Solved: Re: Having problems with certain sites

matlsmith · ‎03-29-2006

Wasn't sure which forum to post this in...hope this one is appropriate.

I have a PTP T1 between two locations with ADSL being fed through the T1.

We recently got it going after getting the telco to fix some wire pair problems in the repeaters.

However I'm still having some issues right now.

I can get out to most internet sites out there (yahoo, google, newegg, etc etc).

However some sites will not load, which include but not limited to hotmail.com mapquest.com, msn.com somehwat.

When I go to these addresses, it says "Finding website" "Website Found" and I get the title in the top bar of the browser..for example when I got to hotmail.com it says "Redirecting" which is to the login servers for hotmail, but it never actually loads the content on the computer screen.

Any ideas why some sites load and others don't?

I'm using Cisco 1841 Modular routers, with a ADSL WIC card in one router for the DSL traffic.

Thanks

Richard Burts · ‎03-30-2006

Matthew

I believe this forum is appropriate for your question.

Your symptoms sound like an issue with negotiation of segment size and fragmentation. Path MTU Discovery is generally used in setting up a TCP connection to discover the largest frame that can be sent without requiring fragmentation. However there are things that can prevent PMTUD from working properly. Most especially there are some networks that block the ICMP response of Fragmentation Required but DF Set. So the end stations do not find the optimum frame size. If the end station sends a maximum frame size (as most would) and somewhere along the path the frame needs fragmentation, then the packet will be dropped.

Your symptoms sound like in trying to go to those sites you open the TCP connection, the first several frames are relatively small and succeed, so you see the initial message from the site, but then are sending large frames which get dropped.

One way around this is to configure ip tcp adjust-mss and set a smaller size. I have encountered symptoms like yours, have set the size to something around 1360, and had things work very well. I suggest that you give this a try and let us know if it helps.

HTH

Rick

HTH

Rick

View solution in original post

Richard Burts · ‎03-30-2006

Matthew

I believe this forum is appropriate for your question.

Your symptoms sound like an issue with negotiation of segment size and fragmentation. Path MTU Discovery is generally used in setting up a TCP connection to discover the largest frame that can be sent without requiring fragmentation. However there are things that can prevent PMTUD from working properly. Most especially there are some networks that block the ICMP response of Fragmentation Required but DF Set. So the end stations do not find the optimum frame size. If the end station sends a maximum frame size (as most would) and somewhere along the path the frame needs fragmentation, then the packet will be dropped.

Your symptoms sound like in trying to go to those sites you open the TCP connection, the first several frames are relatively small and succeed, so you see the initial message from the site, but then are sending large frames which get dropped.

One way around this is to configure ip tcp adjust-mss and set a smaller size. I have encountered symptoms like yours, have set the size to something around 1360, and had things work very well. I suggest that you give this a try and let us know if it helps.

HTH

Rick

HTH

Rick

matlsmith · ‎03-30-2006

So the command on the cisco router would be.

ip tcp adjust-mss 1360 correct?

under the appropriate interface which in this case would be the T1 interfaces I belive.

matlsmith · ‎03-30-2006

Its working now.

I did what you suggested on the ethernet port that all LAN traffic goes through, and it resolved the issue.

Thanks

Richard Burts · ‎03-30-2006

Matthew

I am glad that we were able to help solve your issue. I hope you will continue to be active in the NetPro forums.

HTH

Rick

HTH

Rick