Subject: [Isis-wg] RFC 3784!

From: Abhishek Verma <[email-masked]>
Date: Mon, 27 Sep 2004 14:50:25 +0530

Hi,

The max size of a TLV in ISIS can be 255. Thus the number of IP
prefixes that we can fit inside one TLV 128/130 is given by:

Bytes needed to encode each prefix = 12 (metric+address+mask)
255/12 = 21 approx

However, if we use 3784 encoding then number of IP prefixes that we
can encode in each TLV 135 is 255/9 = 28 approx.

Obviously, i assume in the latter case that there are no sub TLVs used. 

Does this imply that i can carry greater number of IP prefixes if i
use TLV 135 (wide metrics) if no other TE feature is used?

Regards,
Abhishek V.

P.S.
For the sake of simplicity, i have ignored all the TLV header overheads, etc.

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From: Henk Smit <[email-masked]>
Date: Mon, 27 Sep 2004 14:50:16 +0200 (CEST)

> Bytes needed to encode each prefix = 12 (metric+address+mask)

  Correct.

> can encode in each TLV 135 is 255/9 = 28 approx.

  It's even better. I see you count 5 bytes required to describe a prefix.
 One byte for prefix length, and 4 bytes for the prefix itself. Note that
 you only need 4 bytes if the prefix is 25 bits or longer. So for most
 routes in the Internet you will need 3 bytes or less. Therefor I think
 you can say that on average, you can fit 255/8 = 31 prefixes in a TLV.

> use TLV 135 (wide metrics) if no other TE feature is used?

  Yes.
 This was one of the design considerations of the new TLVs.

      Henk.

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From: Abhishek Verma <[email-masked]>
Date: Tue, 28 Sep 2004 11:05:51 +0530

[clipped]

> This was one of the design considerations of the new TLVs.

Which means that we can carry more number of adjacencies in each TLV
if we use wide-metrics!

Thanks a lot!

Abhishek.

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

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From: Jeff Learman <[email-masked]>
Date: Tue, 28 Sep 2004 08:15:19 -0400

Just keep in mind that your system may not interoperate well
with older implementations that don't support the TE TLV.  Remember
that the rule is you're allowed to ignore TLVs you don't recognize.
This might lead to some serious black-hole issues.  I haven't studied
the TE RFC and I don't know how these issues are addressed.  I'd guess
it has something to do with transitioning all routers in a subdomain
to wide metrics.

Jeff

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From: "Parker, Jeff" <[email-masked]>
Date: Tue, 28 Sep 2004 10:01:32 -0400

Not sure that this mail will make it through to the wg list, as this is from
my "wrong" account.

The issue of interoperation is flagged in the IP Interoperation RFC, and a
transition strategy was described

- jeff parker