Brief version: A 12.4 system cannot use an accept-dialout vpdn on a12.2 system for modem access. See previous postings for details.
I constructed an L2TP filter to run on an intermediate host. It waits for the OCRQ packet from the 12.4 system and removes the Cisco-specific AVP 102 (Username). Everything else it forwards to the 12.2 system. This makes the connection work fine, so the (extraneous, null) Username AVP is indeed the problem.
I then made my filter change the Username AVP to some unrecognized Cisco-specific attribute. This was also handled correctly the 12.2 system and the connection worked.
Finally, I made my filter leave the AVP 102 in the packet but make the value non-null (I made it "x"). Again this made the 12.2 system happy.
Does anyone know how to convince 12.4 to omit the AVP 102 or make it non-null?
Dan Lanciani ddl@danlan.*com