[c-nsp] MTU / BGP

Gert Doering gert at greenie.muc.de
Tue Jul 21 03:06:37 EDT 2015


Hi,

On Tue, Jul 21, 2015 at 08:55:23AM +0200, Mark Tinka wrote:
> > Typical issues that are very much a recipe for trouble is when you have
> > a single layer2 network where different devices have different layer 3
> > MTUs.  Always ensure that *all* adjacent L3 devices agree on the
> > IP/IPv6/MPLS MTU (and take into account the subtle differences between
> > IOS, IOS XR and JunOS...).
> 
> The problem is MTU discovery when you're leasing your Layer 2 network.
> I've had issues with this, which is why I've had to disable that
> capability in past jobs, due it being global but some of your leased
> Layer 2 services have MTU issues.

If you have a link where the L3 MTU is set higher than the L2 technology
can handle, you're run into deep shit anyway - like, ISIS not coming up
(due to padding to full MTU size), L3 packets spontaneously disappearing
into a black hole (because your routers assume they can send them
unfragmented...), etc.

> If you run your own dark fibre network, then MTU discovery is easier.
> But since you do, you probably don't need it in the first place :-).

There's always the possibility for MTU differences in different parts of
the network, like "this access segment here which hosts a xyz-box which
can only do a MTU of 1900" - path MTU discovery will ensur that BGP 
from a box in this network to a box elsewhere on a MTU 9000 link will
still nicely work (and fall down to a MSS of 1860 or so).

gert
-- 
USENET is *not* the non-clickable part of WWW!
                                                           //www.muc.de/~gert/
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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