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Posted by Peter on June 28, 2009, 1:53 pm
The interface to my cable modem connection to shaw.ca
here on Pender Island has MTU = 576 whereas I would
have expected to find MTU = 1500.
joule:/home/peter# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:...:1c
inet addr:24.108...84 Bcast:255.255.255.255 Mask:
255.255.252.0
UP BROADCAST RUNNING MULTICAST MTU:576 Metric:1
Can anyone explain this MTU? Can anyone tell me what
MTU they have for a Shaw connection elsewhere? Is 576
company policy or an erroneous configuration on a router?
Thanks, ... Peter E.
peasthope ... shaw.ca
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Posted by Robert Redelmeier on June 28, 2009, 11:31 pm
> The interface to my cable modem connection to shaw.ca
> here on Pender Island has MTU = 576 whereas I would
> have expected to find MTU = 1500.
>
> joule:/home/peter# ifconfig eth0
> eth0 Link encap:Ethernet HWaddr 00:...:1c
> inet addr:24.108...84 Bcast:255.255.255.255 Mask:
> 255.255.252.0
> UP BROADCAST RUNNING MULTICAST MTU:576 Metric:1
>
> Can anyone explain this MTU? Can anyone tell me what
> MTU they have for a Shaw connection elsewhere? Is 576
> company policy or an erroneous configuration on a router?
All good questions.
Those of us with PPPoE are recommended not use MTUs not to
exceed 1492 so the encapsulation overhead can fit.
I think there is an optimum for MTU depending on underlying
transport layers (ATM or whatever) so there aren't small/short
packets routinely created.
-- Robert
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Posted by Peter on June 29, 2009, 10:16 am
> I think there is an optimum for MTU ...
Thanks Robert.
I had hoped that another Shaw subscriber would tell me
his connection is 1500. I'd like to find that 576 is erroneous.
Regards, Peter E.
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Posted by Rich Seifert on June 29, 2009, 11:42 am
In article
> > I think there is an optimum for MTU ...
>
> Thanks Robert.
>
> I had hoped that another Shaw subscriber would tell me
> his connection is 1500. I'd like to find that 576 is erroneous.
>
> Regards, Peter E.
576 is the largest MTU that is guaranteed never to require fragmentation
in IP. You might try increasing the MTU (but, as Robert said earlier,
not necessarily to the full 1500, to allow further encapsulation) and
see if it causes any problems. You could also talk to Shaw tech support
to see what they say (yeah, I know--a silly suggestion).
Now, if your router and ISP *really* require 576 MTU, you might try
reducing the MTU on your attached system(s) to 576 also, so that there
is no fragmentation being performed in your edge router.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
Send replies to: usenet at richseifert dot com
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Posted by Rick Jones on June 29, 2009, 2:13 pm
> 576 is the largest MTU that is guaranteed never to require
> fragmentation in IP.
That is incorrect. One can toda and at least in the past often had
MTUs smaller than 576 bytes. 576 is the minimum, maximum size for IP
fragment reassembly. That is to say a conforming IV (v4) system must
be able to reassemble IP datagram fragments into full IP datagrams of
at least 576 bytes in size.
Nowadays, and perhaps for a while, certainly in _practice_ using an
MTU of 576 bytes would avoid IP fragmentation, but it is by no means a
guarantee.
rick jones
--
a wide gulf separates "what if" from "if only"
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
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> here on Pender Island has MTU = 576 whereas I would
> have expected to find MTU = 1500.
>
> joule:/home/peter# ifconfig eth0
> eth0 Link encap:Ethernet HWaddr 00:...:1c
> inet addr:24.108...84 Bcast:255.255.255.255 Mask:
> 255.255.252.0
> UP BROADCAST RUNNING MULTICAST MTU:576 Metric:1
>
> Can anyone explain this MTU? Can anyone tell me what
> MTU they have for a Shaw connection elsewhere? Is 576
> company policy or an erroneous configuration on a router?