"ip nhrp map multicast" command and DMVPN

[Sam Brynes]

I am just starting to learn about DMVPN. If we use the command "ip nhrp map multicast <IP>" on the spokes, my understanding is that it will send multicast traffic to the configured NBMA of the hub. Does the hub then re-send this multicast traffic to all the spokes?

If that is the case, if we use a routing protocol that performs dynamic neighbor discovery by using multicast traffic, why don't the spokes become neighbors with each other?

Sam

[MHM Cisco World]

[Sam Brynes]

Thanks MHM Cisco World. Maybe I should have reworded my question to say:

"If that is the case, if we use a routing protocol that performs dynamic neighbor discovery by using multicast traffic, why don't the spokes become neighbors with each other by virtue of the multicast traffic being sent through the hub and back out to the spokes?"

[MHM Cisco World]

Hi friend'

The multicast between spokes must pass through hub' this limitation of dmvpn.

For unicast traffic the next-hop of prefix can point to spoke.

MHM

[paul driver]

Hello
That command is ONLY applicable to NHS (DMVPN hubs), it enables each NHC (spoke) to registrar its NBMA address to the HUB
so from NHS(hub) perspective the spoke NBMA is dynamically learned/created, this solve the administrative burden of statically mapping to each NHC(spoke) from the NHS.

Note: the NHCs will always need to statically map towards the NHS NBMA address for multicast

Example
NHS hub (nbma = 1.1.1.1)
int tunnel 100
ip address 100.100.100.254 255.255.255.0
ip nhrp network-id 1234
ip nhrp map multicast dynamic <  on by default)

NHC1 spoke
int tunnel 100
ip address 100.100.100.1 255.255.255.0
ip nhrp network-id 1234
ip nhrp nhs 100.100.100.254 nbma 1.1.1.1 multicast


NHC2 spoke
int tunnel 100
ip address 100.100.100.2 255.255.255.0
ip nhrp network-id 1234
ip nhrp nhs 100.100.100.254 nbma 1.1.1.1 multicast

etc...