Subject: Second edition 10589 spec question

From: "R. Murali" <[email-masked]>
Date: Mon, 31 May 1999 22:24:49 EDT

With reference to this spec ( revised on 1998-6-16 ), section 8.2.5.2 
Point-to-Point IIH PDU processing, I have the following questions:

1. What is the significance of bullet c) which talks about calculating 
circuit ID status? What is the use of this calculation?

2. Bullet d) states that "If the action is "UP" or "Accept" the IS shall set 
the neighborSystemID to the value of the Source ID field from the PDU.
Isn't it necessary for the IS to generate a new LSP with the new 
neighborSystemID?

Can somebody help me?

Thanks,
Murali

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From: "Lester Ginsberg" <[email-masked]>
Date: Thu, 3 Jun 1999 21:08:29 -0700 (PDT)

First of all, it seems that you are attempting to use the second
edition draft as it were an approved revision of the existing 1992
specification. This is a mistake. Because of the number of issues with 
the draft, it is not suitable as a reference for implementors. It has
been made available to this group in order to obtain comments so that
it may corrected before progressing onward in the ISO approval
process. In discussing the questions you raise below it therefore
makes more sense to refer to the 1992 edition of 10589. In this way
your questions can be addressed without the confusion of disclaimers
because of the unfinished nature of the second edition draft.

Section 8.2.4.2 in the first edition is the relevant section.

 > circuit ID status? What is the use of this calculation?

The significance of the circuit ID status calculation is somewhat
limited. The local circuit ID portion of this item is arbitrarily
assigned by each local system when the circuit is created. Calculation 
of the circuit ID status may provide an indication that the remote end 
has either rebooted or deleted the circuit and recreated it - either
of which would be cause to delete an existing adjacency and eventually 
create a new one. (e.g. system ID hasn't changed, but the Local
Circuit ID did change).

It has been discussed elsewhere (notably "Three Way Handshake for
IS-IS Point-to-point Adjacencies" by Dave Katz - available on the work 
group website) that the significance
of the local circuit ID field on point-to-point circuits is limited.

 > Isn't it necessary for the IS to generate a new LSP with the new 

Yes. That will happen because whenever an adjacency state change event 
occurs (down or up) this will cause LSP generation according to
Section 7.3.6.

  Les

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From: Rajesh Saluja <[email-masked]>
Date: Fri, 04 Jun 1999 10:09:43 -0400

> create a new one. (e.g. system ID hasn't changed, but the Local

Les and Murali,
This calculation does not look like taking care of circuit id change always.
Suppose A and B are two ISs  and A>B. Suppose they both make adjacency on circuit
1.
Since A>B, CircuitID status = A1
Now suppose B has rebooted or deleted the circuit and recreated it with circuit 2.
When A
receives Hello PDU, it will again calculate CircuitID status which will be A1
again because
of higher System ID.

-Rajesh

--

-------------------------------------------------------------
Rajesh Saluja
Nortel Networks Inc 600 Technology Park Billerica, MA  01821
Ph: (978) 288-7791      Fax: (978) 670-8760
--------------------------------------------------------------

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From: "Lester Ginsberg" <[email-masked]>
Date: Fri, 4 Jun 1999 08:21:45 -0700 (PDT)

What you say is strictly correct. However, ISIS makes some explicitly
stated assumptions about the service provided by a point-to-point
subnetwork. (See Section 6.7.2 in the 1992 spec) These require that
the subnetwork be able to detect start-up of the remote end of a link
and send notification to the routing layer that this has occurred. The
routing layer, in turn, would of course take down any adjacency
associated with a circuit on which it has received such a notification.
This would prevent the situation you describe below from ever
occurring.

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From: Rajesh Saluja <[email-masked]>
Date: Fri, 04 Jun 1999 11:59:07 -0400

Fine.  It basically conveys  that any circuit id change will restart
point to point circuit i.e. ISIS will see the interface and hence
adjacency going down and  coming up.
It  means that you can never see a circuit ID change in "Accept"
state.

-Rajesh

--

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From: Dave Katz <[email-masked]>
Date: Fri, 4 Jun 1999 08:59:45 -0700 (PDT)

It's very nice that the spec requires this of the subnetwork, but there
are plenty of subnetworks out there for which this is not possible
or practical in real application.

   From: "Lester Ginsberg" <[email-masked]>
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