My TP-Link Archer C7 was disconnecting client hosts from the internet periodically every five minutes exactly, for eight seconds each time. This was easy to see by pinging a remote host - eight lost pings would show up reliably every 5 minutes. Connections to the router or within the local network (wifi or ethernet) would work fine, but any packets going between WAN and LAN would be interrupted during the outages.

I checked that the upstream internet was not the cause by connecting a laptop to it directly. Steady as a heartbeat.

I looked in the router logs (System Tools > System Log) and noticed that the timeouts coincided exactly with the time that the router renews it's WAN side IP address via the DHCP client. I also noticed a strange set of entries which would appear always at the end of the 8 second disconnect period.

At the beginning of each 8 second outage:

DHCP INFO DHCPC Send REQUEST to server <server_ip> request ip <ip_addr>
DHCP INFO DHCPC Recv ACK from server <server_ip> with ip <ip_addr> lease time 600
DHCP INFO DHCPC:GET ip:<ip_addr> mask:fffffc00 gateway:<gateway_ip> dns1:<dns_ip> dns2:<dns_ip> static route:0
DHCP NOTICE Dynamic IP (DHCP Client) obtained IP successfully

At the end of each 8 second outage:

NULL NULL anonymous_enable=NO write_enable=YES local_enable=YES no_anon_password=YES anon_upload_enable=YES anon_mkdir_write_enable=YES user_config_dir=%s connect_from_port_20=YES asci

That last message is a bit odd. Some googling leads me to think that this set of configuration parameters is associated with the open source FTP server vsftpd. Looking at the Archer's GPL code release for this router I see that vsftpd is indeed included. How is it configured and for what purpose? I don't know.

I messaged TP-Link tech support for advice. They suggested a doing factory reset – press the reset button for 10 seconds. That indeed eliminated the problem, as well as the mysterious vsftpd config message. I'm not sure what the underlying cause of the problem was or what the explanation for the vsftpd message is. I'm putting this here in case anyone else has the same issue and can benefit from any of this diagnostic work. Or conversely if anyone has insight as to the underlying cause.