Multicast Routing Configuration - dummies

By Edward Tetz

Enabling basic multicast routing functionality on your network does not require a lot of work and can be implemented fairly quickly. Some configuration is done in Global Configuration mode and some configuration is done in Interface Configuration mode.

Enabling multicast routing

The first command that you perform in Global Configuration mode is the ip multicast-routing command, which enables multicast routing for the router. Enabling multicast routing initially does not route any data, due in fact that the default configuration leaves all router interfaces without multicast routing in their configuration.

To have multicast routing in a working configuration, you must enter each interface and configure the PIM on those interfaces. To enable PIM on an interface, you use the ip pim command, which accepts either a dense-mode or sparse-mode option. When you enable PIM on an interface, you automatically enable IGMP on those interfaces.

Supporting Sparse-Dense mode

Some groups may require Sparse mode or Dense mode in order to operate properly. Because you configure an interface for one mode or the other, you might find yourself in a catch-22 where you need both installed. To deal with this issue, Cisco supports all modes on its IOS routers, where the mode is applied to the multicast group on the router and not its interfaces.

Because you may end up using Sparse mode for some groups, you require an RP to be configured. When using Sparse-Dense mode, you need to implement it throughout the network to avoid conflicts between Sparse mode and Dense mode interfaces. You implement the Sparse-Dense mode with the command ip pim sparse-dense-mode.

As just mentioned, an RP is a router on your network, so you need to assign this function to one or more routers on your network. Doing so does not require configuration on the RP router, but rather on the downstream router. The downstream router requires the address of an RP, which is accomplished by running the ip pim rp-address rp-address [access-list] [override] command using Global Configuration mode.

Sparse mode implementations require the RP to be specified, although Sparse-Dense mode it is an option as you can use Auto-RP.

Using Auto-RP rather than RPs

Rather than specifying specific RP routers, you can use Auto-RP to automate the selection and configuration of RP routers on your network. Auto-RP allows you to easily use multiple routers on your network because the RPs may be serving different groups or splitting the load over several routers. The routers could be based on the location of a multicast group’s receivers and connectivity issues could be caused by inconsistent configuration across your network.

To configure Auto-RP mode, you need to make two configuration changes, one on the router you want to act as an RP and one on the router that will manage group mappings to RPs. The first command is issued in Global Configuration mode:

ip pim send-rp-announce type number scope ttl-value [group-list access-list] [interval seconds]

whereas the second command is

ip pim send-rp-discovery scope ttl-value

Testing connectivity

You can configure your router to be a member of a multicast group so that you can test connectivity. To add your router as a member, use the ip igmp join-group group-address command in Interface Configuration mode. By default, the router uses IGMP version 2, which maximizes functionality, because version 3 has some restrictions.

To change versions, use Interface Configuration mode and enter this command: ip igmp version {3 | 2 | 1. All routers on the same network segment must use the same version of IGMP, but IGMP version 2 routers will work correctly if IGMP version 1 routers are on the network. So, given the network layout, Router2 may have the following configuration commands to enable PIM routing — Router1’s setup commands then follow.

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#ip multicast-routing
Router2(config)#ip pim rp-address
Router2(config)#interface FastEthernet 0/0
Router2(config-if)#ip address
Router2(config-if)#ip pim sparse-dense-mode
Router2(config)#ip pim send-rp-announce FastEthernet 0/0 scope 16 group-list 1
Router2(config)#access-list 1 permit

To set up Router 1, use these commands:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip multicast-routing
Router1(config)#ip pim rp-address
Router1(config)#interface FastEthernet 0/0
Router1(config-if)#ip address
Router1(config-if)#ip pim sparse-dense-mode
Router1(config)#ip pim send-rp-discovery scope 16