Giants and Dot1Q 1

[chieftan]

We have a connection from a 2950 to a 4506. We are seeing a lot of Giants at either end and I know that one of the causes can be a dot1q tag.

One end of the link is set to "Off" for trunking and the other end is set to "Desirable". Should it be this way or is there something wrong with that config (although I cant see the config, it seems to be doing it automatically)?

Cheers

 

[ADB100]

If the ports haven't negotiated or been hard-oded for trunking then it won't be a trunk and you shouldn't see any giants. Issue the command 'show interfaces trunk' on either switch and make sure it isn't trunking. If you are still seeing giants then I am not sure - it could be a physical problem with the cabling?. Are you seeing giants in the output from the command 'show interface counters errors'?

Andy

 

[chieftan]

Hi Andy,

Yes, seeing it on the interface output.

I understand now why one end is desirable and the other is off, so that part is cleared up.

If a port is set to "Access" then the show int trunk command show trunking as off. However, the default for ports is desirable, so that DTP can be utilised.

Still seeinf an abnormal amount of giants though, and no trunking invlolved. Strange as only (Normally) a tag or VPN tag would do this, but this not VPN'd either.

If the packet comes in larger than the MTU then normally it is fragmented correctly, but when a tag of extra bytes are added it cannot see it as this is after it has entered the switch.

Will continue investigating.

Thanks Andy

 

[unclerico]

what kind of traffic is generating the giants?? have you done a packet capture?? some traffic may have the DF bit set and therefore will not fragment...

 

[chieftan]

Unclerico.... that is a very good point that I had not thought of. Currently we have not been able to complete a wireshark trace, however I am hoping to complete that today and we may see something like this happening. Thanks for the pointer in the right direction. Much appreciated. Will let you know the outcome.