Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Knowing which trunk link is active

Hi friends,

In a dual 6509 setup, is it possible to know which trunk interface is involved in the communication between the two 6509 switches?

Lets say for example that I have three individual trunk links going between the 6509's. Can I know over which link is the communication happening OR which link is currently active and passing packets between the switches? The reason I am asking this question is because I am seeing three links between the two 6509's and all are configured for VLAN pruning except one.

And there is only one trunk link where all VLAN's are allowed and I want to make sure that that is the one being used.

The spanning tree mode is PVST if that might help.

Please advise.

Thanks and Regards


New Member

Re: Knowing which trunk link is active

I suppose it could depend on your configuration. For example, you could have them set up as an etherchannel group. Assuming you don't you should be able to figure it out easy enough just using the show span for each VLAN. Since only one side appears as blocked, you have to check both switches to determine which are in a blocked state. Even so, if you have enough traffic you can more or less verify that by clearing the counters and seeing which ports have the most traffic.



Re: Knowing which trunk link is active

If you have 3 trunk links and they all pass the same vlans most likely spanning tree is blocking at least one or more of these trunks that carry the same vlans . Like previous poster said you might be better off combining all 3 and make a etherchannel group out of them which makes all 3 links look like a single link spanning tree wise .

Hall of Fame Super Blue

Re: Knowing which trunk link is active

Hi Guatam

If there is only one trunk link where all vlans are allowed and the vlan you are interested is not allowed on the other trunks then it would have to be using that one.

If the vlan you are interested in is allowed on all yout trunks then as previous posters have said you can check spanning-tree output.

If the trunk link is being blocked then it can only be being blocked because the 2 6500's have an alternate path for that vlan.



New Member

Re: Knowing which trunk link is active

Hi to All,

Thanks a lot for your responses. I verified the spanning tree configs with show span, show span summary, show span blockedports and found that the trunks were carrying the required VLAN's and there were no ports in blocked state.

It was just my confusion about switches not doing their job.

But actually, it was a FWSM failover issue that was eventually solved after replacing a route with HSRP IP with the actual IP.

Thanks a lot anyways for your responses.



CreatePlease login to create content