We are currently investigating an issue affecting return traffic to a small number of customer IPs, notably those which send traffic sporadically, that are not directly connected to Simwood.
This issue appears to be due to some IPs that are disappearing from the top prefix list, on one router but not on another, which can cause a loop for a few seconds until the lists synchronise. Our network engineers are investigating this as a priority.
There is a helpful presentation discussing how we use Selective Route Download (SRD) at http://blog.simwood.com/2017/09/our-talk-at-linx98-sdn-without-the-bs/
which might better explain how this usually works.
All customers who are directly connected, or connected to us via private peering, are unaffected. Additionally, the vast majority of customers reaching us over the public internet are also unaffected.