Always-on PPPoE but 30 seconds disconnection

So I have a FTTH internet connection and my ISP uses PPPoE – a rather common configuration. But the ISP resets my connection every 12 hours, possibly for accounting purposes.

Now whenever the connection reset was taking place my internet used to vanish for 30 seconds – a very bad situation to be in when you’re in an online meeting, or making a payment. I couldn’t quite figure out why this was happening only on connection resets (terminations initiated by the ISP), when I terminate the connection from my end and reconnect, it was connecting without any lag.

I don’t use a router available off the shelf – it’s a Celeron mini PC with four Gigabit NICs running Ubuntu and I have hand configured all the network related services (firewall, dns, dhcp) on it – including pppd for my PPPoE connection.

Since this is an unlimited connection it doesn’t matter how many hours I am connected or how much data I’ve used, I pay a fixed fee to my ISP. So my pppd was configured with the persist and maxfail 0 options (basically keep trying to connect endlessly if there was any failure). The pppd manual page also lists SIGHUP as a signal which causes it to terminate the current session and restart it if it is configured with the persist option.

Since my issue was only with connection resets I tried to simulate one, using SIGHUP. And there I found the 30 second delay in connecting. After reading the pppd manpage once again I found an option called holdoff which tells pppd to wait for such time before trying to reconnect and the default value of holdoff isn’t documented in example configuration files nor in pppd manpage as of Ubuntu 18.04 which runs on my router. The default value of holdoff is apparently 30 seconds as mentioned in this TLDP guide.

I repeated the same test after setting holdoff 0 and it works fine now – without the 30 second delay.