Hi

We use an ISP that does bonding via ADSL lines and they've installed a Cisco router in our office.
The said that MTU must be set to 1300 in order for browsing etc to work right.
I have my Mikrotik plugged into the Cisco and Ive enabled a PPPoE server with MTU size 1300. When users connect via PPPoE everything works great, however, when I set up DHCP and assign a masquerade rule for specific IPs, browsing does not work correctly. I have changed MTU sizes under my interfaces to be maximum 1300. For some reason I cannot change L2 MTU??

To confirm this, I plugged my Notebook directly into the Cisco router and changed the MTU to 1300 on my notebook and browsing works great.
What else can I set on Mikrotik or where else can i look to resolve this problem

any Ideas please

You can set mangle rule to change TCP mss value.
http://wiki.mikrotik.com/wiki/Manual:IP ... Change_MSS