Subject: [Isis-wg] Changing the System ID

From: Abhishek Verma <[email-masked]>
Date: Tue, 3 May 2005 14:09:43 +0530

Hi,

When a user changes the system ID of an ISIS router isnt that
equivalent of shutting down ISIS running on a router and creating a
fresh ISIS instance there.

I say this because with the new system ID, this ISIS router will now
originate "new" LSPs which will not match with the earlier LSPs that
other ISIS routers had recieved.

I am concerned about some intermediate time till which other systems
dont time out their adjacency with the earlier instance of this ISIS
router.

Let me explain my doubt:

R1 -- R2

System ID of R1 is "A" and its changed to "B" at time t=0

Before t=0, R1 and R2 were ISIS peers and R2 had LSPs from R1 with the
system ID as A. Now at t=0, R1 starts sending HELLOs and all with the
system ID of B. R2 thinks that a new router has come up and starts
initiating the adjecency with this router. Meanwhile, ISIS adjacency
with "A" is also up on R2 as the Hold Time on R2 has not expired.

So R2 maintains adjacencies with "A" and "B" till the Hold time
expires on R2. Can this not create a problem?

One good point that i see in this scheme is that it provides a
seamless transition from system ID "A" and "B" because whatever routes
R2 had installed and learnt from "A" are kept till the Hold Time
expires. It then learns those same addresses from instance "B" and
traffic is never dropped.

Is my understanding of this correct?

Thanks,
Abhishek

--
Class of 2004
Institute of Technology, BHU
Varanasi, India

_______________________________________________
Isis-wg mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/isis-wg

---

From: "Les Ginsberg \(ginsberg\)" <[email-masked]>
Date: Tue, 3 May 2005 22:06:12 -0700

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] On
Behalf
> Of Abhishek Verma
> Sent: Tuesday, May 03, 2005 1:40 AM
> To: ISIS-WG (E-mail)
> Subject: [Isis-wg] Changing the System ID
> 
> Hi,
> 

Abhishek -

A good reference on this topic is RFC 3719 Section 9.

In summary, implementations SHOULD delete the adjacency immediately upon
noting a neighbor system ID change - the nuances of that are well
discussed in RFC 3719.

You can also consult ISO 10589:2002 Section 8.2.5.2d which, contrary to
what is stated in RFC 3719, does actually specify that the adjacency
SHOULD be deleted in cases of receiving a PDU with a new System ID (this
was corrected from the 1992 version).

A system which is going to change its system ID can insure that the
adjacency goes down by sending an IIH using the old System ID and a zero
hold time prior to beginning use of the new SystemID.

   Les