HSRP preempt

[a.azambuja]

document reference:

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_configuration_example09186a0080094e90.shtml

This document show's two HSRP group's configured to provide load sharing... My question is related why the lowest group need the preempt comand ? Is this really necessary ? I think that only highest (active) groups have to be configured with preempt now I'm confused...

Plz help me to understand this question..

[Edison Ortiz]

If you do not use the standby preempt interface configuration command in the configuration for a router, that router will not become the active router, even if its priority is higher than all other routers.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hiap_c/ch20/haiphsrp.htm#wp1098318

[Jon Marshall]

Hi

Just to add to Edison's point. The key thing about preemption is that it is needed if you may be automatically changing priority due to some network event eg.

If router1 is the active HSRP router and it fails then router2 will take over. That is HSRP at it's simplest.

However quite often you need to track another interface with HSRP. When you track another interface you are monitoring it's state. if it goes down you may want the HSRP active gateway to switch to the other router so you decrease the priority of the active router to be lower than the priority of the standby router. But if you don't have preempt on the standby router it will not preempt the active router.

Hope this makes sense

Jon

[williamsdo]

Hi, you use the preempt command to control the behavior of the standby unit, it will allow the primary unit to failback or take up the primary router spot again after it has recovered from a failure. Your primary group doesn't need this command because they are not configured as backup or failover unit.

HTH

[smothuku]

Hi Azambuja ,

Adding some info about preempt.

When this command is configured, the router is configured to preempt, which means that when the local router has a Hot Standby priority higher than the current active router, the local router should attempt to assume control as the active router.

If preemption is not configured, the local router assumes control as the active router only if it receives information indicating no router is in the active state (acting as the designated router).

When a router first comes up, it does not have a complete routing table. If it is configured to preempt, it will become the active router, yet it is unable to provide adequate routing services. Solve this problem by configuring a delay before the preempting router actually preempts the currently active router.

When group number 0 is used, no group number is written to NVRAM, providing backward compatibility.

IP redundancy clients can prevent preemption from taking place. The standby preempt delay sync seconds command specifies a maximum number of seconds to allow IP redundancy clients to prevent preemption. When this expires, then preemption takes place regardless of the state of the IP redundancy clients.

The standby preempt delay reload seconds command allows preemption to occur only after a router reloads. This provides stabilization of the router at startup. After this initial delay at startup, the operation returns to the default behavior.

The no standby preempt delay command will disable the preemption delay but preemption will remain enabled. The no standby preempt delay minimum seconds command will disable the minimum delay but leave any synchronization delay if it was configured.

When the standby follow command is used to configure an HSRP group to become an IP redundancy client of another HSRP group, the client group takes its state from the master group it is following. Therefore, the client group does not use its timer, priority, or preemption settings. A warning is displayed if these settings are configured on a client group:

Router(config-if)# standby 1 preempt delay minimum 300

% Warning: This setting has no effect while following another group.

Examples

In the following example, the router will wait for 300 seconds (5 minutes) before attempting to become the active router:

interface ethernet 0

standby ip 172.19.108.254

standby preempt delay minimum 300

Thanks,

satish