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. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

3750 dot1q trunking problems

Recently we've installed several 3750 pairs in our server LAN. They are stacked with eachother and trunked back to our 6509's. Every so often the LED's on the active links flash amber, on older software revisions it used to increase the runt/input error counters whenever this happened, but on 12.1(19) its not anymore. I assume that's because Cisco considers the behavior to be a cosmetic problem and that it doesn't affect switch performance.

However, we were having problems with our SAP systems and they disappeared when we took the links back to the core out of trunking mode - which also stopped the LED from going amber and the error counters from increasing. Cisco wants me to put a sniffer on the problem, but the tech doesn't seem to understand that the switches are in our production environment that I cannot take down or otherwise cause problems on it. Its a 24x7 system.

All I know is if I have the link in 802.1Q trunking, I get the LED/errors and our SAP guys see the performance problem they were having. If I take it out of trunking, both go away.

Is anyone else seeing this problem? Do you know if the fix in the latest software that stopped the error counters from incrementing actually fixes the problem? I know the LED is still blinking amber quite often still - its just no longer incrementing the error counters... Seems like rather than fixing the problem they just hacked it to not show it as well.

  • Other Network Infrastructure Subjects
New Member

Re: 3750 dot1q trunking problems

What are the trunk modes on 3750 and 6509? Desirable? Trunk?

Try setting them both to Trunk so they don't have to negotiate the trunking and will always be trunks.

Also put a sniffer on the switch, it won't bring down the network, just span the traffic from uplinks to a sniffer port. Then you can capture the traffic and see if the frame header looses it's trunking characteristics.

New Member

Re: 3750 dot1q trunking problems

I have been seeing the same problem too however I have found the following bug:

This is a cosmetic issue and does not affect performance.

CSCec14238 - 3750 connected using dot1q cause Runts increase on interface.


When using dot1q on trunk interface on the Cat3750, runts may be seen on show interface output because valid dot1q encapsulated packets which are 61-64 bytes including the q-tag are counted by the Cat3750 as undersized frames, even though these packets are forwarded correctly. In addition, these packets are not reported in the appropriate category (unicast,

multicast, broadcast) in receive statistics.


This is a h/w limitation of the Cat3750 platform. There is no workaround.

Re: 3750 dot1q trunking problems

Yes, that is the bug that Cisco says that it is. I have tried changing the trunking mode, which we had set as desirable, to "on" and the problem seems to have disappeared on the switch we had upgraded to the latest IOS (12.1(19)). At my next window I will try this on the switches we initially had problems with to see if it helps with the problem or not.

Although Cisco says that the problem is completely cosmetic, if you are getting the LED flashing amber, from what we can tell it was causing degredation in network performance. Definitely upgrade and turn trunking on rather than have it set to desirable, this might fix the slight trouble that it was having.

This widget could not be displayed.