Subject: Re: Problems with wm interface and vlan's
To: Chris Ross <>
From: Steve Woodford <>
List: netbsd-users
Date: 03/02/2005 15:25:40
On Wednesday 02 March 2005 14:51, Chris Ross wrote:

>    Okay.  Interestingly, tho, the fxp reports 1500, while vlan reports
> 1500.  Why does the wm's vlan interface report 1496, unless wm is
> set to 1504?  Not real important, but an odd difference, IMHO.

The mtu on a physical interface like fxp or wm is not related to the mtu=20
of vlans using that interface. The vlan's initial mtu is either 1496 or=20
1500 bytes depending if the *hardware* can handle the additional 4 bytes=20
vlan tag overhead when sending/receiving Ethernet packets.

> wm0 at pci1 dev 6 function 0: Intel i82542 1000BASE-X Ethernet, rev. 3

This chip doesn't support > 1500 byte frames, so any vlans on top of it=20
will be limited to 1496 mtu.

=46rom the vlan(4) manual page:

"vlan can be used with devices not supporting the IEEE 802.1Q MTU, but
 then the MTU of the vlan interface will be 4 bytes too small and will
 not interoperate properly with other IEEE 802.1Q devices, unless the
 MTU of the other hosts on the VLAN are also lowered to match."

Cheers, Steve