Subject: [Isis-wg] LAN ID field in LAN IIH

From: "Sachidananda K" <[email-masked]>
Date: Thu, 15 Nov 2001 11:14:59 +0000

Hello,

I've a question related to the values to be filled in packet format for the 
L1 LAN ISH. As soon as the IS comes up, what value do we fill in the LAN ID 
field. Is it supposed to be left blank ( filled with all 0's). I'm refering 
to the first Hello packet sent on the broadcast interface.

Please help me clarify this doubt.
Thanking you in advance for all the response.
Sachi

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From: "Ken Larmer" <[email-masked]>
Date: Thu, 15 Nov 2001 08:57:54 -0500

Hi Sachi,

> to the first Hello packet sent on the broadcast interface.

	Do you mean L1 LAN IIH PDU? An ISH is sent on a broadcast interface if you
are running IS-IS and/or DECnet Phase V. The ISH is meant to inform OSI ES
or DECnet Phase V ES of the routers available on a given LAN. However, in
Integrated IS-IS (IP-Only) there is no need to send an ISH on a broadcast
circuit (Int IS-IS does not care about ESs).

	With reference to a LAN IIH PDU, when a broadcast circuit is first enabled,
the LAN ID portion is set in accordance with the following ISO-10589
reference:

 8.4.1 Enabling of broadcast circuits

When the broadcast circuit is enabled on an Intermediate system the IS shall
perform the following actions.

a)	Commence sending IIH PDUs with the LAN ID field set to the concatenation
of its own systemID and its locally assigned one octet Local Circuit ID.
...
d)	After waiting iSISHelloTimer × 2 seconds, run the Level 1 and or the
level 2 designated intermediate system election process depending on the
Intermediate system type.

8.4.5 LAN designated intermediate systems

The LAN ID field in the LAN IIH PDUs transmitted by this system shall be set
to the value of the LAN ID field reported in the LAN IIH PDU (for the
appropriate level) received from the system which this system considers to
be the Designated Intermediate System. This value shall also be passed to
the Update Process, as the pseudonode ID, to enable Link State PDUs to be
issued for this system claiming connectivity to the pseudonode.

If this system, as a result of the Designated Intermediate System election
process, considers itself to be designated Intermediate System, the LAN ID
field shall be set to the concatenation of this system’s own system ID and
the locally assigned one octet Local Circuit ID.