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Date: Fri, 27 May 2016 00:57:34 -0400 (EDT)
From: Garrett Wollman <wollman@hergotha.csail.mit.edu>
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To: dmarquess@gmail.com
Subject: Re: Changing MTU on cxgbe
References: <CAJpsHY4vF5Ky6GuAusLOOROgiQuyD2CcRmVxu8x3cArQRZxcbg@mail.gmail.com>
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In article <CAJpsHY4vF5Ky6GuAusLOOROgiQuyD2CcRmVxu8x3cArQRZxcbg@mail.gmail.com> you write:

># ifconfig -m cxgbe0
>cxgbe0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST>

># ifconfig cxgbe0 mtu 9000
>ifconfig: ioctl SIOCSIFMTU (set mtu): Invalid argument

I believe this device, like many others, does not allow the MTU (or
actually the MRU) to be changed once the receive ring has been set up

You may be able to do it if you down the interface first, or you may
have to do it at boot time before anything else (and in particular
before the interface is marked "up" for the first time).

-GAWollman