Source Trees

Source Trees
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
Multicast route tables are somewhat different. A sample of a multicast table follows. Notice
that the notation is different. Instead of having the destination address listed and then the next
hop to get to the destination, source tree uses the notation (S, G). This notation specifies the
source host’s IP address and the multicast group address for which it is sourcing information.
Let’s take the first one, for example. This is seen as (198.32.163.74, 224.2.243.55), which
means that the source host is 198.32.163.74 and it is sourcing traffic for the multicast group
224.2.243.55:
(198.32.163.74, 224.2.243.55), 00:01:04/00:01:55, flags: PT
Incoming interface: POS1/0/0, RPF nbr 208.124.237.10, Mbgp
Outgoing interface list: Null
(198.32.163.74, 224.2.213.101), 00:02:06/00:00:53, flags: PT
Incoming interface: POS1/0/0, RPF nbr 208.124.237.10, Mbgp
Outgoing interface list: Null
(195.134.100.102, 224.2.127.254), 00:00:28/00:02:31, flags: CLM
Incoming interface: POS1/0/0, RPF nbr 208.124.237.10, Mbgp
Outgoing interface list:
FastEthernet4/0/0, Forward/Sparse, 00:00:28/00:02:54

FastEthernet4/1/0, Forward/Sparse, 00:00:28/00:02:31
(207.98.103.221, 224.2.127.254), 00:00:40/00:02:19, flags: CLM
Incoming interface: POS1/0/0, RPF nbr 208.124.237.10, Mbgp
Outgoing interface list:
FastEthernet4/0/0, Forward/Sparse, 00:00:41/00:02:53
FastEthernet4/1/0, Forward/Sparse, 00:00:41/00:02:19
(128.39.2.23, 224.2.127.254), 00:04:43/00:02:06, flags: CLMT
Incoming interface: POS1/0/0, RPF nbr 208.124.237.10, Mbgp
Outgoing interface list:
FastEthernet4/0/0, Forward/Sparse, 00:04:43/00:02:43
FastEthernet4/1/0, Forward/Sparse, 00:04:43/00:03:07
(129.237.25.152, 224.2.177.155), 00:17:58/00:03:29, flags: MT
Incoming interface: POS1/0/0, RPF nbr 208.124.237.10, Mbgp
Outgoing interface list:
FastEthernet4/0/0, Forward/Sparse, 00:17:58/00:02:44
Figure 19.12 gives you a good picture of how source trees work.