The purpose of ip nhrp map multicast dynamic in DMVPN

[Mitrixsen]

Hello, everyone!

I am currently studying DMVPN Phase 1 and I have a question about the following command:

ip nhrp map multicast dynamic

I understand that this is issued on the hub router/the NRHP server, but what exactly does it do? I was a little confused from the explanation that I've read

This command tells the hub router where to forward multicast packets to. Since the IP addresses of the spoke routers are unknown, we use dynamic to automatically add their IP addresses to the multicast destination list when the spokes register themselves.

And then we issue

ip nhrp map multicast nbma-IP 

on the spokes.

This command specifies which destinations should receive broadcast or multicast traffic through the tunnel interface.

I am generally confused about this command. It specifies destinations that should receive multicast/broadcast traffic? But doesn't GRE support multicast and broadcast by default? Why is this 

ip nhrp map multicast dynamic/nbma-IP

command even necessary?

Thank you in advance.

David

[M02@rt37]

Hello @Mitrixsen,

While GRE tunnels can indeed carry multicast and broadcast traffic by default, DMVPN introduces complexity by dynamically establishing direct spoke-to-spoke tunnels over the hub. These dynamically established spoke-to-spoke tunnels are unicast tunnels and do not inherently support multicast or broadcast traffic between spokes. In a DMVPN environment, the hub router plays a key role in forwarding multicast and broadcast traffic between spokes. The hub uses the command

ip nhrp map multicast dynamic

to learn about the dynamically registered spokes and can then forward multicast traffic accordingly.

[paul driver · Accepted solution]

Hello
This command negate NHS the need to statically map to each NHC for multicast traffic, which will obviously cut down on the manual configuration of any new NHC being attached and misconfiguration, the additional command that would be required on the NHC would be to register itself to the NHS 

( ip nhrp nhs  <tunnel ip of NHS> )