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Cisco 3500 XL - Dot1q trunk - mtu is at 1496 for tagged vlan, MTU issue

Posted on 2008-10-14
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Last Modified: 2008-10-20
I have two csico 3500 xl switches that are connected via a 10Mbit point to point radio.  The interfaces are configured for dot1q tag and trunk with vlan 1 being native.

I can ping at 1500 on vlan 1, but only 1496 on vlan 10.

It appears that there is an MTU issue with the 4 extra bytes that are added to vlan 10 for the dot1q tags.

I have adjusted the interface and vlan MTU to 1536, rebooted, and still can't transfer traffic at 1500, which is affecting traffic flows that are on vlan 10.  vlan 1 is not affected.

Any ideas on what could be causing this?
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Question by:cdubbcisco
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Expert Comment

by:Don Johnston
ID: 22716359
I don't think it's your switches. I think it's the radio link equipment. The only work-around I can think of is the reduce the MTU at the source of the packet (the PC and server).
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Expert Comment

by:kyleb84
ID: 22716676
What brand/model is your Radio link?

""I have adjusted the interface and vlan MTU to 1536""

You don't need to increase your MTU, you need to decrease it.

If you set your Cisco's interface/vlan MTU to 1496, does this help?

The MTU is the max amount of data put in each Ethernet frame, the VLAN header (Those extra 4 bytes) are included as "data" and appended to this, therefore an MTU of 1496 will end up as 1500 on the wire with VLAN information.

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Expert Comment

by:Don Johnston
ID: 22716783
Changing the MTU on the switch isn't going to help. The 3500XL is a layer 2 switch and won't fragment the packet.
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cdubbcisco earned 0 total points
ID: 22725982
The 10M connection is the problem, as it's a nature of 10Mbit. The max MTU on a 10M link is 1500.

The problem if i lower to 1496 is that some traffic types (SSL, VPN's in particular) require to have 1500, and if they don't it fails.

We are upgrading our radios to 100M radios which will fix our problem.

Thanks for the feedback.
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