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New Member

Does HDLC provide a form of authentication?

Hi.I've read that It provides no authentication.I know one of the reasons ppp Is used over HDLC Is because HDLC doesn't provide authentication.

  I took one of the ICND1 module tests on this site and one of the answers concerning HDLC states that It dows provide a form of authentication.

Now I'm confused !

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Accepted Solutions
Cisco Employee

Does HDLC provide a form of authentication?

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

4 REPLIES
Cisco Employee

Does HDLC provide a form of authentication?

Hello,

To my best knowledge, there is no authentication support in HDLC. Is the module test online? If so, can you post the URL to it?

Best regards,

Peter

New Member

Does HDLC provide a form of authentication?

Hi.Thanks for the quick reply!

https://learningnetwork.cisco.com/docs/DOC-1752  Module 5 test , question 28

Cisco Employee

Does HDLC provide a form of authentication?

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

New Member

Does HDLC provide a form of authentication?

Thanks for the detailed answer , that question threw me off a bit!

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