DMVPN Dual Cloud Path Selection Using IGP Metrics
In a DMVPN implementation where multiple DMVPN clouds exist, spoke-to-spoke communication path selection can be controlled through the metric of the underlying Interior Gateway Protocol (IGP) being used. In a dual-hub DMVPN design, spokes typically have equal-cost paths to other spokes via both hubs. The routing table on the spoke determines which hub will be queried for NHRP resolution, and thus, which cloud will be used for data transmission.
For example, in a case where EIGRP is being used, if a spoke needs to reach destination 4.4.4.4 behind another spoke, and has the following routes:
D 4.4.4.4 [90/28288000] via 172.16.2.2, Tunnel2 [90/28288000] via 172.16.1.1, Tunnel1
The EIGRP metrics can be adjusted to prefer one path over another. When the spoke queries the preferred hub using NHRP, it will receive the next-hop IP address of the remote spoke within that specific DMVPN cloud. This allows administrators to control which DMVPN cloud will be used for spoke-to-spoke communication.