Traffic not always routing through x86 machine

So I finally got around to building a big x86 based system. And a beauty she is too…

The important things are the three ethernet cards and the Level 5 licence.

The problem is that something’s wrong.

The machine’s running RouterOS ver 4.3 for x86 and the version of user manager that was shipped with ver 4.3 of RouterOS.

One ethernet (E1) port goes to the rest of the network.

One ethernet (E2) port goes to a Cisco router an on to a fibre optic connection to an upstream service provider.

The last ethernet (E3) port goes to a Linksys ADSl router in bridge mode for specific local only traffic.

I have source NAT set up so that all traffic leaving on E2 is NATted and all traffic leaving on E3 is natted.

E2 has a static IP address supplied by the service provider
E3 has a PPPOE connection and gets a dynamic IP address.

There are some sites that I simply can’t browse.

If I go (for example) to http://forum.landerovernet.com I get the following error:

The connection was reset

The connection to the server was reset while the page was loading.

  • The site could be temporarily unavailable or too busy. Try again in a few
    moments.
  • If you are unable to load any pages, check your computer’s network
    connection.
  • If your computer or network is protected by a firewall or proxy, make sure
    that Firefox is permitted to access the Web.

IE gives:

Internet Explorer cannot display the webpage

What you can try:
Diagnose Connection Problems

More information

This problem can be caused by a variety of issues, including:

•Internet connectivity has been lost.
•The website is temporarily unavailable.
•The Domain Name Server (DNS) is not reachable.
•The Domain Name Server (DNS) does not have a listing for the website’s domain.
•There might be a typing error in the address.
•If this is an HTTPS (secure) address, click Tools, click Internet Options, click Advanced, and check to be sure the SSL and TLS protocols are enabled under the security section.

For offline users

You can still view subscribed feeds and some recently viewed webpages.
To view subscribed feeds

1.Click the Favorites Center button , click Feeds, and then click the feed you want to view.

To view recently visited webpages (might not work on all pages)

1.Click Tools , and then click Work Offline.
2.Click the Favorites Center button , click History, and then click the page you want to view.

There are many other sites that give the same error.

Where should I start looking for the fault?

Thank you.

I need to add that I can trace to any of the sites that give this problem from my pc.

You mean http://forum.landrovernet.com, right?

Can you ping the sites that don’t work?

I have seen where the MSS is too large will cause some sites to not work. This is due to some MTU along the way being too small. If ICMP is blocked on their side then the web site will not be able to send the ICMP fragmentation needed packet. This is why some sites would work because they sent the ICMP fragmentation needed packet.

Maybe your problem, maybe not.

Yes, but not just that site.

Look what Sky News looks like:
skynews (Medium).jpg

Yes

How would you suggest I go about fixing this?

Ether1, 2 & 3 all have an MTU of 1500. ROS gets upset if I try to change that.
Ether1 has an L2MTU of 1600. The other two cards have blank MTUs.