[VLAN] MPU Problems with e100 and VLANs

[Emil Micek]

Hello list,

I've got a Linux router with 2 NICs:

- eth0 is external interface connected to Internet, lspci reports it as
  01:08.0 Ethernet controller: Intel Corporation 82801DB PRO/100 VE
  (CNR) Ethernet Controller (rev 82)
- eth1 is internal interface connected to our local network, lspci line 
  looks like this
  01:0c.0 Ethernet controller: Intel Corporation 8255xER/82551IT Fast 
  Ethernet Controller (rev 10)

Now i want to do VLAN tagging of internet traffic passing through the
router, so i created VLAN interface eth1.101 with IP 10.146.154.62 
(our local network has 10.146.154.0/26 adress range).

Everything is working fine except small packets.
Minimal ethernet frame cannot be shorter than 64B (according to the
specification). If the payload of the frame is not big enough, the frame
is padded with random data to 64B. If such padded frame arrives to our
router, the padding is removed, the VLAN ID tag is added and the frame
is sent to our local network without the padding added back, resulting
in frame smaller then 64B. Such frames are then discarded by our cable
modem equipment located in our local net.

Wire shark dump of such small packet can be seen on
http://www.tkrjasek.cz/mie/etth/packet.txt

The router is running CentOS version 5 with distrib kernel
2.6.18-8.1.4.el5. Both of the NIC's are handled by e100 kernel module.

Now i'd like to ask what is the best aproach to solve this problem?

Thanks

Emil