Campus LAN Design Issue

Unanswered Question
Jun 4th, 2008

Hi Friends,

For a Dual core campus LAN design, we know that HSRP or GLBP will give the router redundency. with one of them enabled on distribution switches for a switch block, if one Ethernet link between one access switch and distribution switch failed ( say disconnect)how will that access switch forward its frames to the standby router

as it will not become active even this link fail? At normal operation do these switches forward its frames using both redundant links or do they use only the link to the currently active gateway??


I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
jrensink78 Wed, 06/04/2008 - 05:11

Let's just talk HSRP for a frame of reference.

In this scenario, I assume you have a layer-3 connection between the distribution switches (which is recommended). So there is not an alternate layer 2 path to the active router.

In this type of a setup, what you want to do is setup interface tracking within HRSP on your active router. Interface tracking allows HSRP to decrement its priority when a given interface goes down. You can specify how much the priorty is altered when you set this up.

So say your active router has a priority of 120 and your standby has a priority of 100. Setup interface tracking on the active router to track that uplink from the access switch and set the decrement value to 30 (or any number above 20). Lastly, ensure that the preempt option is enabled on both routers.

So when the link goes down, the active router changes its priority to 90. That is now less than the standby's priority of 100. Preempt takes over and the active and standby roles switch. Once the interface comes back up, the original active router's priority goes back up to 120 and it takes over again.

You can do similar things with the other gateway redundancy protocols.

Hope this helps!

rangaabey Wed, 06/04/2008 - 20:56

All you explained is ok Jeffry. But can we make interface tracking for the inerfaces that are defined for HSRP operation?.I am refering the latest cisco press book for BCMSN.

As I understood from that book we normally set interface tracking for the interfaces that connects the switch block to the out side network. Not for the interfaces participating the HSRP process.

When HSRP is enabled what specific interfaces HSRP will use to send HSRP Hello msges? Only L3 or both L3 and L2?

jrensink78 Thu, 06/05/2008 - 06:07

Now that you pose these questions, I believe I was mistaken in my last post. Since HSRP relies on both switches or routers being on a shared layer-2 medium, the hellos (which are multicast to should only go out the configured interface. Other VLANs do not get the same advertisements (whic is evidenced by the ability to share standby groups. Sorry if I caused any confusion.

Here is what I should have posted...

When the link between the access switch and the active HSRP distribution switch goes down, the 2 distribution switches would lose connectivity on that VLAN. So the standby switch would no longer be receiving HSRP hellos. After 10 seconds (by default), HSRP on the standby switch would determine that the active HSRP switch is no longer around and it would make itself the active switch. So you would actually end up with each distribution switch saying that it owned the virtual IP. But the access switch is only connected to 1 of them, so there isn't a problem.

When the connection is re-established, HSRP would then have to go through the election process again and go back to only one of the distribution switches owning the virtual IP.

Where this wouldn't work is if the VLAN was shared across multiple access switches (an end-to-end VLAN). In that case, nothing would change in the HSPR state and the access switch that lost the link to the primary HSRP switch would be cut off on that VLAN.

Sorry if I caused confusion earlier. Hopefully I got it straight this time.

rangaabey Fri, 06/06/2008 - 02:51

Thank you very much for your valuable contribution. I am too busy today...

There is a bomb blast near my working place

Sri Lanka.

Thanks friend.I got a print out of u'r msg

and will post you a msg if I need some thing further.


This Discussion