DMVPN Point to Point GRE and mGRE - DMVPN spokes can use either point-to-point GRE tunnels or multipoint GRE tunnel interface. Recently, I received a question regarding DMVPN.
In fact, the Reader asked me two questions:
When is GRE used in network design?
When is mGRE used in network design?
Answering the aforementioned questions are the basics that you must know if you are planning to design DMVPN network.
As you might have known, DMVPN is a hub, spoke type of topology. And the most useful, important functionality of DMVPN is that it provides excellent scalability by reducing the number of tunnel interfaces configured on the hub and spokes.
I mentioned the DMVPN phases in one of my articles. Because of that, I will not explain them here again. However, if you don’t understand the meaning of DMVPN phases, I would recommend that you peruse the article on DMVPN basics before reading this article. Point-to-Point GRE interface is used, only in Phase 1, on the spokes. In all the Phases, mGRE interface type is always used on the hubs.
In Phase 2 and Phase 3 of DMVPN implementation, spokes also use mGRE (not multicast GRE, but multipoint GRE) interface types. Compared to the point-to-point GRE interface, mGRE provides scalability; it reduces the configuration complexity, thus making troubleshooting very easy. The more the troubleshooting gets easy, the more the availability time of the network increases.