NHRP and EIGRP

[JustasP]

Hello, I am a complete newbie at this and I wanted to ask for clarification. Using NHRP on a DMVPN is a must, but routing protocols such as EIGRP are also used in configurations of DMVPN. From my understanding NHRP maps out the network through registering all spokes connected to the hub and then helps other spokes learn optimal paths to other spokes. My question is, what is the purpose of routing protocols like EIGRP in DMVPN since it is also used to create optimal routes from one device to the other? I apologize in advance if my question doesn't make full sense, but let me know and I'll try to expand on my question.

[balaji.bandi]

As per my understanding, in DMVPN 

Server and Client

The server is act as Hub and has all the information when the clients like to interact.

Client - this will be a branch and it will connect to Server all time.

Server (NHRP) keeps all the IP address information of Clients in the cache.

Client ( NHRP client) - they register with NHRP Server to register themselves

Normal porcess : 

Depending on DMVPN Phases, when the client needs to talk to another client it requests NHRP Server information, Server will direct the client to client communication.

You use EIGRP over VPN to enable Dynamic Routing.

[Georg Pauwen]

Hello,

actually, in a DMVPN, you could use static routes only, but that would defeat the 'dynamic' part of Dynamic Multipoint Virtual Private Network. After all, the entire purpose of having DMVPN is to build on-demand, dynamic tunnels. That is why EIGRP, BGP, OSPF (and ODR as well) are being used.

So using a dynamic routing protocol is basically an integral part of DMVPN.

[Richard Burts]

Perhaps one way to look at this question is that NHRP works to help the remote find and connect to other participants in DMVPN. NHRP does not deal with what subnets are available from the hub(s) and what subnets are available from other remotes. As @Georg Pauwen points out you need some routing information, preferable a dynamic routing protocol, to optimize access to those other networks/subnets.

[Joseph W. Doherty]

This might also help: https://learningnetwork.cisco.com/s/article/dmvpn-concepts-amp-configuration