SRP 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 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. 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 establishes 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. 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. Chapter 13: First Hop Redundancy New York Los Angeles NY-Primar