Cisco Systems 78-11741-02 manual Configuring an IP Multicast Static Route, IPC-429

Configuring IP Multicast Routing

Configuring an IP Multicast Static Route

Configuring an IP Multicast Static Route

IP multicast static routes (mroutes) allow you to have multicast paths diverge from the unicast paths. When using PIM, the router expects to receive packets on the same interface where it sends unicast packets back to the source. This expectation is beneficial if your multicast and unicast topologies are congruent. However, you might want unicast packets to take one path and multicast packets to take another.

The most common reason for using separate unicast and multicast paths is tunneling. When a path between a source and a destination does not support multicast routing, a solution is to configure two routers with a GRE tunnel between them. In Figure 68, each unicast router (UR) supports unicast packets only; each multicast router (MR) supports multicast packets.

Figure 68 Tunnel for Multicast Packets

MR 1 UR 1 UR 2 MR 2

Source

Link

Tunnel

Destination

43278

In Figure 68, Source delivers multicast packets to Destination by using MR 1 and MR 2. MR 2 accepts the multicast packet only if it believes it can reach Source over the tunnel. If this situation is true, when Destination sends unicast packets to Source, MR 2 sends them over the tunnel. Sending the packet over the tunnel could be slower than natively sending the it through UR 2, UR 1, and MR 1.

Prior to multicast static routes, the configuration in Figure 69 was used to overcome the problem of both unicasts and multicasts using the tunnel. In this figure, MR 1 and MR 2 are used as multicast routers only. When Destination sends unicast packets to Source, it uses the (UR 3, UR 2, UR 1) path. When Destination sends multicast packets, the UR routers do not understand or forward them. However, the MR routers forward the packets.

Figure 69 Separate Paths for Unicast and Multicast Packets

	UR 1	UR 2	UR 3
Source	MR 1		MR 2

Link

Tunnel

Destination

43279

To make the configuration in Figure 69 work, MR 1 and MR 2 must run another routing protocol (typically a different instantiation of the same protocol running in the UR routers), so that paths from sources are learned dynamically.

Cisco IOS IP Configuration Guide

IPC-429