HSRP Question

[abhishek_nandavat]

Hi!

I was wondering whether the pre-empt command should be used on both the devices which are on HSRP or should it be used only on the active device. Is at all it is to be used on both the devices please give me an example scenario.

Thanks,

AbhisheK

[devang_etcom]

abhishek,

normally preempt command used for to retain active mode in HSRP...in HSRP one router is active and other is standby...when active rotuer goes downa then standby router will take the role of the active router...and now if that old active router come up it will not become active...so if you want to make your old active router as active router after standby you need to give preempt command on that active router during the configuration...

now some time you need to enter preempt command both router when you are having multiple group of standby on HSRP group router which is used for the load sharing or you can say proper utilisation of the resorces...mean one router is the active for one standby group and other router is the active router for the other group...at that time you are configuring preempt on both the router...

hope this help you

rate this post if it helps

regards

Devang

[abhishek_nandavat]

Devang,

Thanks for the reply.

In the probable scenario that you have mentioned for using pre-empt on both the devices, in that case we would need to configure pre-empt on different groups, right? What I want to know is, in what condition we need to configure pre-empt in the same group on two different devices. We've had a discussion on this earlier, I am not able to recall it :( Thats the sad part of it.

[devang_etcom]

read this

http://www.cisco.com/en/US/products/hw/switches/ps646/products_configuration_guide_chapter09186a00801cdf27.html

i think here you will have your answer but again here it his using two different group

rate this post if it helps

regards

Devang

[atif.awan]

You configure preempt on an HSRP enabled router that you want to take over the active role when it's priority is higher than the current active router. This can happen when you have a pair of routers configured for HSRP and are doing interface tracking on the current active router. A failure of the tracked interface will decrement the priority of the active router to a value below the current standby router. For the standby router to takeover it has to be configured with the preempt option. Another thing to note is that when the interface comes back online on your primary router, in order for it to resume the active role it will have to be configured with preempt option also.

[abhishek_nandavat]

Atif,

If the standby router has to be configured with pre-empt so that it can become the active router when the primary active device goes down, then what is the function that HSRP would be performing??? Is'nt it supposed to do that failover automatically without pre-empt being mentioned?

[devang_etcom]

to get active role in HSRP for the standby router there is not requirement for the preemt on the standby router...when you configure the HSRP with out preempt it will work...but preempt is used to get the active role only...

hope this will clear your idea

rate this post if it helps

regards

Devang

[atif.awan]

I think you misunderstood my post. I was not referring to normal HSRP opertion when the standy becomes active on losing HSRP keepalives from the primary. What I was referring to was a situation when both primary and secondary are up and are receiving keepalives from eachother but the priority of the primary goes below that of the secondary on account of a tracked interface failing. In this situation the standby will not take over the active role unless it is configured with the preempt option. I hope this clears things.

[devang_etcom]

ya that i know and it is true if the active will loss the connection to isp which is track by the HSRP then you will need to have preempt on secondary...

regards

Devang

[abhishek_nandavat]

Atif,

It did clear a few things. But you forgot to mention the reason that why the standby router would not take over the active role in the situation that you mentioned. Can you let me know the reason for this particular behavior???

Regards,

AbhisheK

[atif.awan]

Because this is how Cisco's HSRP implementation works without the preempt command. The preempt command is what tells a router with a higher priority to immediately take over the active router role. If this command is not entered on the standby router it will not become active even if it sees the current active router's priority decrease below it's own.

[devang_etcom]

when you enable the interface tracking...and when the router find the interface which is track by the HSRP router down...thne HSRP priority of the router is decreased...so here HSRP will take care that if the track interface goes down then that route should not be a active router...so at that time active router priority will becaome less than the standbay...and as priority of the active router decrease at that time standby router will become active...

hope this will help

regards

Devang

[Anand Narayana]

this is the working configuration of HSRP

interface Vlan5

ip address 10.2.9.254 255.255.255.0

standby 5 ip 10.2.9.252

standby 5 timers 5 15

standby 5 priority 110

standby 5 preempt

hope this helps,

rate this post

[abhishek_nandavat]

Anand,

I DO NOT need working configuration of HSRP. Perhaps you forgot to read my first post.