HSRP Question

Answered Question
Aug 21st, 2007
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Hi everyone,


I have a question about how HSRP works.

I have two routers, A and B, running IOS 12.4, and tracking on an ICMP Echo. (relevant configs for both routers below). From what I have read so far, I am under the impression that as soon as the tracked object changes state, the swtichover from Stanby to active is immediately. From what I have seen, it takes about 30 secs to switchover. It seems to correspond to the Hello/Hold timers, although they are supposed to be independent events. Why does it take that long to switchover? Does the Hello packet (when the routers can communicate with each other) carries the info to switchover?

Any help will be much appreciated...

here are the configs:

router A:

boot system flash c2800nm-entservices-mz[1].124-3g.bin


ip sla monitor 1

type echo protocol ipIcmpEcho AAA.BBB.CCC.DDD source-interface FastEthernet0/1

frequency 10

ip sla monitor schedule 1 life forever start-time now

!

track 100 rtr 1

!

!

!

interface FastEthernet0/0

description LAN Ruteador A Navega

ip address xxx.yyy.96.66 255.255.255.224

ip nat inside

ip route-cache flow

duplex auto

speed auto

standby 1 ip xxx.yyy.96.65

standby 1 timers 30 35

standby 1 priority 110

standby 1 preempt delay minimum 3600

standby 1 track 100 decrement 20

!



router B (not doing any tracking, only backup):

!

interface FastEthernet0/0

ip address xxx.yyy.96.67 255.255.255.224

ip nat inside

duplex auto

speed auto

standby preempt

standby 1 ip xxx.yyy.96.65

standby 1 preempt delay minimum 5

Correct Answer by sundar.palaniappan about 9 years 9 months ago

The problem is the 'standby 1 timers 30 35' on routerA. I believe the default hello/hold timer is 3/9 secs respectively.


The router doesn't initiate any adhoc hello packet when the priority changes. It waits till the next hello is due and that's the reason why it can take up to 30 seconds for the alternate router's higher priority to take effect when the tracked object goes down on the primary router.


HTH


Sundar

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jwdoherty Tue, 08/21/2007 - 19:43
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From various Cisco documentation (e.g. http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094afd.shtml#topic1) priority is sent in a hello packet. What's not clear is whether the hello packet would be sent immediately when the priority changes or with the next "scheduled" hello. From your results, it appears that the initial change in priority goes with the next scheduled hello packet. You could probably confirm this by using debug.


To speed up the switch over, decrease the hello time interval. (You might also want to adjust your holdtime to be the usual 3x the hello time - allows for 3 missed hellos.)

Correct Answer
sundar.palaniappan Tue, 08/21/2007 - 19:50
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The problem is the 'standby 1 timers 30 35' on routerA. I believe the default hello/hold timer is 3/9 secs respectively.


The router doesn't initiate any adhoc hello packet when the priority changes. It waits till the next hello is due and that's the reason why it can take up to 30 seconds for the alternate router's higher priority to take effect when the tracked object goes down on the primary router.


HTH


Sundar

e-alvarez Wed, 08/22/2007 - 16:14
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That explains the time it takes for the switchover. I was playing with the hello/hold timers, but I had the impression that a hello would be sent as soon as the priority changed, no "as scheduled"...It was just for academic purposes.


Thanks to all

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