Re: [nsp] cisco hdlc packet formats

From: Sonam Thapa (
Date: Mon Jun 04 2001 - 22:19:47 EDT

Hi Shankar,

Thanks for your reply! ISIS operation over
cisco-hdlc seems to be different from that
over 802.3-ethernet which has a packet format
of [da][sa][length][LLC/SNAP][NLPID]
In 802.3, the LLC/SNAP is 0xFEFE03 and
NLPID has the values that you list for ISIS, ESIS,
CLNP etc.

So over cisco-hdlc the 03 from 0xFEFE03 is skipped
as per your message and the 0xFEFE is immeidately
followed by the NLPID in the data part of frame.
Is this correct?

This raises one other question -
how (or if) ISIS can run over regular ethernet
which does not have the LLC/SNAP header or space
for an NLPID. Has such an encapsulation been
defined by cisco or the ietf?

Once again, TIA,

--- Shankar Vemulapalli <> wrote:
> Dont' have the HDLC packet format handy -
> But in general:
> For ISIS/CLNS/ESIS - the DLC header is 0xFEFE (for
> OSI family)
> and the protocol specific fixed header for ISIS
> after DLC header is: 0x83
> for ESIS is: 0x81
> and for CLNS is: 0x80
> (as per the ISO10589, ISO/TR9577)
> /Shankar

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