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Date:      Sat, 7 Feb 2015 02:42:24 +0000
From:      "Pokala, Ravi" <rpokala@panasas.com>
To:        "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>
Subject:   Changing the MTU on a lagg device
Message-ID:  <D0FABB8B.12C626%rpokala@panasas.com>

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Hi folks,

Let's say you have lagg0, consisting of if0 and if1. If you want to change
the MTU, you have to remove if0 and if1 from the lagg, change their MTUs,
and add them back; that is:

1) ifconfig lagg0 -laggport if0
2) ifconfig lagg0 -laggport if1
3) ifconfig if0 mtu 9000
4) ifconfig if1 mtu 9000
5) ifconfig lagg0 laggport if0
6) ifconfig lagg0 laggport if1

It would be nice if this could be done with a single command:

1) ifconfig lagg0 mtu 9000

Panasas implemented that functionality for our older base FreeBSD, and
we're looking to port it forward and push it upstream. However, it looks
like someone thought about this case and explicitly decided not to do it;
if_lagg.c has:

	case SIOCSIFMTU:
		/* Do not allow the MTU to be changed once joined */
		error =3D EINVAL;
		break;


Does anyone know why that is? Would anyone object to a patch that lets you
change the MTU on the lagg device, and having the lagg driver change it on
all the component interfaces for you?

Thanks,

Ravi




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