CCIE Practical Studies, Volume I

Chapter 16. Using Hot Standby Routing Protocol (HSRP)

You might think that Cisco should have named HSRP (the Hot Standby Routing Protocol) as HSDG instead, for Hot Standby Default Gateway. This is exactly what HSRP provides to the hosts residing on local LANs. It seems that the name HSRP can imply a lot of things about "backup" that it actually doesn't provide. Many times, it has been necessary to explain exactly what HSRP can do for a customer's network besides give it a great name .

HSRP provides a consistent and reachable network layer address for IP and, in limited forms, IPX. By providing a constant address, even upon failure, a form of redundancy can occur. The most common deployment of HSRP is in LAN environments, where two routers "share" a common host address between them. This common address, called the hot standby address, is used as the default gateway for all the local hosts on that LAN segment. One router acts as the primary router and actually receives traffic destined for the hot standby address. The secondary router ignores this traffic until a configured set of circumstances is reached that affects the router's HSRP priority or standby priority. When the secondary router's priority exceeds the primary, it starts servicing requests for the hot standby address becoming the primary router. From the workstation's point of view, it has a single default gateway, pointing to the hot standby address. Figure 16-1 illustrates a common HSRP deployment.

In Figure 16-1, all the TCP/IP clients have a default gateway set to 172.16.1.1. The default gateway tells the IP client to forward any traffic not found on a local subnet to a specific IP address. The 172.16.1.1 address then is used as the HSRP virtual address. In this example, the router caladan is configured to track the serial interface to arakas. This means that if the serial interface enters a down state, it decrements the router's priority by 10 or by another configured value. While the serial interface is up, caladan receives and forwards IP packets from the workstations. If the serial interface on caladan drops , giedi_prime becomes the primary HSRP router. It immediately starts to service IP packets from the workstations, preserving all active sessions. The loss of the caladan router is completely transparent to the workstations on the LAN segment.

HSRP is most effective in scenarios that involve hosts defaulting traffic to a single network address, such as in Figure 16-1.