This is actually a SmartOS/illumos issue. At some point I'll probably fix it there. Basically, something happens when NAT'ing an extra ip. Sometimes the gateway for that extra ip is dropped and unable to be added. It's definitely a SmartOS or illumos bug because dtrace doesn't pick up that the route is dropped, so it's some kernel trickery for sure.

The workaround is to just remove the interface and add it again. I was thinking a simple cron job could check every minute to see if the route was dropped and remove/add the interface.

It doesn't happen all the time, and as soon as I have some time I'd like to investigate it properly and fix illumos directly.