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MTU

On 2016-07-22 20:20, Phil Rosenthal 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 tested Cisco CRS-1 (or maybe already upgraded to CRS-3) to Juniper 
MX480 or MX960 on about 10 ms latency link. It was iBGP carrying 
internal routes plus full BGP table (both ways).
I think the bottleneck was CPU on the CRS side and maxing MSS helped a 
lot. I recall doing later on tests Juniper to Juniper and indeed the 
gain was not that big, but it was still visible.

Juniper command 'show system connections' showed MSS around 9kB. I 
haven't checked TCP Window size.

-- 
Grzegorz Janoszka