Subject: [Isis-wg] FW: Voids in LSP

From: Abhishek Verma <[email-masked]>
Date: Sat, 2 Oct 2004 19:19:08 +0530

In case my previous mail got lost!

> Hi,
> 
> One thing which i noticed in OSPF was that each individual route can
> be advertised in a separate LSA. And multiple LSAs can be clubbed
> inside one LSUpdate message. If i understand correctly, then this for
> some reason seems to be missing in ISIS.
> 
> Say, i receive a route. I try fitting this into one LSP. I find that
> it cant fit, so i create a new LSP and put this route there. I thus
> end up advertising two LSPs. Now, suppose some routing information
> that i had stored in the first LSP goes away, and makes enough space
> for me to fit the route that i have in the new LSP here in the first
> LSP.
> 
> Will i remove the information from the 2nd LSP and try to fit this in
> the first one?
> 
> If not, then cant it lead to a lot of waste spaces inside an LSP? I
> mean, routes can come and go and cant that create voids in an LSP?
> 
> I think, now if i get some new routing information i will try to fit
> that into my original LSP. If it can fit there, then i will advertise
> this LSP. Only, if it cant fit, then i will use my next new LSP which
> i had created.
> 
> Is this how it works?
> 
> Thanks,
> Abhishek V.
> 
> P.S.
> 
> BTW is there a document/URL which can tell tell me of how and where
> the two protocols ISIS and OSPF differ?
> 
> --
> Class of 2004
> Institute of Technology, BHU
> Varanasi, India
>

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From: Shankar Vemulapalli <[email-masked]>
Date: Mon, 4 Oct 2004 11:18:10 -0700 (PDT)

Hi Abhishek -

> > the two protocols ISIS and OSPF differ?

Take a look at Henk's e-mail @:
http://www.merit.edu/mail.archives/nanog/1999-01/msg00012.html

This is from Dave Katz @: http://www.nanog.org/mtg-0006/katz.html

Also, check the John Moy's & Radia Perlman's books too..

Hope it helps.

Thanks,

/Shankar

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From: Jeff Learman <[email-masked]>
Date: Mon, 04 Oct 2004 16:03:45 -0400

>> the two protocols ISIS and OSPF differ?

diff-ospf-isis group:

We wrote a document on this.  Anybody know where it ended up?

Jeff

---

From: Jeff Learman <[email-masked]>
Date: Mon, 04 Oct 2004 16:14:20 -0400

>> mean, routes can come and go and cant that create voids in an LSP?

Not really.  You're sending two LSPs where you could send one, but
LSPs aren't fixed size, so other than the overhead of the additional
LSP fragment, you're not wasting space.  If other info comes in,
you can put it there in some circumstances.

ISO 10589 has some language about how it's nice not to make info
"jump" from fragment to fragment.  As I recall, it's not easy
advice to follow.

I think the silence you've received might be due to the fact that
the behavior here is very implementation-dependent, and that
you can do a very simple thing and your routers will interoperate,
but might be less efficient than other implementations.  So it's
up to you to work out the issues here since it's not an
interoperability problem.

You need to work out your own compromise between minimizing the
movement of info from LSP to LSP versus sending the minimum number
of LSPs.

In general, ISIS's flooding mechanism is very efficient and scalable
from a bandwidth perspective.  I know this from studies I've done
for low-bandwidth lines like SONET SDCC (192 kBit/sec).  Regardless,
you still want to strike a good tradeoff between the two extremes.

Regards,
Jeff

Disclaimer: I haven't worked on ISIS at Cisco, and I don't speak for
Cisco in this WG.

---

From: Abhishek Verma <[email-masked]>
Date: Wed, 13 Oct 2004 15:26:30 +0530

Jeff,
 
> diff-ospf-isis group:

I tried searching for this group but was unable to find one! Is there
a way to join this?

> We wrote a document on this.  Anybody know where it ended up?

I believe you are talking of
draft-bhatia-manral-diff-isis-ospf-00.txt. Right? I see that it has
expired. Are you guys planning for the next version?

This draft is an excellent resource for beginners like us, who often
get confused with the different link state routing protocols that we
have! And this helped straighten out a lot of things for me!

Thanks,
Abhishek.