Subject: [Isis-wg] The first very simple question

From: "Daniel Yang" <[email-masked]>
Date: Wed, 6 Dec 2006 06:15:31 -0800

Hello everyone,

This is my first to use mail list, I even don't know whether this mail would
reach you and any of you would give me a reply.

I am a Chinese boy,  I'm working for Huawei Company's ISIS development team
now.

I'm reading RFC3784 now. Below is some words quoted from RFC3784:

   To preclude overflow within a traffic engineering SPF implementation,
   all metrics greater than or equal to MAX_PATH_METRIC SHALL be
   considered to have a metric of MAX_PATH_METRIC.  It is easiest to
   select MAX_PATH_METRIC such that MAX_PATH_METRIC plus a single link
   metric does not overflow the number of bits for internal metric
   calculation.  We assume that this is 32 bits.  Therefore, we have
   chosen MAX_PATH_METRIC to be 4,261,412,864 (0xFE000000, 2^32 - 2^25).

And my question is: Why can't we choose MAX_PATH_METRIC to be 0xFF000000?

In my opinion, the greatest metric of a single link is 0xFFFFFE. If the
MAX_PATH_METRIC is 0xFF000001,
the MAX_PATH_METRIC + 0xFFFFFE = 0xFFFFFFFF. and 0xFFFFFFFF does not
overflow the internal metric.

And why we choose MAX_PATH_METRIC to be a single link metric less than the
overflow value?

Best regards,
Daniel Yang
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---

From: "Tony Li" <[email-masked]>
Date: Wed, 6 Dec 2006 07:59:37 -0800

Hi Daniel,
 
Welcome to the list.
 
We felt that it was prudent to leave some additional safety margin and
that we had ample dynamic range, so we were generous with the guard
band.
 
Tony

  _____  

From: Daniel Yang [mailto:[email-masked]] 
Sent: Wednesday, December 06, 2006 6:16 AM
To: [email-masked]
Subject: [Isis-wg] The first very simple question

Hello everyone,

This is my first to use mail list, I even don't know whether this mail
would reach you and any of you would give me a reply.

I am a Chinese boy,  I'm working for Huawei Company's ISIS development
team now. 

And my question is: Why can't we choose MAX_PATH_METRIC to be
0xFF000000?

And why we choose MAX_PATH_METRIC to be a single link metric less than
the overflow value?

Best regards, 
Daniel Yang