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HSRP Tuning Example

Today I will discuss about HSRP Tuning. As you see in the article “HSRP (Hot Standby Router Protocol)”, it is quite simple configure Hot Standby Router Protocol. [boxads]
Some cases require a custom configuration, using priority, track, preempt etc…and these are:

• The standby preempt interface configuration command allows the router to become the active router when its priority is higher than all other HSRP-configured routers in this Hot Standby group. The configurations of both routers include this command so that each router can be the standby router for the other router. If you do not use the standby preempt command in the configuration for a router, that router cannot become the active router.
• The standby priority interface configuration command sets the router’s HSRP priority (the default priority is 100).
• The standby timers interface configuration command sets the interval in seconds between hello messages (called the hello time) to five seconds and sets the duration in seconds that a router waits before it declares the active router to be down (called the hold time) to eight seconds. (The defaults are three and 10 seconds, respectively.) If you decide to modify the default values, you must configure each router to use the same hello time and hold time.
• The standby track command allows you to specify another interface on the router for the HSRP process to monitor in order to alter the HSRP priority for a given group. If the line protocol of the specified interface goes down, the HSRP priority is reduced. This means that another HSRP router with higher priority can become the active router if that router has standby preempt enabled.
• The standby authentication interface configuration command establishes an authentication string whose value is an unencrypted eight-character string that is incorporated in each HSRP multicast message. This command is optional. If you choose to use it, each HSRP-configured router in the group should use the same string so that each router can authenticate the source of the HSRP messages that it receives.

Suppose to have this physical configuration:

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HSRP (Hot Standby Router Protocol)

The HSRP (Hot Standby Router Protocol) is a Cisco proprietary first-hop redundancy protocol (FHRP) designed to allow for transparent fail-over of the first-hop IP router, and has been described in detail in RFC 2281.

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HSRP provides high network availability by providing first-hop routing redundancy for IP hosts on Ethernet, Fiber Distributed Data Interface (FDDI), Bridge-Group Virtual Interface (BVI), LAN Emulation (LANE), or Token Ring networks configured with a default gateway IP address. HSRP is used in a group of routers for selecting an active router and a standby router. In a group of router interfaces, the active router is the router of choice for routing packets; the standby router is the router that takes over when the active router fails or when preset conditions are met. HSRP active and standby routers send hello messages to the multicast address 224.0.0.2 using UDP port 1985.

The virtual router is simply an IP and MAC address pair that end devices have configured as their default gateway. The active router processes all packets and frames sent to the virtual router address. The virtual router does not process physical frames and exists in software only. The active router physically forwards packets sent to the MAC address of the virtual router. The virtual router MAC address is a well know mac-address and it is 0000.0c07.acxx, where xx is the HSRP group member. For example, if the group is 20 the virtual MAC address is 0000.0c07.ac14 (remember that the number in the mac address is expressed in HEX!!!).

When the active router fails, the other HSRP routers stop seeing hello messages from the active router. So, the standby router will be the new active router and, if possible, a new standby router will be elected. Because the new active router assumes both the IP and MAC addresses of the virtual router, the end stations see no disruption in service. The end-user stations continue to send packets to the virtual router MAC address, and the new active router delivers the packets to the destination.

HSRP has 2 timers:

• Hello interval time: Interval between successive HSRP hello messages from given router. Default is 3 seconds.
• Hold interval time: Interval between the receipt of a hello message and the presumption that the sending router has failed. Default is 10 seconds.

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