Ethernet LAN MTU for cable modem connection to shaw.ca

Bookmark this page:  YahooMyWeb Yahoo!  Google Google  Windows Live Favorites Windows Live  del.icio.us del.icio.us  digg digg  Add to Netscape Netscape
Subject Author Date
MTU for cable modem connection to shaw.ca Peter 06-28-09
Posted by Peter on June 28, 2009, 1:53 pm
Please log in for more thread options
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

Posted by Robert Redelmeier on June 28, 2009, 11:31 pm
Please log in for more thread options

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



Posted by Peter on June 29, 2009, 10:16 am
Please log in for more thread options

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.


Posted by Rich Seifert on June 29, 2009, 11:42 am
Please log in for more thread options
In article


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

Posted by Rick Jones on June 29, 2009, 2:13 pm
Please log in for more thread options

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...

Similar ThreadsPosted
MTU for cable modem connection to shaw.ca June 28, 2009, 1:53 pm
Putting a switch between router and cable modem August 17, 2009, 2:43 pm
Who is DHCP server for my Router's WAN port connected to Cable modem? April 9, 2005, 4:21 pm
Re: Ethernet connection sensitive to cable length May 3, 2008, 12:37 pm
WakeONLAN did not work for direct cable connection ? (Nics were shutdown after a night of sleep ?) February 9, 2008, 4:56 am
Problem: 2 ISP, 1 router, 1 modem, 1 modem/router September 20, 2006, 10:14 am
modem not trained August 4, 2004, 1:12 pm
10Mpbs vs 100Mpbs DSL Modem January 4, 2006, 7:15 am
SHDSL 4.6 modem set for sale $600 for the set October 29, 2004, 5:31 pm
Ethernet LED winks on DSL modem & router March 25, 2008, 7:52 am
ForSALE: SHDSL 4.6 modem set for sale : open to Offers November 26, 2004, 7:06 pm
I have PC->Router->DSL Modem->ISP, Does ISP Router learn the PC MAC address? April 25, 2005, 9:33 am
Losing connection... August 25, 2005, 10:58 am
unnumbered connection & DSL March 2, 2006, 12:52 am
Problem with WAP and LAN connection September 4, 2006, 7:21 am
Latest PostsForumRSS
NEWS: Samsung takes on the Apple iPad with the 7 inch Galaxy... Wireless Networking
c3560 port configuration Cisco Systems
Broadband 2010: A Big Slowdown [telecom] General Telecommunications Forum
Control Hot Water Circ Pump With X10? General Home Automation
Official Course CCNP TSHOOT 642-832 / Foundation Learning Gu... Cisco Certification
Speedflow Communications Honored for Innovation Voice-Over-IP
USB _to_ RJ45 (not from) connection Ethernet LAN
FAQ: Maximizing cable modem or DSL speed Cable Modems
CASH FOR CISCO - I BUY USED AND NEW EQUIPMENT & LOTS MOR... Telecom Technical
FAQ: Maximizing cable modem or DSL speed Digital Subscriber Line
How to set up Meridian 1 to "provide clock" to a C... Nortel Networks
New Discovery about WDM LAN and Telecom Cabling
Control Hot Water Circ Pump With X10? Home Automation
Text file to automate restoring a dropped VPN connection. Virtual Private Networks
Home Theater Installation Home Theater
Re: The Turkic Languages in a Nutshell Fiber Optics
sip Video Conferencing
Residential Cabling Guide Home Cabling Guide

Finally, an instantly downloadable book that saves you thousands in home improvement dollars! Enjoy living in 21st century technology-advanced home while increasing its selling value and competitive advantage on the real estate market. Whether your cabling is for home office or high-tech leisure, you can wire your home yourself or learn "wirish" to speak with your cabling contractors in their language!

Click Here to learn more