runt frames reported on cisco switch

I am getting many (>10%) runt frames on a cisco switch on the port that
is connected to a linux router running with vlans. Despite the reported
errors, there are no observed problems with the network.

I have two other servers, both using vlans, plugged into the same switch
with no reported problems.

I have tried the linux router on other ports on the cisco switch and the
fault migrates with the change of port.

The Ethernet driver is on the router 'via-rhine', and I have observed
the following relevant bits in the module (drivers/net/via-rhine.c):

"The chip does not pad to minimum transmit length."

And:

"
        if (skb->len < ETH_ZLEN) {
                skb = skb_padto(skb, ETH_ZLEN);
                if (skb == NULL)
                        return 0;
        }
"

This says to me that the driver is only checking for and padding frames
< 60 bytes long. But isn't the minimum length of a vlan tagged frame 64
bytes? I assume this because at some point the frame may be sent out on
a port without the vlan tag and will need to maintain the minimum frame
size after the vlan header has been removed...

So is there a problem here? If so, I assume that to fix it I would
detect a vlan frame (proto == 0x8100) and pad to 64 bytes instead...
 
Thanks

James