[GTALUG] Rogers and MTU size

James Knott james.knott at rogers.com
Sun Jan 4 19:16:42 UTC 2015

On 01/04/2015 01:43 PM, Scott Allen wrote:
> On 4 January 2015 at 12:55, James Knott <james.knott at rogers.com> wrote:
>> 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.
> What does this pertain to? The router WAN side, router LAN side, or
> client machine(s)?
Well, since I said Rogers DHCP, it's the MTU they assign to your
firewall, for your connection to them.

> My Internet connection, provided by Teksavvy, is on the Rogers cable
> network. Running ifconfig on my desktop machine I get:
> eth0    Link encap:Ethernet  HWaddr xx:xx:xx:xx:xx:xx
>           inet addr:xxx.xxx.xxx.xxx  Bcast:xxx.xxx.xxx.255  Mask:
>           inet6 addr: fe80::bexx:xxff:fexx:xxxx/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           RX packets:260045 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:221024 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:1000
>           RX bytes:250671109 (250.6 MB)  TX bytes:33714522 (33.7 MB)
> lo        Link encap:Local Loopback
>           inet addr:  Mask:
>           inet6 addr: ::1/128 Scope:Host
>           UP LOOPBACK RUNNING  MTU:65536  Metric:1
>           RX packets:20203 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:20203 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:0
>           RX bytes:2392082 (2.3 MB)  TX bytes:2392082 (2.3 MB)
In there, I was getting only a 576 byte MTU for eth0, even though the
interface was configured for 1500.  I also wasn't getting the IPv6 link
local address.  I guess Teksavvy is providing different config for their
DHCP settings.  You can verify this with Wireshark.  You can force DHCP
negotiation with "dhcpcd eth0 -n" so you don't have to wait for it to

You'll notice the 65536 MTU on lo.  That's the maximum for IP, since
there's no MTU limit on the loopback interface.  Back when I was at IBM,
where they used token ring, I set my MTU to about 4K, as I was often
moving large files to/from local servers.

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