[Xen-users] RE: [Xen-devel] Ethernet MTU

["Sylvain Coutant" <sco@xxxxxxxxxx>]

> So when configuring a VLAN on an interface, you say that Linux
> automatically adds 4 to the MTU of the underlying physical interface?
> 
> Since Linux has no way of synchronizing the MTU with the interface at
> the other end of the wire, that's really stupid.  It should just lower
> the MTU of the virtual interface by 4 bytes instead.

No, that would also be stupid in some way. The goal is each interface have the 
same mtu on each side.
If you some have a 1500 bytes mtu interface without tagging at the other end of 
the Ethernet segment (I mean the switch removes the tag before sending the 
packet to the other computer), you would then have different mtu sizes (1496 on 
one side and 1500 on the other).

It is usual to manage oversized Ethernet frames when dealing with tags. So you 
keep a basic mtu of 1500 and add a varying size Ethernet header to this. Your 
Ethernet packet will then be something *starting at* 1518 (with a basic 
header). In the case of 802.1q tagging, it will be larger. This is the way you 
ensure each ethernet interface has the same mtu at each end.


BR,

--
Sylvain COUTANT


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