Subject: [Isis-wg] area address in Source NSAP.

From: ramanathan rams <[email-masked]>
Date: Wed, 20 Mar 2002 10:55:08 -0800

hi all,

     I have the following doubt, could u please explain me?

* we r maintaining 2 area address list in an IS,
  * manual area address list and
  * another list, which is having all the area addresses present in that 
area( formed from L1 LSPs).

  my doubt is, if an IS originates a packet, in the source NSAP field, can 
we use an area address such that it is present in the second list but not 
in the manual area address list of that IS?
  second doubt is, similar to the frst doubt. When an IS originates the 
ISH, 10589 says, we have to use the lowest available area address in the 
NET field. For finding the lowest available area address, do we have to 
search only the manual area address list or can we search the second list 
also?

thanks
rams.

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From: Jeff Learman <[email-masked]>
Date: Wed, 20 Mar 2002 14:52:38 -0500

Answer to second question first:

  You must use the lowest-valued manual area address in your NET.

As for the first question, I assume you mean a CLNS user datagram.
The rules for network addresses (I forget the ISO doc numbers for
"naming and addressing", and "netwok layer addressing") mean that
you're not supposed to play mix and match with pieces of NSAP
addresses.  A system is assigned an NSAP address and should use it.
This includes an IS.  The implication is that the NSAP for an IS
network service user should have an area address matching one of
its manual area addresses, but it needn't be the lowest one.
Now for the tricky part -- you could probably get away with using
any of the dynamic area addresses and it would work.  However (a)
you'd be breaking the addressing rules, and (b) you'd get into
trouble in various dynamic conditions, such as when the area gets
partitioned, or when a dynamic area address is dropped.  I can't 
think of any good reason for using a dynamic area address as part
of a source NSAP address.

While I'm on the soap-box, I should point out another source of
confusion concerning IS-IS and NSAP addresses of network service
users on the IS.

If you have area address A and B, and system ID I, and an NSEL of 1,
that means you could have two different NSAP addresses: A-I-1 or B-I-1.
According to how IS-IS works, it will deliver packets to either just
the same way.  HOWEVER, you have no right to assume the CLNS engine
and/or Transport implementation on another IS will be able to accept
packets with mix-and-match addresses.  Some implementations will,
some won't.  Both are within the letter and spirit of the law.  You
aren't allowed to play mix-and-match with NSAP addresses, so if you do,
be prepared to expect either outcome.  This complicates autodiscovery
mechanisms based on the IS-IS FDB, for example.

Regards,
Jeff