Doing something on the [ ROUTER ] leads to a state where it appears that DNS / PING response traffic does NOT come back from the exchange
Inspection of traffic through smart switch [ X ] shows no DNS / PING response traffic from the [ MODEM ] to the [ ROUTER ]
Correct. The smart switch was only inserted temporarily to provide a way of capturing traffic before it hit the router's wan port. I wanted to verify the capture performed on the router itself (using tcpdump on the wan port). The captures matched so I've removed the switch from the equation.
Is there any diagnostic interface / facility in the [ MODEM ] which might show if there is inbound traffic on the [ VDSL ] ? Is it possible that the symptom is something happening in the [ MODEM ] - that is there is [ VDSL ] traffic, but it is not being processed properly?
Afraid not. There are plenty of stats on VDSL line status but no indication of traffic. It shows packets transmitted and received for the LAN but not for the WAN connection.
When in this state is ANY inbound traffic being received at the [ ROUTER ] ? TBB_BQM might help here.
I've set up TBB_BQM as suggested by @corringham (sorry I didn't reply to you). When I did the packet capture at the switch between the router and modem there were a few packets coming in but hardly any. They were mainly duplicate ACKs and retransmissions. I guess this does make sense. My router isn't sending out traffic because it can't do DNS resolutions and it thinks the wan has gone down. So external hosts aren't going to continue TCP conversations until they receive ACKs for packets already sent. And, in the absence of those ACKs, they'll retransmit. There are one or two incoming packets that aren't duplicate ACKs or retransmissions but they quickly dry up. In the end, the vast majority of the traffic is outgoing dns requests and pings from my router as it desperately looks for life on the internet before it gives up and restarts the pppoe session.
@MisterW made reference as to where is the VLAN insertion here? If done in the [ ROUTER ] is it possible that the something trigger has modified the VLAN thereby causing the traffic to be subsequently sent over a non-existent logical connection?
The link @MisterW provided shows 2 bridge modes, simple and full. The 166 only provides the simple option. My 166 is set up to insert VLAN tag 101 - that's the default setting and it hasn't changed. The UDM does have the option to insert a VLAN tag but that option isn't set and never has been.
If we're looking for potential issues with the Vigor 166 or trying to eliminate it, I could dig out the old Openreach modem and put it in place of the 166. It's a simple box but it always worked. I wonder where it is?
