DMVPN - multicast commands

When deploying a DMVPN there are some specific commands that are issued on both the hub and the spokes in order to allow the topology to deal with Multicast traffic. DMVPN deals with multicast traffic in a particular way, due to the use of NHRP.

On the hub, the following command is used for multicast:

ip nhrp map multicast dynamic

This command tells the router to dynamically learn and map multicast sources to remote sites. When the hub receives multicast traffic, it will use NHRP to determine which remote sites are interested in the multicast group and then forward the multicast traffic only to those interested sites. If the command is not issued, the hub router will treat multicast traffic unicast traffic, sending a copy to each remote site regardless of whether they have active receivers interested in the multicast group.

Since the IPv4 addresses of the spoke routers are unknown, we use the dynamic keyword to automatically add their IP addresses to the multicast destination list when the spokes register themselves.

On the spoke, the following command is issued for multicast:

ip nhrp map multicast <ip_address>

...where <ip_address> is the address of the hub. This command essentially defines a mapping for multicast traffic, so that the spoke will send all multicast traffic to the hub. If the command is not issued, the spoke may send the multicast traffic to multiple hubs (if they exist) or it may even broadcast it over the entire network.

Particularly in Phase 2, it is interesting to note that multicast traffic will not trigger an NHRP redirect. Only unicast traffic will do so.