HSRP default behavior

Answered Question
Dec 23rd, 2011

Hi All,

The default behavior of HSRP(with Priorities to default) should make the router active if it has highest interface IP address. But it is not happening practically (note that R1 with IP 192.168.10.2 has been configured first)

R2(config)#interface vlan 10

R2(config-if)#ip add 192

*Mar  1 00:04:42.979: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan10, changed state to up

R2(config-if)#ip add 192.168.10.3 255.255.255.0

R2(config-if)#st

R2(config-if)#standby 10 ip 192.168.10.1

R2(config-if)#do ping 192.168.10.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.10.2, timeout is 2 seconds:

.!!!!

Success rate is 80 percent (4/5), round-trip min/avg/max = 12/31/48 ms

R2(config-if)#exit

R2(config)#

*Mar  1 00:05:32.727: %HSRP-5-STATECHANGE: Vlan10 Grp 10 state Speak -> Standby

R2(config)#do show standby

Vlan10 - Group 10

  State is Standby

    1 state change, last state change 00:00:24

  Virtual IP address is 192.168.10.1

  Active virtual MAC address is 0000.0c07.ac0a

    Local virtual MAC address is 0000.0c07.ac0a (v1 default)

  Hello time 3 sec, hold time 10 sec

    Next hello sent in 2.808 secs

  Preemption disabled

  Active router is 192.168.10.2, priority 100 (expires in 7.360 sec)

  Standby router is local

  Priority 100 (default 100)

  IP redundancy name is "hsrp-Vl10-10" (default)

Any reason for this behavior? Please let me know if I am wrong.

Regards,

Ravi

I have this problem too.
0 votes
Correct Answer by Peter Paluch about 2 years 3 months ago

Dear friends,

Indeed, what you see is expected. The IOS Configuration Guide for HSRP at

http://www.cisco.com/en/US/docs/ios-xml/ios/ipapp_fhrp/configuration/12-4t/fhp-hsrp.html#GUID-1B905648-DF2B-4B68-9821-594EA794A176

puts it quite clearly:

A standby router with equal priority but a higher IP address will not preempt the active router.

Best regards,

Peter

Correct Answer by krahmani323 about 2 years 3 months ago

Hi Ravi

Thanks for the detailed presentation of your tests :-)

I am totally agree with the results. Please find my comments for each step.

Conditions/router     status

R1      192.168.10.2(f0/0)

R2      192.168.10.3(f0/0)





to   check interface ip   priority

R1

R2


R1   configured first and   then R2

active

backup

Expected as there is no pre-emption   (and even with pre-emption enabled because of  CSCdv91652)

both   rebooted at same time

backup

active

Expected as the routers boots at the same time and in this case the highest IP @ of R2 is the tie breaker for the HSRP master election.

Priority

R1

R2


110   to R1(r2 still on   default)

backup

active

Expected as the preemption is not   enabled on R1.

restarted   both routers

active

backup

Expected as the routers boots at the same time and in this   case the highest HSRP priority makes R1 active.

Preempt

R1

R2


made   r2 priority 115 and   issued preempt on R1

active

backup

Expected as the preemption is not enabled (yet) on R2 although R2 priority is higher.

removed   preempt on r1 and   configured on r2

backup

active

Expected after preemption has been   enabled on R2 which has higher priority.

Note The result would have been the same even without removing preemption on   R1.

Hope that helps.

Kind regards.

Karim

Correct Answer by krahmani323 about 2 years 3 months ago

Hello Ravindra, Cflory,

This behaviour should be the expected one as documented by bug CSCdv91652 - HSRP should not preempt if priorities are equal

Before in earlier versions it should have preempted, but the behaviour has been changed since.  

http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCdv91652

This ddts is an enhancement to the HSRP preempt mechanism.

An Active HSRP router may be preempted by an equal priority non-Active router if the non-Active router has a numerically greater interface IP address.

This is the current expected behaviour, since the interface IP address is used as a tie-breaker in situations where the prioritues are equal.

This ddts changes that behaviour such that an equal priority non-Active router will never preempt the current Active router.

Hope that helps.

Kind regards.

Karim

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cflory Fri, 12/23/2011 - 12:22

"Preemption disabled"

Enable preemption on your interfaces with 'standby 10 preempt".  I could be wrong here, but I believe that is the issue.

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8c.shtml

!--- Allows the router to become the active router when the priority 
!--- is higher than all other HSRP-configured routers in the hot standby group. 
!--- If you do not use the standby preempt command in the configuration 
!--- for a router, that router does not become the active router, even if 
!--- the priority is higher than all other routers. 
ravindra.krishna Fri, 12/23/2011 - 12:31

Hi Cflory,

I wanted to check the default behavior of the HSRP with default priorities. Since the IP address of the Vlan 10 on R2 is higher than R1,  R2 should have been the active one. But went to backup. It does not make any sense to me why R1 is still active.

Any help will be appreciated.

