Cisco's HSRP provides a way for IP workstations to keep communicating on the internetwork even if their default router becomes unavailable. HSRP works by creating a phantom router that has its own IP and MAC addresses. The workstations use this phantom router as their default router.

HSRP routers on a LAN communicate among themselves to designate two routers as active and standby. The active router sends periodic hello messages. The other HSRP routers listen for the hello messages. If the active router fails and the other HSRP routers stop receiving hello messages, the standby router takes over and becomes the active router. Because the new active router assumes both the IP and MAC addresses of the phantom, end nodes see no change at all.

They continue to send packets to the phantom router's MAC address, and the new active router delivers those packets.

HSRP also works for proxy ARP. When an active HSRP router receives an ARP request for a node that is not on the local LAN, the router replies with the phantom router's MAC address instead of its own. If the router that originally sent the ARP reply later loses its connection, the new active router can still deliver the traffic.

Figure 4-5 shows a sample implementation of HSRP.

Figure 4-5 An Example of HSRP: The Phantom Router Represents the Real Routers


Central Park

Central Park

In Figure 4-5, the following sequence occurs:

1 The Anderson workstation is configured to use the Phantom router as its default router.

2 Upon booting, the routers elect Broadway as the HSRP active router. The active router does the work for the HSRP phantom. Central Park is the HSRP standby router.

3 When Anderson sends an ARP frame to find its default router, Broadway responds with the Phantom router's MAC address.

4 If Broadway goes off line, Central Park takes over as the active router, continuing the delivery of Anderson's packets. The change is transparent to Anderson. If a third HSRP router was on the LAN, that router would begin to act as the new standby router.

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