IP Fragmentation

Wed Aug 20 17:24:40 UTC 2008

In a message written on Wed, Aug 20, 2008 at 09:43:44PM +0530, Glen Kent wrote:
> Do transit routers in the wild actually get to do IP fragmentation
> these days? I was wondering if routers actually do it or not, because
> the source usually discovers the path MTU and sends its data with the
> least supported MTU. Is this true?

Yes.

A GigE jumbo frames host (9120) to a standard POS interface (4420)
to a DS3 customer (1500) happens, and the GigE->POS and POS->DS3
routers must both do fragmentation.

> I would wager that the vendors and operators would want to avoid IP
> fragmentation since thats usually done in SW (unless you've got a very
> powerful ASIC or your box is NP based).

As far as I know the "big" routers all do it in hardware with no real
performance penality; but I haven't studied in detail.

-- 
       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
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