Subject: [Isis-wg] Areas in ISIS

From: "Metz, E.T." <[email-masked]>
Date: Mon, 19 Jun 2000 15:40:56 +0100

Hi all,

is it possible to have an ISIS network with different areas, but without an
explicit backbone area?

I thought it could be done because the L1/L2 routers in each of the areas
will form (L2) adjacencies and exchange routing info. So even without a real
backbone area, traffic will still flow between differents areas.

Is the above correct? Does it make any sense?

cheers,
	Eduard

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From: Abe Martey <[email-masked]>
Date: Mon, 19 Jun 2000 09:43:36 -0700

> is it possible to have an ISIS network with different areas, but without an

Sure! Each router belongs to one area and there is no need for a 
physically distinct backbone area. The backbone refers to the level-2
routing domain which is a logical area....the group of routers involved 
in level-2 routing (as you describe below) form the isis backbone...the
backbone stretch just has to be contiguous...

Cheers.

- abe

-- 

==.==.==.==.==
Abe Martey
San Jose, CA
408 527 9149

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From: Dave Katz <[email-masked]>
Date: Mon, 19 Jun 2000 13:19:50 -0700 (PDT)

I fail to see the distinction between what is described and a "physically
distinct backbone area."  In particular, if there are going to be distinct
L1 areas, there must be a contiguous L2 backbone (which could be as small
as a single link) to connect the L2 routers.  An L1/L2 router is
essentially the same thing as an OSPF router with at least one interface
in the backbone area and at least one interface in another area.

--Dave

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   Cheers.

   - abe

   -- 

   ==.==.==.==.==
   Abe Martey
   San Jose, CA
   408 527 9149

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From: Abe Martey <[email-masked]>
Date: Mon, 19 Jun 2000 15:29:33 -0700

Dave Katz wrote:
> essentially the same thing as an OSPF router with at least one >interface in the backbone area and at least one interface in another >area.

Technically yes...but IS-IS would allow a linear layout of multiple
areas whereas OSPF needs all areas touching a _common_
centrally located area 0...unless you wanted to play with
virtual links. The IS-IS backbone is not as clearly carved out as the
OSPF area 0...a difference probably rooted in ISO node-based addressing
vs IP link-based addressing.

I'll agree however that in a real network, the IS-IS backbone should be 
as distinctly carved out as an OSPF backbone....

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From: Mike Truskowski <[email-masked]>
Date: Mon, 19 Jun 2000 18:04:28 PDT

Sounds to me like you are saying the same things here only
in different ways... 

The difference occurs at the logical layer...  an area in OSI is
dictated by the nsap while an area in ospf is a logical mapping...

OSI backbones can have multiple osi areas while an ospf area 0
is one area with the backbone belonging to it...

mike