vlan with higher MTU (jumbo frame) than underlying Ethernet interface

[Erik Fair]

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?

The goal would be to use jumbo frames between consenting hosts on a switch that supports jumbo grams, but segregate the traffic by VID (and IP address) such that a jumbo-supporting host won't send jumbograms to a non-supporting host by default.

Erik <***@netbsd.org>


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[Manuel Bouyer]

Post by Erik Fair
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
vlans.

Post by Erik Fair
The goal would be to use jumbo frames between consenting hosts on a switch that supports jumbo grams, but segregate the traffic by VID (and IP address) such that a jumbo-supporting host won't send jumbograms to a non-supporting host by default.

But you can have another vlan(4) interface with 1500 MTU

[Erik Fair]

Post by Manuel Bouyer

Post by Erik Fair
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
vlans.
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 they send."

Erik <***@netbsd.org>


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[Matt Thomas]

Post by Erik Fair

Post by Manuel Bouyer

Post by Erik Fair
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
vlans.
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.

set the mtu on the interface route.
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