I'm partially posting this for my own future reference and for anyone else having this problem. I'll also be re-posting this on the Ubiquity Unifi Networks Community Forums and forwarding my findings on to Draytek.
Over the weekend, I replaced my Plusnet Hub One (rebranded BT Homehub 5) and Cisco router with a Unifi USG and Draytek Vigor 130.
Prior to this, my line was stable, syncing at ~65Mb down and 19.99 up. Since installing the Draytek, I've been finding the connection drops out regularly, with the line failing, retraining and connecting once more. This isn't an uncommon problem it would seem, with lots of people having similar issues. Lots gave up and switched back to other hardware.
https://community.plus.net/t5/Fibre-Broadband/Draytek-vigor-130-Can-t-hold-a-connection-for-long/td-...
https://community.plus.net/t5/Broadband/Draytek-Vigor-130-modem-Connection-Drops/td-p/1348428
https://community.plus.net/t5/Fibre-Broadband/Config-for-Ubiquiti-ER-X-EdgeRouter-X-on-Plusnet-FTTC/...
https://community.ubnt.com/t5/UniFi-Routing-Switching/Vigor-130-amp-USG-WAN-drops/td-p/2255159
https://community.ubnt.com/t5/UniFi-Routing-Switching/WAN-drop-outs-USG-3P-Draytek-Vigor-130-Sky-Fib...
https://community.ubnt.com/t5/UniFi-Routing-Switching/VDSL-dropping-USG/td-p/1847005
I noticed that certain activity and events triggered the failure, particularly when I started my SSH tunnel to RDP to my server. This event would trigger the line drop within about 10-20 seconds of establishing the connection. Often the line wouldn't come back up until the modem was rebooted.
I realised I hadn't yet configured one thing I did have on the old setup - baby jumbo frames. For those who don't know, this is where the modem supports frames with an MTU of 1508, less the PPPoE portion 8bytes, less the header 28 bytes, you should be able to ping a server, e.g. www.yahoo.com with a packet of 1472.
C:\Users\michael>ping www.yahoo.com -f -l 1472
Pinging atsv2-fp.wg1.b.yahoo.com [87.248.98.7] with 1472 bytes of data:
Reply from 87.248.98.7: bytes=1472 time=34ms TTL=55
Reply from 87.248.98.7: bytes=1472 time=27ms TTL=55
Reply from 87.248.98.7: bytes=1472 time=27ms TTL=55
Reply from 87.248.98.7: bytes=1472 time=27ms TTL=55
Ping statistics for 87.248.98.7:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 27ms, Maximum = 34ms, Average = 28ms
After following the instructions in this post, I pasted the json code to my controller's /usr/lib/unifi/data/sites/default/config.gateway.json, provisioned and restarted the USG. The restart was key.
https://community.ubnt.com/t5/UniFi-Routing-Switching/config-gateway-json-review-USG-1508-pppoe-MTU-...
Following this, my thoroughput has stabilised and the issues with the line dropping out have gone away. At a guess, I would say something in the modem's firmware doesn't like having to fragment the packets and was causing the drop out. Certainly it was not a line issue.
Speed tests before and after:
20-06-2018 19:24:24 (GMT) 53.8 Mbps 18.1 Mbps Plusnet
20-06-2018 17:46:11 (GMT) 48.7 Mbps 18.4 Mbps Plusnet
Hope this helps someone. If I need to add anything else, I'll edit this.
