netdev - Re: warnings from MTU setting on switch ports

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Message-ID: <2d4024ef-f80a-475f-a247-e02971b55152@prevas.dk>
Date:   Mon, 30 Nov 2020 23:50:34 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>
Subject: Re: warnings from MTU setting on switch ports

On 30/11/2020 23.35, Vladimir Oltean wrote:
> On Mon, Nov 30, 2020 at 11:13:39PM +0100, Rasmus Villemoes wrote:

>> FWIW, on a 4.19 kernel, I can do 'ping -s X -M do' for X up to 1472
>> for IPv4 and 1452 for IPv6, but I don't think that tells me much about
>> what the hardware could do.
> 
> IP will get fragmented by the stack to the interface's MTU.

Nope, that's what the '-M do' is for:

# ping -M do -s 1880 169.254.246.13
PING 169.254.246.13 (169.254.246.13) 1880(1908) bytes of data.
ping: local error: message too long, mtu=1500

Rasmus