mple design with a redundant pair of routers acting as a default gateway. The normal design in such a case would dictate that one router is a primary, and one is a secondary (or, in HSRP terms, one is active and one is standby). However, this diagram does not contain enough information to determine how the network is behaving. Which router is actually forwarding packets? Are they both forwarding packets? Is one a primary and the other a secondary router? Internet Router A Router B Ethernet Figure 13-1. Simple HSRP design To configure HSRP for this network, we'll need to determine three things ahead of time: the IP address of Router A's Ethernet interface, the IP address of Router B's Ethernet interface, and a Virtual IP address (VIP) that will act as the gateway for the devices on the network. The IP addresses of the router interfaces never change, and neither does the VIP. The only thing that changes in the event of a failure is who owns the VIP. The VIP is active on whichever router has the highest priority. The priority defaults to a value of 100, and can be configured to any value between 0 and 255. All routers that are in the same HSRP group (the default group is 0) send out HSRP packets to the multicast address 224.0.0.2 using UDP port 1985. All HSRP packets have a time-to-live (TTL) of 1, SO they will not escape the local Ethernet segment. When a router with an interface running HSRP starts that interface (or at any time the interface comes up), HSRP sends out hello packets and waits to see if any other HSRP routers are found. If more than one HSRP router is found, the routers Chapter 13: Resilient Ethernet negotiate to determine who should become the active router. The router with the highest priority becomes the active router, unless there is a tie, in which case the router with the highest configured IP address becomes the active router. In our example, we'll apply the three IP addresses needed as shown in Figure 13-2. Internet Router A Router B Ethernet 192.168.100.0/24 Figure 13-2. IP addresses assign