You may have heard that you should not run OSPF over DMVPN but do you actually know why?
There is actually no technical reason why you cannot run OSPF over DMVPN but it does not scale very well.
So why is that?
The reason is that OSPF is a link state protocol so each spoke router has to contain the topology database. Also as the DMVPN cloud has to be a single subnet all the spoke routers have to be in the same OSPF area.
Typical design rules specify between 50-100 routers per area. This is not such an issue any more with newer routers but you do need to ensure your branch router has the memory / cpu requirements.
What is the alternative to OSPF over DMVPV?
According to the Cisco Design Guide the preferred routing protocol to use is EIGRP.
EIGRP does not need to have a topology database of the entire network, it is only concerned with routing to its neighbors.
I will be posting a series of posts on DMVPN configuration in the coming weeks and will go into more depth on this subject and how to scale DMVPN to 1000’s of remote sites.