MTU

Fri Jul 22 18:20:41 UTC 2016

> On Jul 22, 2016, at 1:37 PM, Grzegorz Janoszka <Grzegorz at Janoszka.pl> wrote:
> What I noticed a few years ago was that BGP convergence time was faster with higher MTU.
> Full BGP table load took twice less time on MTU 9192 than on 1500.
> Of course BGP has to be allowed to use higher MTU.
> 
> Anyone else observed something similar?

I have read about others experiencing this, and did some testing a few months back -- my experience was that for low latency links, there was a measurable but not huge difference. For high latency links, with Juniper anyway, there was a very negligible difference, because the TCP Window size is hard-coded at something small (16384?), so that ends up being the limit more than the tcp slow-start issues that MTU helps with.

With that said, we run MTU at >9000 on all of our transit links, and all of our internal links, with no problems. Make sure to do testing to send pings with do-not-fragment at the maximum size configured, and without do-not-fragment just slightly larger than the maximum size configured, to make sure that there are no mismatches on configuration due to vendor differences.

Best Regards,
-Phil Rosenthal