Subject: [Isis-wg] DR election

From: "Ken Larmer" <[email-masked]>
Date: Thu, 12 Jul 2001 15:17:08 -0400

Hi,

	I have a question pertaining to the DR election process within Int IS-IS.
ISO-10589 states the following:

Broadcast subnetworks

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.

b)	Solicit the End system configuration as described in 8.4.6

c)	Start listening for ISO 9542 ESH PDUs and acquire adjacencies as
appropriate. Do not run the Designated Intermediate System election process.

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.

LAN designated intermediate systems

The IS shall run the Level 1 and/or the Level 2 Designated Intermediate
System election process (depending on the Intermediate system type) whenever
an IIH PDU is received or transmitted as described in 8.4.4. (For these
purposes, the transmission of the system’s own IIH PDU is equivalent to
receiving it). If there has been no change to the information on which the
election is performed since the last time it was run, the previous result
can be assumed. The relevant information is

f)	the set of Intermediate system adjacency states;

g)	the set of Intermediate System priorities (including this system’s); and

h)	the existence (or otherwise) of at least one “Up” End system (not
including manual adjacencies) or Intermediate system adjacency on the
circuit.

	I understanding waiting iSISHelloTimer X 2 seconds when a circuit is first
enabled, however, what happens when we have a circuit failure, which times
out all of our adjacencies and then the circuit comes back. Do we then run
the DR election process after we receive our first LAN IIH with an adjacency
reference to us in its IS Neighbors field or do we wait the iSISHelloTimer X
2. If we wait iSISHelloTimer x 2, our convergence times may be delayed
considerably?

	Secondly, our iSISHelloTimer may be different from our neighbors, which we
may be unaware of because we have yet to receive an IIH from them. I can see
the DR changing multiple times given the correct configuration prior to the
correct DR being elected. Should there be some check against the LAN ID
contained within the IIH prior to running the DR election process?

	Your comments greatly appreciated!

Cheers,
Ken Larmer

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---

From: "Ken Larmer" <[email-masked]>
Date: Thu, 12 Jul 2001 15:40:05 -0400

Hi,

	I believe I have answered my own question?

	If there is more than one IS on a LAN, there is already a DR. In accordance
with ISO-10589, the DR will use the dRISISHelloTimer = 1 second. So, with
this considered, we will receive a LAN IIH from the DR advertising an
adjacency to us prior to electing a new DR. This is because we send our LAN
IIH to the allL1/L2 multicast address, so all ISs on the LAN will receive it
at the same time. Consequently, the IS
with the shortest ISISHelloTimer will be the first system we form an
adjacency with. If we do not have a higher priority than the current DR,
than we will not elect ourselves as the DR.

Thanks,
Ken

> -----Original Message-----
> From: [email-masked]
> [mailto:[email-masked]]On Behalf Of Ken Larmer
> Sent: Thursday, July 12, 2001 3:17 PM
> To: Isis-Wg@Juniper. Net
> Cc: Ken Larmer
> Subject: [Isis-wg] DR election
>
>
> Hi,
>