Network Warrior

HSRP Interface Tracking

While HSRP is a wonderful solution that enables recovery from router or Ethernet interface failures, its basic functionality falls short in another scenario. Figure 13-3 depicts a more complex problem than the one considered previously. In this scenario, the serial link connecting Router A to the Internet has failed. Because the router and Ethernet interfaces are still up, HSRP is still able to send and receive hello packets, and Router A remains the active router.

Figure 13-3. Primary Internet link failure without interface tracking

The network is resilient, so the packets will still get to the Internet via the F0/1 interfaces—but why add another hop when we don't need to? If we could somehow influence the HSRP priority based on the status of another interface, we could fail the VIP from Router A over to Router B based on the status of S0/0. HSRP interface tracking allows us to do exactly that.

By adding a couple of simple commands to our HSRP configurations, we can create a design that will allow the Ethernet interfaces to failover in the result of a serial interface failure:

• Router A:

  interface f0/0
   ip address 192.168.100.2 255.255.255.0
   standby ip 192.168.100.1
   standby preemptstandby track Serial0/0 10

• Router B:

  interface f0/0
   ip address 192.168.100.3 255.255.255.0
   standby ip 192.168.100.1
   standby priority 95
   standby preemptstandby track Serial0/0 10

On each ...