Re: [RRG] Re: [RAM] Tunneling overheads and fragmentation

Gert Doering <> Wed, 12 September 2007 20:08 UTC

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Date: Wed, 12 Sep 2007 22:08:37 +0200
From: Gert Doering <>
To: Dino Farinacci <>
Subject: Re: [RRG] Re: [RAM] Tunneling overheads and fragmentation
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I seem to have missed that e-mail.  Sorry for replying not in sequence.

On Tue, Sep 11, 2007 at 02:53:50PM -0700, Dino Farinacci wrote:
> >I did a survey about a month ago and it is true. I will yield to  
> >those folks who responded to me to protect their privacy. ;-)

We haven't been asked in this survey - and our data looks quite different.

> >o We are going to 9K MTUs on all our internal links.
> >o Where we don't have 9K MTUs, we use 4470.
> >o Virtually no one runs ISP links at 1500.

In nearly all locations we have devices that still have Fast Ethernet 
links (because upgrading to GigE is expensive and the amount of
packes flowing through the link doesn't require it).

While we don't run 1500 on links where MPLS is used, most Cisco gear
in use cannot go over 1520...1530 on FastEthernet ports.  This limits
the GigE machines to 1530 as well, as things really break (today) if 
you share a layer2 segment between machines with different MTUs.

So there is no way our network could run on a MTU of 4470 or even 
higher any time soon.

>    In practice, this is not really a problem.  Hosts typically do not
>    originate IP packets larger than 1500 bytes.  And second, a survey
>    has been taken (from a list of ISPs, see Acknowledgement section)
>    where nearly all ISP link MTUs are either 4470 bytes or support
>    Ethernet jumbo frames of 9000 bytes.  Therefore, we don't anticipate
>    any problems with prepending additional headers.

This is handwaving, and not a good basis for research work.

Gert Doering
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