Xfinity modem in bridge mode not working well with OPNsense

[darp12345]

I had a continuous problems with monitoring my WAN gateway when using the Xfinity provided modem when the modem is set in bridge mode. The connection would start fine, work for a few min and then the modem will stop responding. Took me a bit of time to figure out what was wrong.  Seems that the Technicolor modem CGM4331COM that Xfinity supplies stops responding if there isn't an ARP request for the modem's IP within preceding 5 min. This is very weird limitation as the ethernet address of the modem doesn't change so the response of each ARP request gives you back the same ethernet address each time. The problem doesn't seem to happen with Linux and Windows as by default these two OSes have ARP cache entry lifetime of less than a minute. For FreeBSD the default is 20 min. Once I found out the cause of the problem, the fix for the OPNSense firewall was to simply add net.link.ether.inet.max_age=120 in the tunables. As I posted previously - the Motorola MB8611 doesn't have this problem nor has any other modem that I have seen before. A very weird and undocumented behavior that should be fixed.
Perhaps for OPNSense we can set by default net.link.ether.inet.max_age to 1 or 2 minutes so people don't have to deal with these issues.

[jspikeball123]

dude thank you so much, I was pulling my hair out trying to figure what my problem was. This fixed it. As a fellow bridged xfinity modem user, any other tips or settings you have found to improve performance? Appreciated!

[MysteryIron]

I am planning to switch to bridged mode for my nic. This definitely helps.

[slackadelic]

That right there is one of the biggest reasons I ditched their modem and got my own.