Hi,
In my personal opinion, the question is wrong on the subject. HDLC never had an authentication mechanism - it was a descendant from more complex SDLC and used initially on bus architecture between a computer and its peripherals (hence the point-to-point and point-to-multipoint modes of operation).
In my opinion, the correct answers are:
HDLC includes support for both point-to-point and multipoint configurations.
HDLC is compatible between devices from different vendors.
HDLC specifies an encapsulation method for data on synchronous serial data links using frame character and checksum.
The second answer may be surprising, as it is often stressed in many textbooks that if different vendors' routers should communicate, PPP should be used instead of HDLC. The gotcha, however, is in the fact that Cisco does not implement true HDLC on its serial interfaces, rather a very, very light bake-off that implements just the basic framing structure but not the higher operations of a fully-fledged HDLC specification - hence, in fact, Cisco speaks its own dialect of HDLC (called sometimes cHDLC) and this can be incompatible with other vendors. If Cisco and other vendors spoke true, clean HDLC, there would be no issues in interoperability even with HDLC.
More details about Cisco HDLC format can be found here:
http://www.nethelp.no/net/cisco-hdlc.txt
Best regards,
Peter
