for the encapsulation choices, I'm referring to the IOS help output (cut for clarity):
Router(config-if)# encap ?
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
ppp Point-to-Point protocol
for the PPP stack, I'm referring to the below:
PPP Protocol stack compared to OSI Reference Model:
OSI Layer PPP Protocol Stack 3 Upper-layer Protocols (such as IP, IPX, AppleTalk) 2 Network Control Protocol (NCP) Link Control Protocol (LCP) High-Level Data Link Control Protocol (HDLC) 1 Physical layer (such as EIA/TIA-232, V.24, V.35, ISDN)
the image is not showing.
Anyway, hdlc is cisco proprietary, and doesnt have the options and features that PPP has.
If you enable ppp with authentication between two routers, then run debug ppp auth and debug ppp neg you will see it invoke the LCP and NCP - they are PPP related protocols
HDLC does not invoke them, as its a much less sophisticated protocol.
Thanks for the quick reply.
My understanding (from the image) is that the PPP protocol stack is everything listed in the layer 2 section: HDLC, LCP and NCP, because the title of the image is "PPP protocol stack". Am I mis-understanding something? Or does PPP only consist of LCP and NCP?
(added color to initial image, for clarity)
its just a wording thing...PPP, HDLC, NCP, LCP are all layer 2 protocols, thats the important thing to remember. PPP uses LCP and NCP; HDLC does not.
For ccna thats about all you need to know, plus frame relay.
As I mentioned PPP was created about 20 years ago or so, as an alternative to older Point to Point protocols like hdlc and slip that were proprietary and/or lacking in features, and was in a large degree based on HDLC. Even the the packet structure of PPP looks a lot like HDLC.
No HDLC is not Cisco proprietary. However, Cisco's version of HDLC "is" proprietary.
Here is a brief history of HDLC (source: WIKIPEDIA)
HDLC is based on IBM's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP, while ANSI named their essentially identical version ADCCP.
Derivatives have since appeared in innumerable standards. It was adopted into the X.25 protocol stack as LAPB, into the V.42 protocol as LAPM, into the Frame Relay protocol stack as LAPF and into the ISDN protocol stack as LAPD.
HDLC was the inspiration for the IEEE 802.2 LLC protocol, and it is the basis for the framing mechanism used with the PPP on synchronous lines, as used by many servers to connect to a WAN, most commonly the Internet.
A mildly different version is also used as the control channel for E-carrier (E1) and SONET multichannel telephone lines. Some vendors, such as Cisco, implemented protocols such as Cisco HDLC that used the low-level HDLC framing techniques but added a protocol field to the standard HDLC header. More importantly, HDLC is the default encapsulation for serial interfaces on Cisco routers. It has also been used on Tellabs DXX for destination of Trunk.
In regards to PPP, it was designed after the original HDLC specifications, but included many new features such as authentication, encyrption and compression.
PPP over serial links is encapsulated at the lower layer in HDLC type frames. This is most likely why they show HDLC as part of the PPP protocol stack.
Here are just some of the RFC's that cover PPP.
RFC 1661 - Standard 51, The Point-to-Point Protocol (PPP)
RFC 1662 - Standard 51, PPP in HDLC-like Framing
RFC 1963 - PPP Serial Data transport Protocol
RFC 2687 - Proposed Standard, PPP in a Real-time Oriented HDLC-like Framing
Hope this helps.