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I have a system with two physical interfaces, combined into a LACP aggregation group. That LACP channel has two VLANs, one untagged (the "native vlan") and one using VLAN tagging. This gives us:

lagg0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=19b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4>
        ether 00:25:90:1d:fe:8e
        inet 10.243.24.23 netmask 0xffffff00 broadcast 10.243.24.255
        media: Ethernet autoselect
        status: active
        laggproto lacp
        laggport: em1 flags=1c<ACTIVE,COLLECTING,DISTRIBUTING>
        laggport: em0 flags=1c<ACTIVE,COLLECTING,DISTRIBUTING>
vlan0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=3<RXCSUM,TXCSUM>
        ether 00:25:90:1d:fe:8e
        inet 10.243.16.23 netmask 0xffffff80 broadcast 10.243.16.127
        media: Ethernet autoselect
        status: active
        vlan: 610 parent interface: lagg0

Is it possible to set a 9K MTU on lagg0 while preserving the 1500 byte MTU on vlan0? Normally I would simply try this out, but this is actually on a vendor-supported platform and I am loathe to make changes "behind the back" of their administration interface.

This system is roughly FreeBSD 7.3.

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2 Answers 2

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Yes, that is possible. The VLAN parent must be equal to or larger MTU than the VLAN itself.

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I used different MTUs vor vlan and parent Interface in FreeBSD 4.x times (1496 and 1500 in my case), so i think it should work.

You could just use two FreeBSD machines connected to each other and try it out.

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