>Question came up on the CCIE group revolving around the meaning of the term
"tunnel"
>
>I think I am seeing where the author of the below quote is going. I'm
>wondering if one of the folks on this group might be willing to offer some
>insight.
>
>The question originated with someone calling an OSPF virtual link a tunnel.
>After some back and forth, someone offered the following (edited for
>brevity, and clarity):
>
>"...understand the basics of software tunneling. TUNNELING PROVIDES
>ALTERNATIVE TO THE NATIVE CONNECTIVITY PROCEDURE. In this case there is no
>physical/datalink connectivity, so connectivity is achieved through software
>interface.
>
>" Virtual Links are not tunnels, you can't transport traffic over them, they
>just carry routing information. They are TUNNELING this routing information
>through area 1. THIS IS THE PRINCIPLE OF TUNNELLING. IT IS NOT ONLY
>APPLICATION DATA THAT IS TUNNELED : IN THIS CASE ROUTING UPDATES ARE BEING
>TUNNELED!
>
>"you seem to know only Cisco exam material (tunnel interfaces) please read
>further afield and grasp generics/basics of software programming principles,
>"
>
>Can anyone offer further clarification here?
In the most general sense, a tunnel is a means of taking a protocol
data unit payload of OSI layer N of protocol family P1: (N,P1)-PDU,
and transmitting it with a delivery header at layer M of protocol
family P2. What is actually transmitted is, minimally, a (N,P1)-PDU
encapsulated in a (M,P2)-PDU. There may be a "shim" between the end
of the delivery header and the beginning of the payload header;
there's no good OSIRM term for the shim.
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