Multicast Static Routes

Multicast Static Routes

When using PIM, unicast and multicast routes are congruent. In other words, the unicast and multicast packets follow the same path. This makes sense because PIM uses the unicast routing table to make multicast routing decisions. Occasions can arise where you may want the unicast and multicast routing tables to diverge. For whatever reason, to accomplish this route divergence, use a static multicast route (mroute).

ip mroute source mask [protocol process-number] rpf-address | interface [distance]

source

IP address/mask of the multicast source.mask

protocol

Optional. The unicast routing (OSPF, EIGRP, and so on).

process-number

Optional. The process number of the routing protocol that is being used.

rpf-address

The incoming interface for the mroute. If the Reverse Path Forwarding address, rpf-address, is a PIM neighbor, PIM Joins, Grafts, and Prunes are sent.

interface

The interface type and number for the mroute (ethernet 0, serial 1, and so on).

distance

Optional. This determines whether a unicast route, a DVMRP route, or a static mroute should be used for the RPF lookup. The lower distances have better preference. If the static mroute has the same distance as the other two RPF sources, the static mroute takes precedence. The default is 0.

Static multicast routes are not exported or redistributed; they are local to the router on which they were configured. The first example of a static mroute is in a network in which a tunnel is used to maneuver around a non-multicast capable router (see Figure 9-8).

Figure 9-8: A static mroute is needed to direct multicast traffic over the tunnel.

Routers A and C would be configured with an mroute that directs multicast traffic to the tunnel.

ip mroute 0.0.0.0 0.0.0.0 tunnel 0

The next example involves a tunnel that drops multicast traffic right in the middle of your network from an external source (see Figure 9-9).

Figure 9-9: Static mroute needed for multicast traffic not originating in the internal network

When the RPF check is made, routes are looked up in the unicast and the static mroute tables. If we use a simple default mroute like we did in the last example, all RPF checks would point to the tunnel. We may also have internal multicast sources in our network and we would want the RPF interface to be determined from the unicast routing table and not the static mroute table. The way to accomplish this is with the following router commands:

ip mroute 172.16.0.0 255.255.0.0 null0 255

ip mroute 0.0.0.0 0.0.0.0 tunnel 0

For sources in the 172.16.0.0 network, we will have an RPF route from the unicast routing table and the mroute table. The administrative distance for the mroute is greater than that for the unicast routing table, so the unicast route will be used as the RPF. Because there is a match in the mroute table, there is no need to check any other mroutes, so the default mroute will not take affect.

For external sources, there is no route in the unicast routing table and the first mroute does not match, so the default mroute will be used. This technique is a bit strange, but it does come in handy. If you only wanted to check a particular unicast (OSPF, EIGRP, IGRP, RIP) routing protocol, use the following form:

ip mroute 0.0.0.0 0.0.0.0 ospf 100 null0 255

ip mroute 0.0.0.0 0.0.0.0 tunnel 0.

Be careful, because if you reverse the order of the ip mroute statements, then the default route will always be taken.