is it an HSRP Bug?

[ric_nsgdata]

Hi,

I have implemented HSRP betwen Cisco 1841 & Cisco 1700. and used Standby Track command.

1841>

interface FastEthernet0/0

ip address 10.4.0.2 255.255.252.0

duplex auto

speed auto

standby 1 ip 10.4.0.1

standby 1 priority 254

standby 1 preempt

standby 1 track Serial0/0/0 100

1751>

interface FastEthernet0/0

ip address 10.4.0.3 255.255.252.0

duplex auto

speed auto

standby 1 ip 10.4.0.1

standby 1 priority 250

The show standby Command on 1841 is as follows:

1841#show standby

FastEthernet0/0 - Group 1

State is Active

4 state changes, last state change 01:49:46

Virtual IP address is 10.4.0.1

Active virtual MAC address is 0000.0c07.ac01

Local virtual MAC address is 0000.0c07.ac01 (v1 default)

Hello time 3 sec, hold time 10 sec

Next hello sent in 0.244 secs

Preemption enabled

Active router is local

Standby router is 10.4.0.3, priority 250 (expires in 8.108 sec)

Priority 154 (configured 254)

Track interface Serial0/0/0 state Down decrement 100

IP redundancy name is "hsrp-Fa0/0-1" (default)

YB-MUMNP-P-1841#

I have made the serial 0/0/0 link down (not by Serial Shutdown command, but by switching off the link)

If you see, the priority of the 1841 router is 154, as compared to 250 of 1751,

Yet 1841 is Active router.

Is there somethin i hv missed, or the router is misbehaving?

rgds

Sumedh

[nethelper · Accepted solution]

Hello Sumedh,

you need to configure 'standby 1 preempt' on the 1751 router as well. Without the command, the 1751 will only become the active router when there is no other active router, and in your case, the 1841 is the active router, regardless of the priority.

Regards,

Nethelper

[ric_nsgdata]

Thank You very much,

You r my personal Jesus Christ!

You have solved the issue!

[ankurbhasin]

Hi Nethelper,

I will like to confirm on this? Once the priority of 1841 went down by 100 and came to 154 the hellp packet should have told the 1751 router that the priority is low in comparison to yours and immediatley 1841 should have gone to standby state after comparing the priority becaue the hsrp hello packets carries the priority in its packet.

Then why preemt keyword is playing role out here. I always think that this keyword should only be configured on my primay router so that it can again be actve once it comes up after any down attack on my primar router?

Please confirm my doubt what preemt keyword is doing with the hello pclats that 1751 is not able to know about the lower priority value once the serial interface was made off.

Regards,

Ankur

[nethelper]

Hello Ankur,

although the priority on the 1841 router is lowered (because the tracked serial interface is down), the 1841 is still the active router. Without the 'preempt' configured on the 1751, the 1751 would only become the active router if the 1841 is down, that is, if the link between both goes down. Without the 'preempt' configured on the 1751, the priority does not matter, as long as the active router is still active.

Does that make sense ?

Regards,

Nethelper

[ankurbhasin]

Hi Nethelper,

What I am trying to understand is that hello packets carry the priority infrmation so when the priority is reduced definetely the hello packet will carry the 1841 reduced priority value to 1751 router. Please correct me if I m wrong?

What I am thinking is does the priority value only plays the role at the time of election and later does not play any role?

WHY I am asking this question is because lets say we have not implemented any tracking and peempt configs and we manually reduce the priority value on active router will it affect the standby router or not. AFAIK it will definetely make the standby router as active because the hello pacte will carry the priority value in its packet header?

Regards,

Ankur

[amit-singh]

Hi,

I do agree with Ankur. There is something more in this rather than just giving the preempt keyword on 1751. Without that keyword it should also make the 1751 as the active router.

When the interface of 1841 goes down, it will reduce the prority with 100 and will let the standby router know via its Hello packet that my prority is low and you can be an active router. It is not necessary to give " standby preempt " keyword on both the routers. You can set it on either of them.

The main use of " standby preempt " keyword is only to force the election and make the router as active which has prevously gone down and was an active one. For example 1841 is active and 1751 is standby.OIf 1841 goes down, 1751 will become active and If wee want that when 1841 comes back it should become the active router, then we can give the preempt keyword on 1841.

So still, I am in doubt that giving the preemption keyword on both the routers ??? there is something more in it.

let us know the IOS on the routers.

regards,

-amit singh

[st_kannan]

Hi,

The standby preempt feature means that the router with the higher priority is always the active router if it is available. The protocol handles this feature by allowing routers to send HSRP coup packets (Coup messages are sent when a router wishes to become the active router) So I think if preempt option is not configured it won’t intiate the election that is it won’t send coup message. Since in this case ethernet interface is not down so router router 1751 won't come as active one.

As per RFC 2281,

Preemption capability

If a router has higher priority than the active router and preemption is configured, it MAY take over as the active router using a Coup message.

I hope this will help.

Regards,

Kannan.S.T.

[amit-singh]

Thanks Kannan,

It has certainly cleared the doubts. Thanks again for the post.

regards,

-amit singh

[amit-singh]

Hi Kannan,

I went back to RFC 2281 and read about your mentioned point.

Please read this :

==>>> The following information MAY be configured on any router:

Preemption capability

If a router has higher priority than the active router and

preemption is configured, it MAY take over as the active router

using a Coup message.

http://www.cse.ohio-state.edu/cgi-bin/rfc/rfc2281.html

In RFC 2281 its mentioned that it can be configured on any of the router. Its not mentioned that it has to be configured on both the routers.

Sorry to say but it again has raised the doubts. Can you please put some more light on this.

regards,

-amit singh

[st_kannan]

Hi Amith,

For example, take this scenario if 1851 router is down and immediately router 1751 becomes as active using hold time interval. After sometime if router 1851 come up during that time it send hello packet with higher priority. If preempt option is not configured in router 1851 it won’t send coup message to start election. So the router 1851

never come active one. If configured it immediately become active.

In this scenario router 1751 sees hello packet from router 1851 with lower priority that self. But here preempt option is not configured also the router is not down. So it won’t send coup message to start election. So as per show output the router 1851 still in active state.

So we need to configure preempt option in both router if you configured “Track interface” option.

I hope it clear your doubt.

Regards,

Kannan.S.T.

[arvindchari]

Hi amit,

I was having the same dilemna while preparing for my BCMSN - CCNP exam :) Consider this scenario

Router A with HSRP priority 50 with no preempt configured

Router B with HSRP priority 25 with preempt enabled.

Assume router A is the first HSRP router on the LAN segment it becomes the default active forwarder and begins forwarding data. Now router B comes up and since it has preempt configured, it will preempt router A. However since it has lesser priority it will fail.

Assume router A fails (i.e the hold down timer expires after not recieving 3 consecutive hellos) now router B becomes the default active forwarder.

So far so good.

If router A comes back online, it will start sending hellos, but since it is NOT configured with the preempt command, router A will not set the preempt flag and hence router B will continue being the active forwarder until the time it actually goes down or an administrator manually does the needful.

Now if Router A was configured with standby preempt, Router A would correctly set the preempt flag and since it has higher priority it would correctly be set as the active forwarder once again

Hope I understood your problem correctly and gave the right explanation.

:)

[amit-singh]

Thanks guys, for your posts.

regards,

-amit singh