Member Avatar for dan@dani

Hi,

I have 1 PC and 1 Laptop in a configuration below:
Internet -- ADSL Modem + Router - Laptop
|
PC

Everything is fine with laptop, the connection to the internet is normal. But things are different on the PC.

I can connect to the network and can ping the internet, i get a reply from pinging the www, eg. www.yahoo.com
But from the browser, not all website i can browse. for example i can browse to www.asia1.com but i can't browse to www.yahoo.com.
The error page from IE shows: IE cannot display the webpage

Some troubleshooting that i have done:
- replace the ADSL modem + router (1 device)
- swap the network cable
- install new NIC on the PC
- use different browser (firefox)
- reinstall Windows XP SP2

Currently my PC is filled only with OS Windows XP SP2 only.

Please help... I'm getting desperate now

cheers - han

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  • 5 Years Discussion Span
  • Latest Post Latest Post by andy326733

Dani AI

Generated

This thread is a classic MTU/MRU/PMTUD problem on an encapsulated link (PPPoE or similar). ’s diagnosis — the modem reverted to its default MRU after a reboot so large packets were being dropped — explains why small ICMP pings succeeded while HTTP sessions from a browser failed. When "Fragmentation Needed" messages are blocked or the MRU is wrong, TCP connections hang or fail even though ping looks fine.

Quick, repeatable test (Windows):

ping -f -l 1472 www.example.com

If the reply says the packet needs fragmentation, reduce the -l size (try steps of 10) until the ping succeeds. Add 28 to the largest successful -l value to get the usable MTU for the path (ICMP adds 28 bytes). Use that MTU/MRU as the target setting.

How to fix it

  • Apply the discovered MTU/MRU on the modem/router and save the configuration to NVRAM so it survives reboots.
  • If the router supports TCP MSS clamping, enable it and set MSS = MTU - 40 (IP+TCP headers = 40 bytes). MSS clamping often fixes broken PMTU without changing per-host MTU.
  • If the router cannot be changed, set the host MTU (or use vendor NIC utilities). If settings keep reverting, check modem firmware and update or contact the vendor.

Notes and why the laptop worked

  • Different OS/driver/firewall behavior can let one device discover a working path MTU while another fails (PMTU replies blocked, different TCP stacks, or automatic MSS adjustments).
  • ’s telnet idea is useful to confirm a raw TCP connection to port 80, but the MTU ping test above pinpoints fragmentation issues.
  • Defaults vary by modem model (check the modem manual or web UI); for PPPoE links 1492 is a common starting point, but the correct value should be determined with the ping method.

Recommended Answers

All 3 Replies

Member Avatar for John A

Have you tried manually getting the websites with Telnet?

When you reinstalled, did you format the hard drive first?

Other than that, I don't know.

Member Avatar for dan@dani

Thanks for the reply.
The problem is solved already.
It turns out that the MRU setting wasn't saved during configuration, so when the modem was rebooted, the setting was back to the default. Default value is 1492 and the working configuration value is 1432.

One thing that I still don't understand is why my laptop able to connect to the internet perfectly.
Weird...

Anyway thanks again.

cheers - dan

Member Avatar for andy326733

i have a teracom modem what is the default value for mru in that

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