On 2016-07-22 20:20, Phil Rosenthal wrote:
On Jul 22, 2016, at 1:37 PM, Grzegorz Janoszka <Grzegorz@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