We have been evaluating new DDoS mitigation partners and are at the stage of trialling one with live traffic to tune / evaluate their detection algorithms. As such we will be announcing all Slough (and anycast) prefixes through them during this window, causing some traffic to reach us this way. This has been selected to be a quiet time on the network. The following should be noted though:
- this only affects people reaching us over IP Transit and to some extent public peering; those who are directly connected and/or privately peer will be unaffected and their traffic will continue to flow directly. Those who are directly on-net in Slough will only be affected in relation to traffic reaching them over IP Transit. 
- traffic traversing this route will see slightly reduced MTU for this test which may cause fragmentation issues for those still sending SIP over UDP rather than TLS/TCP. Those affected by this and unwilling/unable to change transport may prefer to send traffic to Manchester for the duration of this test. Changing transport remains our advice. 
This is an important further step in ensuring we're able to defend the network against an attack larger than the already substantial on-net provision in-place. We appreciate your understanding.
Naturally, we'll be monitoring throughout and will roll back if any issues are observed.