I'm bench testing a mikrotik PPPoE concentrator (posting through it now). I was bench testing it last week with a netgear WGR614 router and all seemed to be working so I tried to test it in the field. Of the two customers I hooked up on it, one customer also had a WGR614 and their connection worked. The other had an older D-Link DI-604, it did not. It connected and authenticated with PPPoE, but I could only seem to get out to google.

We supply off-the-shelf WiFi routers to customers, so I switched that DI-604 with our current TrendNet TEW-432BRP, same symptoms. If I plugged in my Mac directly using the built in PPPoE client, it worked perfectly. Those are the only routers I had on hand, so I pulled the MT PPPoE server and put in static addresses to get them back on the net until I could bench test again today.

So today, I have the MT RB450G on the bench into a switch with four different brands of routers on it, with four different PPPoE test accounts:
Netgear WGR614v8
D-Link DI-604
Buffalo WHR-G125 (running DD-WRT 24-sp1 mini)
TrendNet TEW432BRP

This time, the Netgear works AND the D-Link works, but the Buffalo/DD-WRT and TrendNet do not. Google will load, I can do searches through google and get results, but when you click on those results only about 1/5 go anywhere. Seems the connection to the page is established, but the progress bar just hangs in loading. I can ping through, I can do DNS lookups, everything SEEMS to work, except actually loading the majority of pages.

So.... help?!

Any ideas where the problem is here? I set up User Manager and PPPoE according to the wiki guides, the RB is not doing routing per-se, it's just handing out IPs from our core RiverStone router. I set up a nat pool on the RS to use with this test.

RS (Natting 172.16.16.0/26, gateway 172.16.16.1) -> RB450G (172.16.16.2, PPPoE pool 172.16.16.3-62) -> WiFi AP (EnGenius EOC-1650) -> WiFi Client (outdoor CPE, also EOC-1650) -> Customer WiFi Router (PPPoE) -> Customer PC

Perhaps you need to change-tcp-mss, ppp profile set used_profile change-tcp-mss=yes.
Set max-mtu/max-mru to 1440 or even lower. Then try the same pages again.

Looks like the default was set for change-tcp-mss=yes. Bumping the MTU down to 1440 seems to work great on the two routers that weren't working. Changed the TrendNet to 1440 and tested. The DD-WRT router had an "auto" setting for the MTU which also worked, but I can't see where it tells me what it's using on auto.

The default MTU on all the routers when using PPPoE was 1492, is there something I can change in the MikroTik or the riverstone so that I don't have to change the MTU on each customer router? Is it as simple as increasing the MTU on the upstream port(s)?

On every router you should have change-tcp-mss=yes and /interface pppoe-server server set <number> max-mtu=1440 max-mru=1440

Perhaps I misunderstood. I changed the MTU on the customer D-Link/Netgear/Trendnet router to 1440, not the Mikrotik.