Hot Standby Router Protocol HSRP

131.108.1.1/24

131.108.1.2/24

Default gateway 131.108.1.100 (for all hosts on this network)

Figure 5.6 Example without HSRP.

devices in Figure 5.6 have been configured with a default gateway pointing to Router R1. If Router R1 goes down or the Ethernet interface becomes faulty, all the devices must be manually reconfigured to use the second default gateway (Router R2 Ethernet address). HSRP enables the network administrator to elect one of the two routers to act as the default gateway. If the elected router goes down, the second router assumes the IP default gateway. The IOS command under the Ethernet interface, standby track <interface of WAN> will allow the router to monitor the WAN link. If the WAN link continuously fails past a threshold, the HSRP default router will decrease its priority to allow a more reliable WAN connection to provide a gateway. For example in Figure 5.6, if the link on R1 to the WAN fails past a threshold then R2 will assume the HSRP address to provide a faster connection to the IP backbone network.

As shown in Figure 5.6, having redundant routers is helpful, but hosts can get confused because they are typically only allowed one default gateway. When using HSRP, as shown in Figure 5.7, both routers retain their unique Ethernet addresses. However, HSRP allows them to share a virtual address. This virtual address is assigned to each host as its default gateway. In the event of a router failure, the other will assume control of the virtual address.