Dual Hubs in Active/Standby Mode

In basic DSVPN networking, all Spokes are connected to one Hub. Spokes cannot communicate with each other if the Hub fails. Multiple Hubs can be deployed to improve network reliability.

In Figure 1, Hub1 and Hub2 are deployed in the headquarters. Routing policies are deployed on Spokes so that routes to Hub1 have a higher priority than those to Hub2. Normally, Hub1 is the active Hub and Hub2 is the standby Hub.

Figure 1 Dual Hubs in active/standby mode

The working mechanism is as follows:

All Spokes are registered with Hub1 and Hub2, and establish active and standby static mGRE tunnels with Hub1 and Hub2 respectively.
The source Spoke sends an NHRP Resolution Request packet to request the public address of the destination Spoke, which is used to establish a dynamic mGRE tunnel.
- When Hub1 and Hub2 work properly, the NHRP Resolution Request packet is sent to the Hub over the active static mGRE tunnel because the route from the source Spoke to Hub1 has a higher priority. Hub2 forwards the NHRP Resolution Request packet to the destination Spoke.
- When Hub1 fails, the priority of the route from the source Spoke to Hub1 is reduced and the NHRP Resolution Request packet is sent to Hub2 over the standby static mGRE tunnel. Hub2 then sends the NHRP Resolution Request packet to the destination Spoke.
- When Hub1 recovers, the NHRP Resolution Request packet is forwarded by Hub1. This is because the route from each Spoke to Hub1 has a higher priority than the route from each Spoke to Hub2.
The destination Spoke sends an NHRP Resolution Reply packet to the source Spoke, and a dynamic mGRE tunnel is set up.
After the dynamic mGRE tunnel is set up, Spokes can directly communicate with each other. In this case, the Hub running status does not affect service flows between Spokes. If the dynamic mGRE tunnel between branch Spokes is torn down because no traffic passes through the tunnel for a long period of time, the Spokes need to reestablish a dynamic mGRE tunnel. The Spokes then determine the Hub to which they send NHRP Resolution Request packets based on the route priority.