I've recently been playing with L2TP tunnels between Cisco and Mikrotik gear in an LAC/LNS type configuration. The L2TP tunnel was coming up but any traffic across the tunnel would fail. Enabling l2tp logging on the Mikrotik router revealed the error "received unsupported protocol 0x2145"
Google didn't give any direct solutions but did give a few clues suggesting it might be related to "Protocol Field Compression" (PFC), and reading RFC 1661 yielded a bit more information on how it should work.
Fortunately I was able to find a fix.
|A Mikrotik bug? Never!
|PPP Encapsulated ping
|PPP LCP Reject message
ppp pfc local forbid