PPPoE needs lower MTU and MSS or some apps and sites will break

Hello

I still remember my first job in the network field, it was for a company called Sonicwall. Most of our phone calls were from really inexperienced admins, and because most of them had pppoe connections, a large chunk of the tickets could be solved by suggesting that mtu should be 1492, not 1500.
Now today my colleague had an interesting case where employees couldn’t log in to skype business. The router logs had a lot of max fragments errors, which clearly showed that it was busy trying to fragment packets but it just couldn’t store all the fragments in the memory.

Adjusting the TCP MSS on the dialer interface solved the problem. This is also described here. It turns out that skype logins use TCP and skype seems to ignore the PMTUD.

https://www.cisco.com/c/en/us/support/docs/ip/transmission-control-protocol-tcp/200932-Ethernet-MTU-and-TCP-MSS-Adjustment-Conc.html

 

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