Re: MTU 1640

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Hello Johan,

Thank you for the response. In fact everything works fine at my site. The
point is why do I have MTU size of 1460 instead of 1492 (1492 = 1500
default MTU - 8 Byte PPPoE Header). Furthermore I cannot change in the MTU
Size in the UI of monowall (I can change but it doesn`t take effect).

A ping gives following result:

$ ping x.x.x.x -M do -s 1432
PING x.x.x.x (x.x.x.x) 1432(1460) bytes of data.
1440 bytes from x.x.x.x: icmp_seq=1 ttl=59 time=46.1 ms
1440 bytes from x.x.x.x: icmp_seq=2 ttl=59 time=37.9 ms
1440 bytes from x.x.x.x: icmp_seq=3 ttl=59 time=35.5 ms
1440 bytes from x.x.x.x: icmp_seq=4 ttl=59 time=37.2 ms

--- x.x.x.x ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 601ms
rtt min/avg/max/mdev = 35.522/39.213/46.193/4.127 ms
$

$ ping x.x.x.x -M do -s 1433
PING x.x.x.x (x.x.x.x) 1433(1461) bytes of data.
ping: local error: Message too long, mtu=1460
ping: local error: Message too long, mtu=1460
ping: local error: Message too long, mtu=1460
ping: local error: Message too long, mtu=1460

--- x.x.x.x ping statistics ---
4 packets transmitted, 0 received, +4 errors, 100% packet loss, time 608ms

Thank you,

Jakob

On 26 December 2014 at 11:18, Johan Vanderwesthuizen <[email protected]>
wrote:

> The default MTU, size of 1500-byte MTU, or somewhat smaller is sufficient
> because Ethernet frames are limited to a standard maximum of 1500 bytes of
> payload data. Various IEEE standards use expanded frame sizes to carry
> additional information. Don't spend to much time with the MTU size at
> first. Verify what are the speed and duplex settings and actually check the
> results first, before troubleshooting layer 3 and 4 issues. Well, this is
> my 2c from dealing with PPPoE/VPN issues in the past. Another test you can
> run, once connected, is to change the MTU size from the command line and
> see at what value does an ping fails when modifying the MTU size/value. You
> will notice if you make the MTU size to small, the ping will fail/timed
> out. Troubleshooting MTU size and fragmented packers are really hard to
> figure out. Wireshark will add some light to the works, depending "where"
> you you capture the network traffic. (Encrypted vs unencrypted) Hope this
> info will put you on the right track.
>
> > On Dec 26, 2014, at 3:12 AM, Jakob Schwienbacher track<
> [email protected]> wrote:
> >
> > Hi,
> >
> > Im running M0n0wall 1.8.1. Im using PPPoE to connect to my ISP. I had
> some
> > trouble with MTU size, then i saw my WAN MUT is set to 1460:
> >
> > $ ifconfig
> > ~
> > ngwan0: flags=88d1<UP,POINTOPOINT,RUNNING,NOARP,SIMPLEX,MULTICAST>
> metric 0
> > mtu 1460
> > ~
> >
> > I went to Interfaces > WAN and set the MTU to 1492 (which should be the
> > Maximum possible on PPPoE links). The MTU setting is visible on the UI
> and
> > in the config file also after a reboot.
> >
> >
> > $ cat /var/etc/mpd.conf
> > startup:
> >
> > pppoe:
> > create bundle static B1
> > ~
> > set link mtu 1492
> > ~
> >
> > I disconnected and reconected my WAN interface, I did a reboot of my
> > m0n0wall. But the MTU doesn`t get changed. It remains on 1460.
> >
> > 1. Does anybody see the same issue?
> >
> > 2. Someon know about the value 1460? I couldn't find anything about that.
> >
> >
> > Thanks in advance,
> >
> > Jakob
>

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