We have used both Mikrotik and Cisco tower routers as PPPoE servers for several years, but have recently been swapping out the remaining Ciscos. I have run into a strange problem that I'm hoping somebody here knows the answer.
We have the max MTU and MRU set to 1492 on the PPPoE server, but in the list of dynamically created interfaces some of them show up as 1480, 1454, etc. I didn't think much of this because I know some router manufacturers have those as default settings. If the client router wants a lower MTU, that should be OK as long as it clamps MSS advertisements to the lower value. (I don't want to get into MSS clamping at the server side.) But I ran into a customer with an old DLink router and he was unable to get to www.aol.com <http://www.aol.com> , other websites and speedtests worked OK. Grasping at straws, I had him log into his DLink to see what the MTU setting was. It was set manually to 1492, the menu did not have an option to negotiate MTU automatically. I had him change it to 1480 and that solved his problem. I don't want to get too deep into solving a problem with an old DLink router, but I'm wondering if I've got something configured wrong. I assume if the Mikrotik PPPoE server has the PPPoE virtual interface MRU set to 1480, that means it won't receive packets larger than 1480 (plus PPPoE headers). Why are a few sessions getting MRU lower than 1492? I assume that is what the client insists on during PPPoE negotiation? And if so, why would a router with MTU set to 1492, and that appears to be doing MSS clamping based on 1492, negotiate 1480 with the server? And why would Mikrotik be handling this different than Cisco? I don't think I ever saw an MTU different from 1492 in the Cisco virtual interface properties. I know occasionally (very rarely) I've had a customer say one particular website won't come up, I think I had somebody complain about Yahoo once. Now I'm wondering if that was an MTU problem also. But I don't really see anything under my control at the server end to fix this, other than maybe to set max MRU lower like 1450 or something. I don't really want to do that, because it would mean less efficiency (fewer data bytes per packet) for no apparent good reason. All clients should be able to use 1492 (1500 minus 8 overhead bytes), and if for some reason they want a lower MTU, that should be OK as long as they clamp MSS advertisements to that lower number.
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