HSRP Behaviour

Answered Question
Apr 5th, 2010

Dear Experts,

I seek your help for clarifying the HSRP concept. I am unable to test this in the Lab. So Kindly share your experience.

Please find the attached Diagram.

I have 2 Cisco 3560 Switch connected and a Trunk link is configured between them allowing all the VLANs.Also HSRP is configured with VIP as 192.168.1.100. A server is connected with Teaming / Multipath configured in its NIC.

SWITCH 1 is ACTIVE & SWITCH 2 is STANDBY.

By Chance if the trunk link between the switches goes down as far as I know both the switches will become ACTIVE.

In the above scenario, how the traffic flow path is going to be. Since the VIP MAC will be given by both the switch, which path the Server NIC will choose.

Hope the diagram and explanation is easy to understand.

Looking for your exper comments

Regards,

sairam

      

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Correct Answer by Jon Marshall about 6 years 8 months ago

snarayanaraju wrote:

Hi Jon,

Thanks for you response. Your comments are very useful.

So, you say that in this scenario it will have unpredictable behaviour and packet loss is expected.

So, shall I say that the solution to avoid this issue is  "connecting another cable between the Switches and configure Etherchannel" ??

Anyother best practise solution is also highly appreciated

thanks

sairam

Sairam

Best practice is always to run more than one physical link between 2 switches doing HSRP, so yes you should definitely look to use etherchannel for link redudancy.

Jon

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jfraasch Mon, 04/05/2010 - 09:18

Sorry, just looked at the drawing. Thought you were going to plug into an access switch that is then plugged into the distribution/hsrp layer.


In this instance, I believe the server would chose whatever path it will take. It will not know which side is active or not. It will just send packets to whatever link IT thinks is active.

But it is interesting, on the receive side it will receive packets from both upstream switches.  I don't think it will care that there is a break upstream.  My guess (and its only a guess) is that it will run unaffected.

Message was edited by: jfraasch

Jon Marshall Mon, 04/05/2010 - 09:30

But it is interesting, on the receive side it will receive packets from both upstream switches.  I don't think it will care that there is a break upstream.  My guess (and its only a guess) is that it will run unaffected.

If both NIC were active then i would agree. But if only one NIC Is active then any traffic coming into the switch with the standby NIC connection destined for the server would need to be sent across the switch interconnect.

Jon

jfraasch Mon, 04/05/2010 - 09:34

Jon,


This is true. We load balance across two nics here so in my case the network performance would become unpredictable on the receive side.


We (my company) are redundant past the HSRP switches in the model shown in this post so traffic being transmitted would still get to its destination.

James

Jon Marshall Mon, 04/05/2010 - 09:24

snarayanaraju wrote:

Dear Experts,

I seek your help for clarifying the HSRP concept. I am unable to test this in the Lab. So Kindly share your experience.

Please find the attached Diagram.

I have 2 Cisco 3560 Switch connected and a Trunk link is configured between them allowing all the VLANs.Also HSRP is configured with VIP as 192.168.1.100. A server is connected with Teaming / Multipath configured in its NIC.

SWITCH 1 is ACTIVE & SWITCH 2 is STANDBY.

By Chance if the trunk link between the switches goes down as far as I know both the switches will become ACTIVE.

In the above scenario, how the traffic flow path is going to be. Since the VIP MAC will be given by both the switch, which path the Server NIC will choose.

Hope the diagram and explanation is easy to understand.

Looking for your exper comments

Regards,

sairam

      

Sairam

If the interconnect fails between the switches this does not affect the active server NIC. The active server NIC will stay the same and therefore the server will continue to use the switch it was using before the failure. Obviously though return traffic may very well not reach the server as it may come in on the other switch.

Jon


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snarayanaraju Mon, 04/05/2010 - 10:58

Hi Jon,

Thanks for you response. Your comments are very useful.

So, you say that in this scenario it will have unpredictable behaviour and packet loss is expected.

So, shall I say that the solution to avoid this issue is  "connecting another cable between the Switches and configure Etherchannel" ??

Anyother best practise solution is also highly appreciated

thanks

sairam

Correct Answer
Jon Marshall Mon, 04/05/2010 - 11:00

snarayanaraju wrote:

Hi Jon,

Thanks for you response. Your comments are very useful.

So, you say that in this scenario it will have unpredictable behaviour and packet loss is expected.

So, shall I say that the solution to avoid this issue is  "connecting another cable between the Switches and configure Etherchannel" ??

Anyother best practise solution is also highly appreciated

thanks

sairam

Sairam

Best practice is always to run more than one physical link between 2 switches doing HSRP, so yes you should definitely look to use etherchannel for link redudancy.

Jon

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