DMVPN Phase 2 BGP Routing

Hello Azm

In DMVPN Phase 2, the hub router has a multipoint GRE tunnel interface, and the spokes have point-to-point GRE tunnel interfaces. This means that on the spokes, each tunnel interface is point-to-point, meaning it is designed to connect to only one other interface. The spokes are configured to establish a point-to-point GRE tunnel with the hub.

So by default, spokes can only “see” the hub and not each other. They are not aware of each other’s existence directly, and thus can’t form neighbor relationships or communicate directly with each other.

Now DMVPN Phase 2 uses NHRP to allow spokes to learn about each other and establish direct tunnels between themselves, bypassing the hub. However, the process to establish these direct tunnels involves the hub and NHRP. It is not automatic and requires the spokes to request the necessary information from the hub.

Note that routing protocols such as OSPF and EIGRP rely on the broadcast and multicast capabilities of the underlying network to form adjacencies. Since the spokes don’t directly see each other and require the hub and NHRP to intervene to “simulate” a direct connection, this arrangement does not support the automatic discovery of neighbors using broadcast/multicast. Thus these routing protocols cannot see each other.

Just a note, RIP does not create neighbor adjacencies.

Now why does BGP allow for the creation of neighbors? Well, this is because, unlike OSPF and EIGRP, BGP uses TCP connections to the specific network address of the neighbor to create its adjacencies. DMVPN phase 2 supports such direct communication implicitly, so there is no restriction there. This applies for both eBGP and iBGP…

I hope this has been helpful!

Laz