What's the importance of supporting network layer protocols?

totts

What is the importance of supporting network layer protocols?

On the wide area side, the ISO standard HDLC does not support multiple network layer protocols, Ciscos proprietary HDLC adds a field that does. PPPs NCP configures many different network layer protocols.

Then on the local area side, part of Ethernets LLC... the 'type' fields DSAP and SNAP both define the type of protocol header that follows.

My question is, why is this important... what needs this information next and why? What's the problem if a L2 protocol such as ISOs HDLC doesn't support multiple protocols?

Netstudent

I don't think it is that big of a deal anymore because almost all networks are IP. Before TCP/IP dominated, there was ipx/spx for novell and others. There are many networks that still run on multiple layer3 protocols. Therein lies the problem. Different vendors equipment has there own version of HDLC, Cisco has its version because it adds a proprietary field in the packet to tell it which layer 3 protocol is being used and how to encapsulate other layer 3 protocol.

You ask "What's the problem if a L2 protocol such as ISOs HDLC doesn't support multiple protocols?"

Well HDLC does support more than 1 protocol, it's just that each vendor has it's own method of doing it. So it's not a problem unless you are trying to interconnect two different vendors equipment using HDLC.

See HDLC needs to have some kind of information in it that tells the next layer (L3) what type of protocol HDLC has encapsulated so that Layer3 knows what to do and how to execute the addressing. I'm not sure if this answers your question, but good luck.