Subject: RE: [Isis-wg] (no subject) area/router boundary

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Mon, 29 Jan 2001 11:09:40 -0500 (EST)

Although all the replies have been correct, let me either clarify
or confuse things with a bit of history. Originally IS-IS was
designed for CLNP addresses, where the top part of the address (everything
but the ID portion) was the "area", so an address specified
an area, and a node could have lots of links in that area all
with the same address, but only be in one area. So a node, by
definition, if
"node" is defined by the address of the node, could only exist in
one area.

With IS-IS for IP, though, this didn't really apply anymore since
a node has an address for each interface.

But even with CLNP, it was possible to implement what looked to
the rest of the world like two nodes by having a router run
multiple instances of IS-IS, one for each (level 1) area, and
then the router could exist in both areas, and even pass routing
summaries between them. This was implemented in NLSP, a variant
of IS-IS for IPX. Such an implementation is straightforward
and completely compatible
with current IS-IS routers, who wouldn't be able to tell the
difference between such a beast and two routers.

But the spec does say a router is in (at most) one level 1 area, and might also
be a level 2 router, as someone on this thread pointed out.

Radia

	> 
	> -----Original Message-----
	> From: Volkan Ozdemir [mailto:[email-masked]]
	> Sent: Thursday, January 25, 2001 3:37 PM
	> To: [email-masked]
	> Subject: [Isis-wg] (no subject)
	> 
	> 
	> 
	> Hi,
	> 
	>   I had a basic question about IS-IS. In IS-IS is a
	> router completely in a single area or is it possible to
	> assign interfaces to different areas?
	> 
	> Thanks
	> 
	> Volkan

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