Multilink PPPoE issues

I am trying to setup dual DSL links using multilink PPPoE at a location and have it working about 99% of the way. This is to replace an existing T1. The only issue I have is that SOME (and it is limited) web sites will not load, one of the being http://www.microsoft.com. Go back to the T1, loads right up. If the site supports pinging then I am able to ping the address from either the multilink PPPoE or T1. Also, is seems like all other services - FTP, SSH, Telnet seem to be working on either link. Speed tests from our web site do show speeds that let us know the multilink is functioning.

The configuration between the two is the same as far as IP addresses and they are both terminating on the same network as well for the other end. I have fiddled with the MTU/MRU/MRRU in the Mikrotik to no avail at this point based on some stuff I have seen searching. The far end of the DSL connection is a DS3 ATM connection on a Cisco 3700. This is also hardware under our control/configuration and we have tried a couple of things there as well.

It is driving me crazy and any help would be appreciated. We are running Mikrotik v3.17. Thanks!

Hate to followup to my own post, but I continued searching and searching forums for similar problem. Found somebody that was having something similar using transparent proxy. I “tweaked” their change and sure enough, now ALL the sites we had problems with are loading. Here are the commands used:

/ip firewall mangle
add action=change-mss chain=postrouting comment="" disabled=no new-mss=1402 out-interface=MultiPPPoE protocol=tcp tcp-flags=syn tcp-mss=1403-65535

While I am happy it is fixed, I am still one of those people that would like to know why. Anybody explain why this was necessary to me. Hope this helps out somebody else. Thanks!

you simply had mtu issue