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Webcast-Catalyst9k
New Member

HSRP preempt

Hi,

I would need some advise to clarify on the following HSRP preempt concept. Please let me know if the following is correct.

1) both active and standby routers NOT configured with HSRP preempt. When active router goes down, standby will NOT take over due to NO HSRP preempt configured.

2) both active and standby routers configured with HSRP preempt. When active router goes down, standby will take over as the active. When the active goes down again, the standby will take over as active. However, there will be a short downtime during the HSRP transition states. How long is the transition state?

3) only standby router configured with HSRP preempt. When active router goes down, standby will take over to become the active. However, when the active router goes down again, the standby router will NOT take over as the active due to NO HSRP preempt configured.

Thanks

Christina

11 REPLIES
Hall of Fame Super Gold

Re: HSRP preempt

Christina

1) Not correct. If both routers do not include preempt and the active router goes down then the standby router WILL take over as the active. Preempt is not an issue here. If the currently active router goes down and if there is a standby then the standby will take over as active. In a failure situation the standby will take over without consideration of preempt.

2) I am not entirely clear what you are describing when you say the active goes down and then when the active goes down again. So let me re-phrase it in terms of rtrA and rtrB. Assume that rtrA has a higher priority and becomes active and rtrB is standby. Then rtrA goes down. rtrB then becomes active. At that point I do not follow your scenario. I am not sure whether you assume that rtrA comes back into service or if you assume that there is rtrC which became standby.

If rtrA comes into service again and it has higher priority and it has preempt then rtrA becomes active and rtrB returns to standby. This is because preempt on rtrA allows it to displace rtrB. There is some very brief period of transition. I am not sure how long but I know it is quite short.

If rtrB became active and there is rtrC who becomes standby and then rtrB fails then rtrC becomes active and there is no period of interruption.

3)I have the same problem with this description. You describe active and standby, you assume that the active fails, and then you assume that the active fails again without specifying whether the original active router came back in service again.

So let me go back to my example of rtrA and rtrB. Assume that rtrA has higher priority and becomes active and does not have preempt. Assume that rtrB has lower priority and does have preempt. Then rtrA goes down. rtrB becomes active. (Note that this really has nothing to do with preempt - rtrB takes over because the active failed.) If rtrA comes into service then it is in the HSRP group and it has higher priority but it does not become active because it does not have preempt.

Preempt really has to do with enabling a router that has higher priority but is not active to displace the current active router. Preempt is a factor when some router attempts to displace the current active router. It is not a factor when the active router fails.

Let me describe another example. Assume rtrA has priority of 105 and rtrB has priority of 100. So rtrA becomes the active router. Then assume that rtrA is tracking interface serial0/0 and the interface goes down. Now rtrA has priority 95 and rtrB has priority 100. Will rtrB become active? With preempt configured it will and without prempt configured it will not.

HTH

Rick

New Member

Re: HSRP preempt

Hi Rick,

let me know if i understand you correctly. thanks.

assume there are only 2 routers, rtrA(active) and rtrB(standby). default priority configured on both routers, only using the highest ip address to determine the active router.

with preempt configured on both rtrA and rtrB. when rtrA goes down, rtrB will become the active without any disruption. when rtrA comes up again, rtrA will take over as the active and there will be a short disruption.

without preempt configured on both active and standby routers, rtrA and rtrB will failover to each other without any disruption as long as one of the router is up to become the active router.

thanks for your patience.

christina

Re: HSRP preempt

hi Christina,

see when ever you are dealing with the preempt option then you are not daling with the whole router status... here you are not concerning with the whether whole router is down or other issue... Preempt will come into the picture when you configure the TRACKING future... when you track some interface of the router with HSRP then PREEMPT will be your hero...

first of all ask que to your self why would you like to have HSRP?

Ans. let assume you have two different ISP connection connected with two different router of your local network. Now when you configure the HSRP with out PREEMP and tracking then one router will become ACTIVE and otehr will STANDBY as pernormal election process...right? now assume your routerA is active and routerB is standby... now your routerA goes down then your routerB will become active router from standby and take care of traffic flow...up to this okay...

now lets say your routerA didn't goes down but ISP connection connected to your routerA or interface of routerA whcich is connected with ISP goes down...then what happen? here your routerA is working but its one of the interface to the ISP is down... so what happen to data transfer...? so now here TRACKING and PREEMPT will come into the picture...

now you tracking the interface of both the router connected to the different ISP... it means you have configure the TRACKING as well as PREEMPT feature... now lets assume your link to the ISP of routerA goes down now because of you have configure the HSRP tracking, HSRP active router come to know that its interface to the ISP goes down and as because of PREEMPT configuration on both router your other standby router will take over the ACTIVE role...

hope i try my best to explain...

rate this post if it help.

regards

Devang

Hall of Fame Super Gold

Re: HSRP preempt

Christina

I believe that your understanding is correct.

One note: I am not sure that I tested the scenario where both routers have equal priority and the selection of active was based on address. I am not clear whether preempt enables rtrA to resume being active when it returns to service. It is clear that preempt allows a higher priority router to displace a lower priority router as active. Perhaps we need to test to clarify whether this also extends to comparison of address when priority is equal.

HTH

Rick

Re: HSRP preempt

yes rick...

when you have router configure with the equal priority the active will be elected on the base of address or i think router who configure first will become active.... and yes when you configure the routers with preempt then you can have periviously active router will again take over the role of the active router what it become up or its interface will become up/up...

rate this post if it helps

regards

Devang

New Member

Re: HSRP preempt

Hi ,

Just a quick question regarding this tracking serial interface scenario.

Assuming preempt and tracking serial int commands have been given on the respective routers , what happens when the serial goes down and the priorities become equal?????

Will the router whoz serial is down still act as an active router or will the standby router(configured with preempt) take over as active????

Re: HSRP preempt

hi pallavi

whatever we discuss about equal priority its all about the configuration at very first time...

so normally during configuration you have to take this factor in to considration or you have to make sure that when ever the failur take place then priority of the both router should not become same... otherwise router will failed serial interface will remain in active mode...

hope this help you

rate this post if it helps

regards

Devang

New Member

Re: HSRP preempt

That means the active router with the serial int down will continue to be an active router

bcoz the priorities are same.

And its the humans who have to ensure that the priorities don't end up with the same value after the serial goes down , right????

Thank you.

Re: HSRP preempt

yes...

you have to take care of the priority and you can also configure the decrement value of when the router interface goes down using following command"

standby 3 track serial0 20

here you configure router to decrease the priority by 20 when it find the track interface is down...

"so make sure both router should not reach to same priority when you have track configuration"

rate this post if it helps

regards

Devang

New Member

Re: HSRP preempt

dear all,in my lab

router a ip address 1.1.1.1

router b ip address 1.1.1.2

both router a and router b no priority config(Default 100)

router b config as preempt

when router b goes down,router a take over

but when router b active again,active can't switch back to router b

why

very tks

Hall of Fame Super Gold

Re: HSRP preempt

It is my understanding that preempt works when there is a priority difference and allows the higher priority router to take over. When the priorities are equal I am not convinced that preempt works based on address relationship. Devang is convinced that preempt does work based on address relationship.

Would you be able to run debug standby, repeat the process of B failing and then becoming active again, and post the debug output? This might shed very helpful light on this question.

HTH

Rick

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