Subject: [Isis-wg] Regarding manual area address

From: "Amudhavalli Narayanan" <[email-masked]>
Date: Thu, 17 May 2001 13:06:53 +0530

Hi,

Can some one help me in understanding the attached flag?

Will it be set for this setup?

R1 - L1L2 router in manually configured  area a1
R2 - L1L2 router in manually configured areas a1, a2.

Will R1 set the attached  flag as it can reach a2 via R2? 

Thanks,
Amudha

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From: "Premal Ashar" <[email-masked]>
Date: Thu, 17 May 2001 01:07:16 -0700

Hi Amudha,

When ever a L1 router knows about a L2 router it will set the attached flag
for it's L1 LSP according to 7.2.9.1 of the 10589

--<EOM>
Premal Ashar
Member of Technical Staff               Email: [email-masked]
Riverstone Networks                     Tel: (408)878-6515
5200 Great America Parkway              FAX: (408)878-6501
Santa Clara, CA 95054                   http://www.riverstonenet.com

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of Amudhavalli
Narayanan
Sent: Thursday, May 17, 2001 12:37 AM
To: [email-masked]
Subject: [Isis-wg] Regarding manual area address

Hi,

Thanks,
Amudha

---

From: mike shand <[email-masked]>
Date: Thu, 17 May 2001 10:27:59 +0100

>for it's L1 LSP according to 7.2.9.1 of the 10589

Errr... not quite. The attached flag is set by a level 2 router in its 
level 1 LSP to indicate that it can reach other areas. A level 1 (only) 
router shouldn't be setting the attached bit, and nothing should be setting 
it just because it "knows about" an L2 router. The critical thing is that 
it can reach other areas. There are various ways it could know about this 
involving manual routes, but typically it is because when it runs its L2 
SPF calculation it can "reach" one or more areas other than its own. And in 
the context of the original question, this really does mean other areas, 
not just other area addresses within the same area.

7.2.9.1 isn't saying anything about this. It is describing what an L1 
router does with the attached bit information it sees in LSPs from other L1 
routers in the area. It finds the (set of) L2 routers (within the area) 
which have the attached bit set, and chooses the set (possibly only one) 
with minimal cost. It then uses this as a "default route" for forwarding 
any data which is unreachable according to its L1 database. (In the CLNS 
case, that is if and only if the area address portion of the destination 
NSAP address does not match any of the local area addresses.)

         Mike

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From: Jeff Learman <[email-masked]>
Date: Thu, 17 May 2001 09:22:00 -0400

Amudha,

In your example, you have configured two area addresses for one area.
Therefore the attached flag would not be set.

Setting multiple area addresses does not place a L2 router
in multiple areas, it causes multiple area addresses to
be configured for the area.

According to the spec, and as far as can be observed by looking
at packets, no L2 router is ever in multiple areas.  While there
may be implementations where one can run the same router in
multiple areas, these routers must make it appear to other
routers as though it is a collection of routers, each in a different
area.

Regards,
Jeff

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From: "Larmer" <[email-masked]>
Date: Thu, 17 May 2001 09:43:34 -0400

Hi,

	There is one other situation, which is not described
by ISO-10589 where the attached bit "may" be set, depending
upon the implementation. When a vendor implements multiple
level-1 IS-IS instances on one IS and you want to get from
one instance to another, should you set the attached bit?
Well, if you set it, you can indeed get between the instances.

	Though, this may create problems? As long as you keep
the L2 area contiguous, you should still be able to reach
all other L1 areas. However, there are situations where you
may take an extra hop or two to get to another area. This will
happen if the L2 process does not have a direct connection to
another L2 area as depicted below?

	_________________________________
	     |L2			|L2
	------------	------------		------
	|		|	|		|		|	|
	|L1  L1   L1|	|L1	    L2|-----------|L2	|
	------------	------------		------
		     |	 |
		_____|_______|____
			   |
			-------
			|  L1  |
			-------

	Mike???

Cheers,
Ken

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of mike shand
Sent: Thursday, May 17, 2001 5:28 AM
To: [email-masked]
Cc: [email-masked]; [email-masked]
Subject: RE: [Isis-wg] Regarding manual area address

         Mike

---

From: mike shand <[email-masked]>
Date: Thu, 17 May 2001 15:06:26 +0100

Ken,

The attached bit mechanism is actually a very crude indication. The 
original idea was that your L2 domain should be "well connected", but when 
it broke you could end up with L2 routers which were isolated from the 
"core". You didn't want to choose one of these as your "nearest" L2 router, 
when there might be an "attached" router further away.

But there are all sorts of scenarios which are not as black and white as 
this. Where the domain gets partitioned into a "big" bit and a "small" bit. 
How do you choose the "big" bit? Count the number of areas you can see? 
Propagate summary routes of the bits you can see and have multiple 
"nearest" L2 routers depending on which route contains the destination? 
etc. etc.

Any attempt to solve complex problems with the existing single bit is 
pretty much doomed to failure. Most implementations have a combination of 
heuristics and manual settings which let you squeeze a bit more mileage out 
of this, but you can always come up with a case you can't deal with.

         Mike