View Full Version : [ubuntu] 802.1q (VLAN) minimum packet size.

May 12th, 2008, 04:38 PM
Hi All

Lately I started working with my ubuntu server (7.04) on VLAN tagging - 802.1q.
I managed to define the interface to work with VLANs, but find out that the minimum packet size stayed the same-60, instead of 64 which is the extra 4 Bytes of the 802.1q & 802.1p header.
It appears that there are some network devices, that cut off the VLAN header, and you stay with illegal packet size (60-4=56) so this packet will be dropped (such as ARP)

Do you know how to increase the minimum ethernet packet size?


May 26th, 2008, 10:09 PM
Found it eventually:

Latest IEEE 802.3 definitions specify a minimum frame size of 64 (with CRC) even if 802.1q was implemented.
It specifies the minimum payload size to 4 bytes less if VLAN TAG is implemented.
Maximum frame size is set to 1522 (with CRC).

* Note that CRC is not seen on sniffers.

All this implies that a frame size of 64 bytes (with CRC) is legal even if the frames format is VLAN TAG (802.1q).
The Layer2 devices in the network should pad the frame if needed (when untagging the frame for example).

* Cisco switches will show a 802.1q frame of size 64 bytes (with CRC) as "undersize", however it will not show it as "RcvErr". In practise it will pass it and will pad the frame.