[GTALUG] Rogers and MTU size

James Knott james.knott at rogers.com
Sun Jan 4 17:55:37 UTC 2015

One thing I noticed recently was that my firewall, running Linux, was
getting a MTU of 576 bytes, instead of the normal 1500 bytes.  This was
being set by the Rogers DHCP server (option 26).  576 was common back in
the days of non-error correcting modems, but hasn't been needed since
then.  If you're running dhcpcd, you can edit
/etc/sysconfig/network/dhcp and change the line 'DHCPCD_USER_OPTIONS=""'
to 'DHCPCD_USER_OPTIONS="-M"'.  This will disable the DHCP MTU setting
and use the configured MTU.

Using a 1500 MTU will improve network efficiency, as 576 bytes will
cause 3 packets to be sent, instead of 1, each with an IP header and
also causes a lot more work for all the routers along the path.  You can
verify the MTU with a ping -s 1500 <destination>.  If that works, then
you have a 1500 byte MTU along the entire path to the destination.

I found this because I was curious as to why the Ethernet port connected
to the cable modem was not getting an IPv6 link local address.  IPv6
requires a minimum MTU of 1280 bytes so, with 576, IPv6 wasn't allowed.

I have advised Rogers of this and the person I was speaking to agreed it
should be 1500.  Let's see if they fix it.

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