some website didn't work

[kubajzs197]

Hello everybody,

I have cisco sr520 adsl router i have everything set up and it is ok. Internet work but i can not open some web pages In DNS is not problem NAT work fine. When i try some simple adsl gateway the problem websites work correctly. I think the problem is on sr520 router. Thanks for your reply

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[Richard Burts · Accepted solution]

Jakub

In my experience one thing that will produce symptoms that some web sites work and some do not would be problems with MTU. Is your router doing any PPPOE, or tunneling, or encryption that would add some bytes to packets as they are routed to the Internet?

HTH

Rick

[kubajzs197]

Hi thanks for your reply. I try set lower MTU it isn't work. I had configured MTU 1492 i found this number on my ISP it is not work a try 1400 without changes any idea? Thanks

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[phil.davenport · Accepted solution]

Hi,

'ip tcp adjust-mss 1452' under the dialer interface should solve it, if not just try dropping to 800 and retest. You should set your MTU back to 1492 first. This will rewrite the MSS value sent by the client on your side when it goes out toward the Internet.

There is an overhead of 8 bytes for the PPP header and if the packet being returned from the webserver is over 1492 it may be being dropped rather than fragmented. You mention some sites as working, this is likley because the MSS used in the communication is set low enough by the webserver in the SYN-ACK ( lowest value exchanged between client and server is used) for those sites so you don't run into packet being over 1492 bytes.

Break out wireshark on the local machine and you can see what is being sent in the SYN and what is received back in the SYN-ACK. You could also use 'debug ip packet' if there's not to much going through the router.

please rate if it solves the problem.

--Phil

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[Peter Paluch · Accepted solution]

Gentlemen,

The MTU issues are indeed one of the most common issues related to similar symptoms. However, if the MTU is going to be modified, there are two places for modification:

The LAN interface (FastEthernet or VLAN SVI) should be configured with ip tcp adjust-mss 1452 command.

The WAN interface (Dialer, in this case) should be configured with ip mtu 1492 command.

The ip tcp adjust-mss command can also be applied to the Dialer interface but most configuration examples place it on an internal LAN interface.

Best regards,

Peter

[kubajzs197]

Guys thanks a lot it is solved i will have an extra money for that in my boss thank you very much ip tcp adjust-mss help thanks a lot

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