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Re: [v6ops] PMTUD issue discussion

Mikael Abrahamsson <swmike@swm.pp.se> Tue, 09 September 2014 07:56 UTC

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Date: Tue, 09 Sep 2014 09:56:47 +0200
From: Mikael Abrahamsson <swmike@swm.pp.se>
To: Tom Perrine <tperrine@scea.com>
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Subject: Re: [v6ops] PMTUD issue discussion
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On Tue, 2 Sep 2014, Tom Perrine wrote:

> What MTUs are actually (commonly) seen in the wild?

My reply is late, but let me write some of my experience as I did some 
investigation into what's possible. All values presented here are IP MTU, 
not L2 MTU. Most people mix these values and I've seen a lot of broken 
deployments due to this.

The major end system IP MTU seen in the world is 1500 (huge majority).

The path MTUs seen might be slightly under due to PPPoE, GRE, GTP etc 
encapsulations on the access layer.

Going larger than this (speaking both of path MTU and end system MTUs), 
most opt for 4470, 9000 or 9180 due to older protocols such as AAL5 ATM, 
SMDS, FDDI, POS etc. Unfortunately Juniper MX doesn't support 9180, but 
instead one has to choose one value of 9178, 9174 or 9170 depending on 
number of vlan tags (or MPLS labels I guess) one wants to support, so 
these are also seen.

Personally I think the balanced approach would be to make your core 
support something around the 9180 figure, and give 9000 to end users.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

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