If you unplug one side of a Metro-E circuit, the far side will not go down with most installations. HSRP responds to the down interface on the New York side by making the NY- Backup router active because we're tracking the serial interface on NY-Primary. Packets are forwarded to NY-Backup, and then across the T1 to LA-Backup, which forwards them to their destinations. The return packets have a problem, though. As the LA-Primary router does not recognize the link failure on the primary T1, it has remained the active router. Return packets are sent to the LA-Primary router, and because it believes the link is still up, it forwards the packets out the SO/O interface, where they die because the other side of the link is down. A more robust solution to a link-failover scenario is to incorporate an interior gateway protocol running on all of the routers. A protocol like OSPF or EIGRP estab- lishes neighbor adjacencies across links. When a link fails, the routing protocol knows that the remote neighbor is unavailable, and removes the link from the routing table. Chapter 13: Resilient Ethernet The solution therefore includes a routing protocol in addition to HSRP. Figure 13-6 shows the same network, but with EIGRP included. Now when the NY-Primary side of the primary links fails, EIGRP loses the neighbor adjacency between NY-Primary and LA-Primary, and removes the link from each router's routing table. Because EIGRP alters the routing tables, it can route around the failed link, even though one side reports an up/up condition. HSRP is still required in this design, as EIGRP has no means of making two routers appear to be a single gateway on the local Ethernet segments. New York Los Angeles NY-Primary LA-Primary Up/down Up/up .1 VIP EIGRP 100 VIP .1 NY-Backup LA-Backup Figure 13-6. Better failover design using EIGRP When HSRP Isn't Enough CHAPTER 14 Route Maps Route maps are the bane of many people studying for certification exams. I think the reason for this lies in the way route maps are designed. They're a little bit b