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Bronze

hrsp , active and standby router

Hi every body!

I just want to ask in order for router to determine if it is active hsrp router, it must compare its id with those of others in the group.. That means that every router in hsrp group must send hello to each other at start up.

But this was not the case .

I connected three routers D4,C3, and A20 to switch. a window xp running wireshark is also connectedto switch.

D4 e0 is 196.196.196.4

c3 e0 is 196.196.196.3

A20 e0 is 196.196.196.20

win xp has 196.196.196.1

i attached the file from wireshark .

The first hello was from D4 (196.196.196.4) claiming to be active. How come D4 knows it is active in hsrp though it did not receive any hello to compare its id with others in the group.

One can argue at start up, every router in hsrp group think it is active. But if this were the case, then we don't see any other hellos from C3(196.196.196.3) and A20( 196.196.196.20) claiming to be active at start up in the attachment

Thanks a lot!

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: hrsp , active and standby router

Sarah

I don't have access to my switches at present but i suspect what is happening is -

1) the first router to send out a hello is D4 which has a priority of 200.

2) Because D4 has not heard any hello's from any other router it assumes it is the active router. Note that this is a common step wit a lot of elections eg. DR/BDR, IGMP querier etc.

3) So C3 and A20 now receive a hello from D4 with a priority of 200. Both C3 & A20 have a priority of 100 so they now know they cannot be the active router.

4) Because D4 does not receive a hello back with a higher priority than it's own it carries on assuming it is the active router

What you could do is reconfigure the priority on either C3 or A20 and also configure preemption. Then capture packets and you should see D4 relinquish the role of active router.

Edit - by the way, looks like you now have your own equipment to test with so we all need to be even more on our toes !!

Jon

3 REPLIES
Hall of Fame Super Blue

Re: hrsp , active and standby router

Sarah

I don't have access to my switches at present but i suspect what is happening is -

1) the first router to send out a hello is D4 which has a priority of 200.

2) Because D4 has not heard any hello's from any other router it assumes it is the active router. Note that this is a common step wit a lot of elections eg. DR/BDR, IGMP querier etc.

3) So C3 and A20 now receive a hello from D4 with a priority of 200. Both C3 & A20 have a priority of 100 so they now know they cannot be the active router.

4) Because D4 does not receive a hello back with a higher priority than it's own it carries on assuming it is the active router

What you could do is reconfigure the priority on either C3 or A20 and also configure preemption. Then capture packets and you should see D4 relinquish the role of active router.

Edit - by the way, looks like you now have your own equipment to test with so we all need to be even more on our toes !!

Jon

Re: hrsp , active and standby router

Jon

Edit - by the way, looks like you now have your own equipment to test with so we all need to be even more on our toes !!

Good one :)

Lejoe

Bronze

Re: hrsp , active and standby router

Someone gave me old 4 2500 routers and one 1900 Ios switch. They are old but i try to practise whatever i can manage on them.

I will perform that lab you suggested and post the packet capture today.

Thanks a lot Jon!

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