Subject: [Isis-wg] Zero Cost links in ISIS

From: Abhishek Verma <[email-masked]>
Date: Mon, 14 Feb 2005 16:21:21 +0530

Hi,

When we advertise an interface using the extended IP Reach TLV the
cost is by default 10. So why is it that we advertise our loopbacks
with a cost of 0?

As i understand, because of the nature of ISO addressing all the ISIS
routers end up as nodes in the SPF tree, and all IP reachability
information is attached as leaves to that tree. Now, if an IP
interface goes down i will not see a neighbor anymore. So, i will
remove it from my LSP and announce this to my neighbors. I will also
remove this information from my IP reachability TLV info.

Now the recieving routers will need to run the *full* SPF when they
recieve this LSP as the topology has changed. SO why do we say that in
ISIS if IP reachability changes then other ISIS routers dont need to
run their full SPFs  (as compared to OSPFv2).

I am confused because whenever IP reachability changes (route goes
up/down) the topology would always get affected. I know i am missing
something. Would someone be good enough to point that out?

Thanks a lot,
Abhishek

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

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From: "Don Goodspeed" <[email-masked]>
Date: Mon, 14 Feb 2005 11:20:05 -0800

Abhishek,

You are asking questions regarding one ISIS implementation that
do not necessarily apply to all vendor's implementations.

As such you will need to ask that specific vendor for the answers
as to why they do these particular actions.

Thanks,
Don

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On
Behalf Of Abhishek Verma
Sent: Monday, February 14, 2005 2:51 AM
To: ISIS-WG (E-mail)
Subject: [Isis-wg] Zero Cost links in ISIS

Hi,

Thanks a lot,
Abhishek

---

From: Hannes Gredler <[email-masked]>
Date: Sun, 20 Feb 2005 16:57:08 +0100

On Mon, Feb 14, 2005 at 04:21:21PM +0530, Abhishek Verma wrote:

| I am confused because whenever IP reachability changes (route goes
| up/down) the topology would always get affected. I know i am missing
| something. Would someone be good enough to point that out?

we tend to use the word "SPF run" or "full SPF run" in a very loose
fashion - what most routers do in fact is

1. calculate a topology graph based on (extd) IS reach information
   based on dijkstra's algorithm;   

2. attach the leaf information [IP routes] to each of the
   nodes in the topology.

3. sort the leaf information [IP routes] based on the path
   cost resulting from step [1]

4. populate the kernel/FIB if there has been a change in
   IP forwarding state from the previous run.

---

so to answer your question: no - just a change in IP reachability
information does not require to repeat from step 1 - what routers
need to do is to calculate from step 2 onwards.

in some vendors implementaton is is termed as a "partial" SPF run,
while in fact its not a real SPF run but rather a sort
operation;

HTH,

/hannes