Subject: RE: [Isis-wg] New 10589 spec, PtoP and ISHs

From: Les Ginsberg <[email-masked]>
Date: Tue, 21 Aug 2001 13:39:46 -0700

This issue is a confusing one, some of which is due to the fact that the
original wording in ISO 10589:1992 was revised by Technical Corrigendum I
(published in 1993), but because many people do not have ready access to the
TCs, they rely on the uncorrected original text. In fact, the original
intent was made clear in TC1 by adding:

"An IS shall cause ISO 9542 to send an ISH PDU whenever a point-to-point
circuit is first enabled."

and then changing the original text in 8.2.3a (which Neal refers to in
stating that an IIH was supposed to be sent when the circuit was first
enabled) to read:

"a)the IS receives an ISH PDU"

So the original intent was to wait for receipt of an ISH before beginning to
send IIHs. The latest ISO 10589 draft further clarifies this by specifying
that ISH's should continue to be sent periodically until the adjacency
reaches the UP state. This is a logical requirement discovered when
implementing that was not clearly stated even in the TC1 changes.

In an IP only IS-IS implementation, the use of ISH's is an anachronism. Many
implementations relax the requirements, at least on reception, so that
receiving an ISH is not required. In following the old axiom of being
"strict in what you send and generous in what you receive", a robust
implementation should send ISHs until the adjacency is up, but allow (at
least for IP operation) the adjacency to come up without ever seeing an ISH.
This is easily achieved by starting to send IIHs when you first receive an
IIH.

I can tell you from first hand experience that there are indeed
implementations out there which do not send ISHs.

   Les

> -----Original Message-----
> From: Neal Castagnoli [mailto:[email-masked]]
> Sent: Tuesday, August 21, 2001 1:14 PM
> To: [email-masked]
> Subject: [Isis-wg] New 10589 spec, PtoP and ISHs
> 
> 
> 
> From the original 10589, routers might be able to get around 
> sending ISHs on
> ptop circuits.  The original states routers should transmit an IIH
> immediately upon circuit startup, and so an implementation 
> need not send an
> ISH to get the peer IS to start IIH transmission.  New 10589, however,
> removes this wording, and now it appears routers require 
> receipt of an ISH
> prior to sending the first IIH.
> 
> Yet if there are old implementations that don't send ISHs, 
> ptop adjacencies
> won't come up with strict new implementations.  Does anyone 
> know whether
> there are deployed ISIS implementations that don't send ISHs on ptop
> circuits?
> 
> Thanks,
> 	
>   --Neal
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---

From: Jeff Learman <[email-masked]>
Date: Mon, 27 Aug 2001 00:41:14 -0400

If my memory serves me, on a PTP circuit, an implementation is required
to handle receipt of an IIH without having received an ISH, so Les's
wise general rule doesn't need to be invoked.