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Re: vlan with higher MTU (jumbo frame) than underlying Ethernet interface
On Sep 7, 2012, at 12:53, Manuel Bouyer wrote:
> On Fri, Sep 07, 2012 at 11:21:11AM -0700, Erik Fair wrote:
>> NetBSD's documentation doesn't say explicitly: can one have a vlan interface
>> with a jumbo frame MTU (e.g. 9000), on top of an Ethernet interface which
>> supports jumbo frames, but whose MTU (1500) has not be adjusted up?
> No, I think the underlying interface needs to have the hightest MTU of all
> But you can have another vlan(4) interface with 1500 MTU
Unfortunately, that's backwards: for network booting (SFAIK, no system firmware
supports or uses jumbo frames - it's an unsafe assumption), and for systems
that don't support jumbo frames to be on the same LAN, you need those to be on
the non-vlan'd network (no VIDs) and everything configured so that no one sends
or broadcasts any packets to them bigger than the standard MTU.
The configuration I have in mind is that the vlan have jumbo frames (vlans need
larger than standard MTU anyway, even if only a few bytes for the VID), and
only those hosts that can support it will configure it. That way, those
(consenting) hosts could jumbo away at each other, but would be sending
standard MTU packets to "everyone else" on the same LAN, with no VID and a
different IP network address.
Of course, the NetBSD Ethernet drivers that support jumbo frames need to be
written to be able to receive them always, or a vlan configuration directive as
I discuss passed through in a nominally invisible way), for this to work. In
this sort of scenario, interface MTU becomes more of an API notification to
userland on what can be sent (or to the IP stack for fragmentation, or TCP MSS)
than an actual constraint on the operation of the driver on packet reception.
Or, if one wants to quote ancient (RFC) scripture, NetBSD's Ethernet drivers
need to be coded to be "liberal in what they accept, and conservative in what
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