LLC and MAC headers


As far as I understood 802.3/802.2 standards, the MAC header holds LLC header carrying DSAP and SSAP fields etc. But where in practice this is utilized? In Ethernet networks I always find only MAC headers, followed with IP packet and no LLC at all.

Could you please clarify this?

Reply to
Roman Mashak
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Your observation is correct. In practice, LLC is not used when running IP over Ethernet. It provides no real benefit in that application.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Reply to
Rich Seifert

It's used with ISO protocols when you are running a full ISO stack.

There is also a spec for IP over 802.2, but as Rich said, it's rather pointless. ISTR that HP-UX used to support it.

Reply to
Andrew Gabriel


I used to know an HP-UX machine that was also running Appletalk (Ethertalk). Somehow there was a bug in the Appletalk software which would sometimes cause it to send IP out SNAP encapsulated. (I believe that was the description.) That would cause the Cisco router to switch over to SNAP form, at which point connectivity stopped (at least to other hosts on the same net).

A ping to another host, and sending of ARP, would switch the router back again.

Very strange.

-- glen

Reply to
glen herrmannsfeldt

What a full ISO stack do you refer to? Do you mean 7-layer ISO stack? Can you ive examples of systems/platforms implementing such stacks?

Reply to
Roman Mashak

to refer to the 7-layer model as an ISO stack. ISO is the organization. One of the things they did was publish a

7-layer OSI reference model. It is just a reference model, though. ISO then has a number of protocols that fit each of the layers of the reference model; e.g. IS-IS is an example of a network layer protocol.

IBM's SNA is an example of a protocol that was designed to run over 802.2. It was widely deployed using token ring networks.


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