Re: MTU

[Chris Kane <ccie14430 () gmail com>]

This topic seems to come up more lately. Much like it did often during
IPSec related deployments. I simplify on 9,000 as an easy number and I
don't have to split hairs (read 9,214 v 9,216) that some vendors have.

My experience has been making a view phone calls and agreeing on 9,000 is
simple enough. I've only experienced one situation for which the MTU must
match and that is on OSPF neighbor relationships, for which John T. Moy's
book (OSPF - Anatomy of an Internet Routing Protocol) clearly explains why
MTU became an issue during development of that protocol. As more and more
of us choose or are forced to support 'jumbo' frames to accommodate Layer 2
extensions (DCI [Data Center Interconnects]) I find myself helping my
customers work with their carriers to ensure that jumbo frames are
supported. And frequently remind them to inquire that they be enabled not
only on the primary path/s but any possible back up path as well. I've had
customers experience DCI-related outages because their provider performed
maintenance on the primary path and the re-route was sent across a path
that did not support jumbo frames.

As always, YMMV but I personally feel having the discussions and
implementation with your internal network team as well as all of your
providers is time well spent.

Later,
-chris









On Fri, Jul 22, 2016 at 8:53 AM, Mark Tinka <mark.tinka () seacom mu> wrote:

> On 22/Jul/16 14:01, Baldur Norddahl wrote:
>
> > Obviously I only need to increase my MTU by the size of the GRE header.
> But
> > I am thinking is there any reason not to go all in and ask every peer to
> go
> > to whatever max MTU they can support? My own equipment will do MTU of
> 9600
> > bytes.
>
> See the below:
>
>     http://mailman.nanog.org/pipermail/nanog/2016-March/084598.html
>
> You can reliably run Jumbo frames in your own network core, and also to
> another network that can guarantee you the same (which would typically
> be under some form of commercial, private arrangement like an NNI).
>
> Across the Internet, 1,500 bytes is still safest, simply because that is
> pretty much the standard. Trying to achieve Jumbo frames across an
> Internet link (which includes links to your upstreams, links to your
> peers and links to your customers) is an exercise in pain.
>
> Mark.



-- 
Chris Kane
CCIE 14430
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