[afnog] MTU Size for transit links for ISPs

[Mark Tinka]

On Friday, September 20, 2013 02:12:35 PM Perreau, Luc 
wrote:

> Mohamed and Johan, I'm still waiting for the upstream to
> give me their MTU setting anyway.

In general, the MTU across IP Transit links (i.e., between 
your and your upstream's border routers, or between your 
peering router and those of other networks) is about 1,500 
bytes.

The MTU within your core (or core of the network your 
connected to) would, these days (although not always), be 
larger than what is supported across the peering link.

So if the majority of your traffic is off-net, i.e., it's 
crossing your peering links into another network's domain, 
your backbone must be able to support at least the MTU that 
is supported on your peering/transit links.

As Nishal has pointed out, a large MTU in your core does not 
mean much, as the largest MTU you can support will be the 
smallest MTU any of your network's interfaces supports. This 
is why it helps to have a consistent policy on what your 
core MTU is. Simple advice, if you can, it should be as 
large as possible so you can support additional network 
fetaures in the future, as well as customers that may 
require it for on-net traffic. This is a-whole-nother thread 
unto itself :-).

Mark.
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