multicast group. Therefore, the shared

tree shown in Figure 8 would be written as (*, 224.2.2.2).

Both source trees and shared trees are loop-free. Messages are replicated only where the tree branches.

Members of multicast groups can join or leave at any time; therefore the distribution trees must be

dynamically updated. When all the active receivers on a particular branch stop requesting the traffic for

a particular multicast group, the routers prune that branch from the distribution tree and stop forwarding

traffic down that branch. If one receiver on that branch becomes active and requests the multicast traffic,

the router will dynamically modify the distribution tree and start forwarding traffic again.

Source Tree Advantage

Source trees have the advantage of creating the optimal path between the source and the receivers. This

advantage guarantees the minimum amount of network latency for forwarding multicast traffic.

However, this optimization comes at a cost. The routers must maintain path information for each source.

In a network that has thousands of sources and thousands of groups, this overhead can quickly become

a resource issue on the routers. Memory consumption from the size of the multicast routing table is a

factor that network designers must take into consideration.

Host A

Host D

Host B Host C

Notation: (*, G)

*=all sources

G=group

Rendezvous

Point

192.168.1.1

192.168.4.4

192.168.2.2 192.168.3.3

224.2.2.2 traffic

Source 1

Source 2

Receiver Receiver

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