Subject: 802.1Q & ETHER_MAX_LEN
To: None <>
From: Manuel Bouyer <>
List: tech-net
Date: 10/01/2000 21:35:24
I tested our 802.1Q support with a cisco switch today, and ran into the
problem mentionned in kern/11109 (I expected to run into this anyway :),
which is that on a cisco trunk port we can expect to see packets of
1522 bytes instead of 1518 (the 4 extra bytes being the 802.1Q tag).
If we want to be able to talk with others 802.1Q devices we need to solve

There are several issue here:
- hardware: does the ethernet adapter supports frames of 1522 bytes ?
  I looked quickly at the tulip and smc83c170 sources and found no way to
  change this, it looks like the 1518 bytes values is hardcoded in hardware.
  Does anyone know if there are (10/100) ethernet chips that can send/receive
  frames bigger than the standart len ?

- software: assuming there are adapters that can do something else than 1518
  (gigabit adapters can), we need a way to advertize this to the vlan layer.
  For now the vlan layer uses the MTU advertised by the ethernet driver.
  We can't change this (from 1500 to 1504) because non-802.1Q packets would
  be too big. So we may need a new field in the if_data struct (maybe ifi_rmtu)
  to advertise the real MTU the interface can do, and which can be used by
  pseudo-interfaces like vlan.

Comments ?

Manuel Bouyer <>