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PPP over HDLC?

I have a beginner question. What are the primary advantages of using PPP over HDLC? Thanks, Nube

3 Replies 3

Richard Burts
Hall of Fame
Hall of Fame

Robert

I would say that the first real advantage of PPP is the option to authenticate the connection. This is clearly something that PPP can do that HDLC can not do.

Another advantage (what can PPP do that HDLC can not) is the possibility of doing multi link which takes several PPP circuits and makes one logical entity of them.

Beyond these factors you may get into some intangible aspects, such as the fact that PPP is a standard protocol supported by many vendors while HDLC is not. Technically the Cisco HDLC is a Cisco proprietary adaptation of the HDLC protocol. And so when connecting a Cisco router to a non Cisco router PPP is generally the better choice.

HTH

Rick

HTH

Rick

Hi Robert,

Please reafd the doc

http://www.cisco.com/en/US/tech/tk801/tk133/technologies_configuration_example09186a00800945be.shtml

http://www.cisco.com/en/US/docs/net_mgmt/active_network_abstraction/3.6_sp4/tech_ref/user/guide/8ppp.html#wp1044053

Please rate if it is helpful...

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

Peter Paluch
Cisco Employee
Cisco Employee

Hi Bob,

In addition to nice answers by Rick and Muhammad, there are a few more things I would like to add.

First of all, PPP was created because there was no suitable open standards-based multiprotocol-capable L2 encapsulation protocol for point-to-point links. HDLC did exist but its frame format lacked a Type field to identify the content. Therefore, plain HDLC was unusable for links that carried multiple protocols. Vendors typically extended the HDLC with their own frame modification for the Type field, resulting in a series of possibly non-compatible HDLC implementations. This was clearly an unpleasant state of things. PPP was created to correct precisely this fundamental inability. In fact, on serial point-to-point links, the PPP header is nothing more than a Type field, and the outer header is a HDLC header (yes, the PPP is in fact carried inside HDLC framing!).

The second great feature of PPP is the presence of control layer that negotiates PPP session, its parameters and the upper protocols to be carried over the session, including their properties. When PPP session is being established, the Link Control Protocol (LCP) takes care of negotiating the session, including the negotiation of authentication, quality monitoring, maximum receive unit, reliable transmission using LAPB and some other additional optional features. In addition, each higher-level protocol like IP or IPv6 has its own management protocol (the Network Control Protocol, NCP) that first negotiates whether the particular L3 protocol should be carried over the session, and if so, what are its operational parameters. This is particularly nice because if one side tries to negotiate, say, IPv6 and the other side does not support it, the IPv6 negotiation will not be completed and the IPv6 will not even be carried over the link. If, say, IPv4 is negotiated then for example, the neighbor's IP address is also discovered (or even assigned) during IPv4 protocol negotiation.

There are also other features to PPP like optional compression of headers or payloads, multilink features, but I personally consider the features mentioned above among the most important.

Best regards,

Peter

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