Subject: [Isis-wg] TLV 132 doubt

From: Abhishek Verma <[email-masked]>
Date: Wed, 13 Oct 2004 17:50:22 +0530

Hi,

I have the following setup and am seeing a queer problem.

R1 (x.1/24) ------------------ R2 (x.2/24)

where x = 2.2.2.0/24

R1 and R2 are both configured to be L1 routers. Initially I have added
only one interface on R2 in my ISIS. Thus i see only 2.2.2.2/24 being
advertised in IP Interface TLV (Type 132).This works fine.

I also see 2.2.2.0/24 being advertised in IP Internal Reachability TLV (128).

Now, I add another interface, say 3.3.3.1/24 to ISIS. What i see now
is that when R2 advertises its L1 LSP, it replaces 2.2.2.2/24 in TLV
132 with 3.3.3.1. Shouldnt we be advertising both the IP addresses in
the TLV 132?

What exactly is then the significance of TLV 132?

Thoug I see two addresses 2.2.2/24 and 3.3.3/24 in TLV 128.

Another doubt: On a broadcast LAN, why dont we send PSNPs to
acknowledge the Pseudonode LSPs that have been originated by the DIS?

Regards,
Abhishek.

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

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From: Shankar Vemulapalli <[email-masked]>
Date: Wed, 13 Oct 2004 16:03:30 -0700 (PDT)

Hi Abhishek -

See below -

> 132 with 3.3.3.1. Shouldnt we be advertising both the IP addresses in

Take a look at the RFC 1195 for information on the the TLV 132.
Also, look into RFC 3359 and RFC 3787 (section 10)

> What exactly is then the significance of TLV 132?

See above -

> acknowledge the Pseudonode LSPs that have been originated by the DIS?

Because IS-IS doesnt' require explicit acknowledgments on a LAN.
Actually, DIS summarizes the database by sending a CSNP (complete
sequence numbers packet) and not a PSNP.

The way it works on the LAN is:  if a router detects that in the received
CSNP, that there is a missed LSP, it transmits that LSP on the LAN.   This
is because, it either not listed in the CSNP or listed with a lower seq. #.

On the contrary, if the a router detects that if the DIS more recent info.
in the CSNP, it explicitly requests that LSP via PSNP.

Hope it helps.

/Shankar

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From: Dave Katz <[email-masked]>
Date: Wed, 13 Oct 2004 19:41:08 -0600

> in the CSNP, it explicitly requests that LSP via PSNP.

Yep, and the nice part about this scheme is that under normal 
conditions the LSP traffic is constant, regardless of the number of 
systems on the LAN.  As such, it is the only truly useful application 
of LAN multicast among control protocols.

The unicast ack scheme that OSPF and others use will at best cut the 
packet count in half, so the packet count still grows proportionately 
with the number of systems, making the optimization effectively useless 
from a scaling perspective.

Then there's EIGRP, which in some cases actually generates *more* 
packets (well, one more anyhow) because of its attempt to use multicast 
than it would have had it just unicast everything.  But I digress.

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From: Marc Binderberger <[email-masked]>
Date: Thu, 14 Oct 2004 21:47:35 +0200

Hi Shankar,

>> What exactly is then the significance of TLV 132?

Are these RFCs really explaining the significance of TLV132 in _LSP_? 
RFC1195 just tells us what to do, RFC3787 talks about the IIH.
While I understand it for IIH - ensuring a consistency between both 
sides (p2p) or LAN members - I wonder why this TLV in LSPs?

Playing with a test router (by accident a box from your company ;-) it 
looks like the first loopback address is used. Kind of a "router ID"? 
Even then the question: why?

Regards, Marc
--
Marc Binderberger    <[email-masked]>