ces, and HSRP is implemented with interface tracking on the F0/0 interfaces. The idea here is that if either of the primary routers should fail, the sec- ondary routers will take over for them. Additionally, should the primary T1 link fail, the secondary link should 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: Resilient Ethernet 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. When HSRP Isn't Enough New York Los Angeles NY-Primary LA-Primary Up/down Up/up .1 VIP VIP .1 NY-Backup LA-Backup Figure 13-5. HSRP limitations 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. Metropolitan Area Ethernet (Metro-E) is susceptible to this condition. Because the Metro-E link is usually a SONET transport that's converted to Ethernet, link integrity is local to each side. 