Routing Multicast Traffic
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multicast traffic travels across the Internet (or intranet) from a source on a
remote network to a local router and host.
Unicast data uses routing protocols to accomplish the task of getting data
to and from remote destinations. Multicast does the same, but it goes about
it in a somewhat different manner. Unicast relies on routing tables. Multicast
uses a sort of spanning tree system to distribute its data. The following sec-
tions describe the tree structures that can be implemented to allow multicast
routing. In addition to trees, several different protocol methods can be used
to achieve the desired implementation of multicast.
Distribution Trees
Two types of trees exist in multicast:
Source trees Source trees use the architecture of the source of the multi-
cast traffic as the root of the tree.
Shared trees Shared trees use an architecture in which multiple sources
share a common rendezvous point.
Each of these methods is effective and allows sourced multicast data to
reach an arbitrary number of recipients of the multicast group. Let's discuss
each of them in detail.
Source Tree
Source trees use special notation. This notation is used in what becomes a
multicast route table. Unicast route tables use the destination address and
next hop information to establish a topology for forwarding information.
Here is a sample from a unicast routing table:
B 210.70.150.0/24 [20/0] via 208.124.237.10, 3d08h
B 192.5.192.0/24 [20/0] via 208.124.237.10, 2w1d
B 193.219.28.0/24 [20/0] via 208.124.237.10, 1d03h
B 136.142.0.0/16 [20/0] via 208.124.237.10, 3d07h
B 202.213.23.0/24 [20/0] via 208.124.237.10, 1w2d
202.246.53.0/24 is variably subnetted, 2 subnets, 2 masks
B 202.246.53.0/24 [20/0] via 208.124.237.10, 1w2d
B 202.246.53.60/32 [20/0] via 208.124.237.10, 1w2d
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