Regards,

Ravi

cflory Fri, 12/23/2011 - 12:34

I'm not telling you to change priorities, I'm suggesting you make either router 'preempt' to become active based on the higher IP address.

ravindra.krishna Fri, 12/23/2011 - 12:42

Hi Cflory,

Even adding the preempt it does not work. The preempt only works with priority. Please consider the output

R2(config)#interface vlan 10

R2(config-if)#sw

R2(config-if)#st

R2(config-if)#standby 10 pre

R2(config-if)#standby 10 preempt

R2(config-if)#standby 10 preempt

R2(config-if)#do show stan

R2(config-if)#do show standby

Vlan10 - Group 10

  State is Standby

    1 state change, last state change 01:27:27

  Virtual IP address is 192.168.10.1

  Active virtual MAC address is 0000.0c07.ac0a

    Local virtual MAC address is 0000.0c07.ac0a (v1 default)

  Hello time 3 sec, hold time 10 sec

    Next hello sent in 0.176 secs

  Preemption enabled

  Active router is 192.168.10.2, priority 100 (expires in 9.276 sec)

  Standby router is local

  Priority 100 (default 100)

  IP redundancy name is "hsrp-Vl10-10" (default)

R2(config-if)#do show standby

Vlan10 - Group 10

  State is Standby

    1 state change, last state change 01:27:44

  Virtual IP address is 192.168.10.1

  Active virtual MAC address is 0000.0c07.ac0a

    Local virtual MAC address is 0000.0c07.ac0a (v1 default)

  Hello time 3 sec, hold time 10 sec

    Next hello sent in 1.316 secs

  Preemption enabled

  Active router is 192.168.10.2, priority 100 (expires in 7.432 sec)

  Standby router is local

  Priority 100 (default 100)

  IP redundancy name is "hsrp-Vl10-10" (default)

R2(config-if)#do show standby

Vlan10 - Group 10

  State is Standby

    1 state change, last state change 01:27:48

  Virtual IP address is 192.168.10.1

  Active virtual MAC address is 0000.0c07.ac0a

    Local virtual MAC address is 0000.0c07.ac0a (v1 default)

  Hello time 3 sec, hold time 10 sec

    Next hello sent in 0.236 secs

  Preemption enabled

  Active router is 192.168.10.2, priority 100 (expires in 9.352 sec)

  Standby router is local

  Priority 100 (default 100)

  IP redundancy name is "hsrp-Vl10-10" (default)

Regards,

Ravi

cflory Fri, 12/23/2011 - 13:04

Ok, thanks for trying, I was just curious.  I am also seeing the same thing happen in my lab under 15.1 code.  I am not able to make the higher IP address active, without a priority.

Perhaps this is an old ability and no longer functional.  Maybe Cisco is requiring priorities now?  I suppose opening a TAC case could shed some light.

Their official stance (which confirms the ability - and the post was updated 9/22/11 - Perhaps for the GLBP questionst):

http://www.cisco.com/en/US/tech/tk648/tk362/technologies_q_and_a_item09186a00800a9679.shtml

Q. If there is no priority configured for a standby group, what determines which router is active?



A. The priority field is used to elect the active router and  the standby router for the specific group. In the case of an equal  priority, the router with the highest IP address for the respective  group is elected as active. Furthermore, if there are more than two  routers in the group, the second highest IP address determines the  standby router and the other router/routers are in the listen state.

Correct Answer
krahmani323 Fri, 12/23/2011 - 13:41

Hello Ravindra, Cflory,

This behaviour should be the expected one as documented by bug CSCdv91652 - HSRP should not preempt if priorities are equal

Before in earlier versions it should have preempted, but the behaviour has been changed since.  

http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCdv91652

This ddts is an enhancement to the HSRP preempt mechanism.

An Active HSRP router may be preempted by an equal priority non-Active router if the non-Active router has a numerically greater interface IP address.

This is the current expected behaviour, since the interface IP address is used as a tie-breaker in situations where the prioritues are equal.

This ddts changes that behaviour such that an equal priority non-Active router will never preempt the current Active router.

Hope that helps.

Kind regards.

Karim

ravindra.krishna Fri, 12/23/2011 - 15:10

Below are my results. Please check Karim.

Please note that the new test results in the below table follow the preceding state or preceding result

Conditions/router   status

R1    192.168.10.2(f0/0)

R2    192.168.10.3(f0/0)

to   check interface ip priority



R1   configured first and then R2

active

backup

both   rebooted at same time

backup

active




Priority



110   to R1(r2 still on default)

backup

active

restarted   both routers

active

backup




Preempt



made   r2 priority 115 and issued preempt on R1

active

backup

removed   preempt on r1 and configured on r2

backup

active




These are my test results and seem to match the theory. Please do let me know if there is any change.

Regards,

Ravi

Correct Answer
krahmani323 Fri, 12/23/2011 - 15:51

Hi Ravi

Thanks for the detailed presentation of your tests :-)

I am totally agree with the results. Please find my comments for each step.

Conditions/router     status

R1      192.168.10.2(f0/0)

R2      192.168.10.3(f0/0)





to   check interface ip   priority

R1

R2


R1   configured first and   then R2

active

backup

Expected as there is no pre-emption   (and even with pre-emption enabled because of  CSCdv91652)

both   rebooted at same time

backup

active

Expected as the routers boots at the same time and in this case the highest IP @ of R2 is the tie breaker for the HSRP master election.

Priority

R1

R2


110   to R1(r2 still on   default)

backup

active

Expected as the preemption is not   enabled on R1.

restarted   both routers

active

backup

Expected as the routers boots at the same time and in this   case the highest HSRP priority makes R1 active.

Preempt

R1

R2


made   r2 priority 115 and   issued preempt on R1

active

backup

Expected as the preemption is not enabled (yet) on R2 although R2 priority is higher.

removed   preempt on r1 and   configured on r2

backup

active

Expected after preemption has been   enabled on R2 which has higher priority.

Note The result would have been the same even without removing preemption on   R1.

Hope that helps.

Kind regards.

Karim

krahmani323 Fri, 12/23/2011 - 17:02

Thanks Peter for pointing out the exact sentence from the documentation (more clear than a bug ID :-)

Regards.

Karim

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Posted December 23, 2011 at 11:27 AM
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