Cisco IOS Cookbook, 2nd Edition

DVMRP Tunnels

Problem

You want to create a DVMRP tunnel to bypass a section of network that doesn’t support multicast.

Solution

You can create a DVMRP tunnel from a Cisco router to a nonCisco DVMRP device by using the special DVMRP tunnel mode. This allows you to pass multicast traffic through a section of network that doesn’t support multicast routing:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip multicast-routing
Router1(config)#interface Tunnel0
Router1(config-if)#ip unnumbered FastEthernet0/0
Router1(config-if)#ip pim sparse-dense-mode
Router1(config-if)#ip dvmrp unicast-routing
Router1(config-if)#tunnel source FastEthernet0/0
Router1(config-if)#tunnel destination 192.168.99.15
Router1(config-if)#tunnel mode dvmrp
Router1(config-if)#exit
Router1(config)#interface FastEthernet0/0
Router1(config-if)#ip address 192.168.1.1 255.255.255.0
Router1(config-if)#ip pim sparse-dense-mode
Router1(config-if)#end
Router1#

Discussion

There are several important things to remember about DVMRP tunnels. First, you cannot create a DVMRP tunnel between two Cisco routers because neither router will send DVMRP Probe messages. So this feature is only used to create tunnels between Cisco routers and native DVMRP devices such as nonCisco routers or Unix servers running mrouted.

Second, a DVMRP tunnel is essentially just a standard IP-in-IP tunnel. However, it is just used to transmit multicast traffic. Unicast traffic follows the normal network ...