MTU

Tore Anderson tore at fud.no
Sat Jul 23 08:28:49 UTC 2016


* Baldur Norddahl

> What is best practice regarding choosing MTU on transit links?
> 
> Until now we have used the default of 1500 bytes. I now have a
> project were we peer directly with another small ISP. However we need
> a backup so we figured a GRE tunnel on a common IP transit carrier
> would work. We want to avoid the troubles you get by having an
> effective MTU smaller than 1500 inside the tunnel, so the IP transit
> carrier agreed to configure a MTU of 9216.

You use case as described above puzzles me. You should already your
peer's routes being advertised to you via the transit provider and vice
versa. If your direct peering fails, the traffic should start flowing
via the transit provider automatically. So unless there's something
else going on here you're not telling us there should be no need for
the GRE tunnel.

That said, it should work, as long as the MTU is increased in both ends
and the transit network guarantees it will transports the jumbos.

We're doing something similar, actually. We have multiple sites
connected with either dark fibre or DWDM, but not always in a redundant
fashion. So instead we run GRE tunnels through transit (with increased
MTU) between selected sites to achieve full redundancy. This has worked
perfectly so far. It's only used for our intra-AS IP/MPLS traffic
though, not for eBGP like you're considering.

> 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.

I'd say it's not worth the trouble unless you know you're going to use
it for anything. If I was your peer I'd certainly need you to give me a
good reason why I should deviate from my standard templates first...

> On the other hand, none of my customers will see any actual difference
> because they are end users with CPE equipment that expects a 1500
> byte MTU. Trying to deliver jumbo frames to the end users is probably
> going to end badly.

Depends on the end user, I guess. Residential? Agreed. Business? Who
knows - maybe they would like to run fat GRE tunnels through your
network? In any case: 1500 by default, other values only by request.

Tore




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