MTU

Fri Jul 22 22:45:49 UTC 2016

On 7/22/16, Phil Rosenthal <pr at isprime.com> wrote:
>
>> 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.

I think the Cisco default window size is 16KB but you can change it with
ip tcp window-size NNN

Lee

>
> 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