Enabling IP Multicast Routing

Enabling IP Multicast Routing
As we have said, multicast routing must be enabled on the router. This step is very straightforward,
but without it, multicast will not work. Let’s look at a configuration of a router that
does not have multicast enabled:
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Terry_3640
!
aaa new-model
aaa authentication login default tacacs+ line
aaa authentication login oldstyle line
aaa accounting exec default start-stop tacacs+
enable secret 5 $1$G7Dq$em.LpM4Huem9uqjZDHLe4.
!
!
!
ip subnet-zero
ip telnet source-interface FastEthernet3/0
[output truncated]


Notice that no multicast information is running on this machine. If we were to try to execute
a multicast-related command, we wouldn’t get any information returned. For example, look at
what happens when the show ip mroute command is issued:
Terry_3640#sho ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, C - Connected, L - Local,
P - Pruned R - RP-bit set, F - Register flag,
T - SPT-bit set,J - Join SPT, M - MSDP created entry,
X - Proxy Join Timer Running
A - Advertised via MSDP
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
Terry_3640#
The command is ip multicast-routing, and an example of the execution follows:
Terry_3640#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Terry_3640(config)#ip multicast-routing
Terry_3640(config)#^Z
Terry_3640#
This enables the multicast on the router. Notice that it was executed while in global configuration
mode. However, the router still cannot exchange multicast information with any neighbors
because none of the interfaces have been enabled. This step is next.