From: Jonathan Thibault <jonathan@navigue•com>
To: "netdev@vger•kernel.org" <netdev@vger•kernel.org>
Subject: Re: gretap default MTU question
Date: Thu, 10 Mar 2016 09:30:50 -0500 [thread overview]
Message-ID: <56E1851A.2030407@navigue.com> (raw)
In-Reply-To: <56E05C8F.1060901@navigue.com>
On 09/03/16 12:25 PM, Jonathan Thibault wrote:
> Hello good people of netdev,
>
> When setting up gretap devices like so:
>
> ip link add mydev type gretap remote 1.1.1.1 local 2.2.2.2 nopmtudisc
>
> I'm observing two different behavior:
>
> - On system A, the MTU of 'mydev' is set to the MTU of the 'parent'
> interface (currently 1600) minus 38. All other interfaces on that system
> have a default MTU of 1500, only the parent was forced to 1600 to avoid
> fragmentation. So 'mydev' accurately figures out that its MTU is 1562.
>
> - On system B, with the 'parent' interface MTU set to 1600 and all other
> defaulting to 1500 (same situation as A), the MTU of 'mydev' gets set to
> 1462.
>
> I'm trying to figure out which behavior is normal and what mechanism (if
> any) causes the MTU to be set differently. In both cases the 'parent'
> device has an MTU of 1600. The code in ip_gre.c does this:
>
> dev->mtu = ETH_DATA_LEN - t_hlen - 4;
>
> In this case, system B would have the expected behavior and I need some
> way to explain what goes on with system A.
>
> Of course I can force the MTU on system B but I was rather pleased with
> the 'magic' on system A.
>
> If anyone here familiar with this code can offer an explanation, it
> would greatly ease my curiosity.
>
> Jonathan
Replying to myself to leave a trace.
Turns out that the MTU of the gretap device is based on the MTU of the
interface that has a route to the 'remote' address. In my specific
case, it used the default route on system B and a static route on system
A. The cause of the discrepancy was a missing route.
Jonathan
prev parent reply other threads:[~2016-03-10 14:30 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-09 17:25 gretap default MTU question Jonathan Thibault
2016-03-10 14:30 ` Jonathan Thibault [this message]
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