I will show here is one of my favorite interview questions, because usually only someone with real-world experience in complex networks has seen it. Figure 13-4 shows a deceptively simple HSRP setup. Two locations, New York and Los Angeles, are connected via two T1s. The routers on either side are connected via the F0/1 interfaces, and HSRP is implemented with interface tracking on the F0/0 interfa- ces. The idea here is that if either of the primary routers should fail, the secondary routers will take over for them. Additionally, should the primary T1 link fail, the sec- ondary link will take over, because interface tracking is enabled. Here are the primary Ethernet configurations for each router: NY-Primary: interface fo/o ip address 10.10.10.2 255.255.255.0 standby ip 10.10.10.1 standby preempt standby track Serialo/o 10 Chapter 13: First Hop Redundancy New York Los Angeles NY-Primary LA-Primary .1 VIP VIP .1 NY-Backup LA-Backup Figure 13-4. Two-link failover scenario using HSRP NY-Secondary: interface fo/o ip address 10.10.10.3 255.255.255.0 standby ip 10.10.10.1 standby priority 95 standby preempt standby track Serialo/o 10 LA-Primary: interface fo/o ip address 20.20.20.2 255.255.255.0 standby ip 20.20.20.1 standby preempt standby track Serialo/o 10 LA-Secondary: interface fo/o ip address 20.20.20.3 255.255.255.0 standby ip 20.20.20.1 standby priority 95 standby preempt standby track Serialo/o 10 Should the T1 connecting NY-Primary with LA-Primary go down completely, the NY and LA routers will recognize the failure and the secondary routers will take over. But real-world problems tend to be more complex than theoretical ones, and this design doesn't work as well as we'd like. Figure 13-5 shows what can go wrong. Assume that the link between New York and Los Angeles suffers a partial outage. Something has happened to cause the serial interface on NY-Primary to enter a state of up/down, but the serial interface on LA-Primary has stayed up/up. I've seen this more than once on different kinds of circuits. When H