Here is the breakdown (thanks to George Matey for this!)
--------------
The first ("address") octet is set to 0x0F for unicast packets and 0x8F for
broadcast packets. Broadcast just means that the higher-level protocol
thought this was a broadcast packet; cisco doesn't support multidrop HDLC at
this time.
The second ("control") octet is always 0.
The next two octets are a 16-bit protocol code, sent most-significant-first.
These codes are usually Ethernet type codes. cisco has added some codes to
support packet types that don't appear on Ethernets. The current list of
codes is as follows:
TYPE_PUP 0x0200 PUP
TYPE_XNS 0x0600 XNS
TYPE_IP10MB 0x0800 IP
TYPE_CHAOS 0x0804 Chaos
TYPE_IEEE_SPANNING 0x4242 DSAP/SSAP for IEEE spanning prot.
TYPE_DECNET 0x6003 DECnet phase IV
TYPE_BRIDGE 0x6558 Bridged Ethernet/802.3 packet
TYPE_APOLLO 0x8019 Apollo domain
TYPE_REVERSE_ARP 0x8035 cisco SLARP (not real reverse ARP!)
TYPE_DEC_SPANNING 0x8038 DEC bridge spanning tree protocol
TYPE_ETHERTALK 0x809b Apple EtherTalk
TYPE_AARP 0x80f3 Appletalk ARP
TYPE_NOVELL1 0x8137 Novell IPX
TYPE_CLNS 0xFEFE ISO CLNP/ISO ES-IS DSAP/SSAP
This list is shared between serial and Ethernet encapsulations. Not all
these codes will necessarily appear on serial lines. This list will probably
be extended as cisco adds support for more protocols.
Bytes after this are higher-level protocol data. These normally look the
same as they'd look on Ethernet. Bridging packets include Ethernet/802.3 MAC
headers; no other packets do.
--------------
HTH,
chris
> -----Original Message-----
> From: Sonam Thapa [mailto:aos824@yahoo.com]
> Sent: Monday, June 04, 2001 8:06 PM
> To: cisco-nsp@puck.nether.net
> Subject: [nsp] cisco hdlc packet formats
>
>
> Hi,
>
> I didn't know which other public list to ask this
> on - cisco-nsp seemed the closest so here goes.
>
> I was wondering how one demultiplexes IS-IS
> packets over cisco-hdlc. Cisco-hdlc has a 2 byte
> protcol type which follows ethertype quite closely.
> For IS-IS packets does this protcol field have the
> value 0xFEFE (denoting OSI packets). If this is
> so, how does a system know what type of packet
> it is - IS-IS, ES-IS etc.
>
> Is there any cisco or ietf document (for use
> with protocol analyzers) available that specify
> the packet formats for cisco-hdlc in general and
> OSI packets over cisco-hdlc in particular.
>
> TIA,
> -aos
>
>
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