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What you’re likely to need is a Yacht Devices YDNB NMEA2K Bridge and configure it to act as a firewall. It’s going to take some coding to make happen, but you’ll need to set it up so that it blocks the PGNs between the two victron setups.

I find this interesting that the Cerbo GX does not default back to the Quattro that is plugged into it on the VE.Bus. This should be a default setting in the system. I had the same issue again last night and corrected imediately after turning off N2K out on the upper bank. Will the Maretron NBE100 bridge work as well? We have a mainly Maretron system and it would be better for using a single program to run the whole network. I believe what you are saying is on the Lower battery bank we need to block the a series of PGN's that the Upper Cerbo is tx. I still do not understand why the plugged in VE.Bus doesn't have priority over VE.Can PGN's. Do you have and idea what PGN's should be blocked in the Network Bridge? I was originally thinking of putting both units on there own networks to isolate them from the each other and flooding the N2K main with proprietary PGN's. I may still do this I have not decided yet.

Yesterday, I discovered that one of my Cerbo Relay's was set to Generator Start. I corrected this setting and it seems as of last night the issue of the lower Quattro cutting out has stopped. It is still early but I will update this post in a couple of days to see if this resolved the issue. The dual systems ran normally most of the day yesterday and all night last night.