The primary reason the parent is FFFF in the zbjoin message is if the device is a router and not an end device. End devices have a parent on the network that does all the routing for them but routers do not. There are a couple of other situations where the FFFF will show up including a minor bug I found recently that only affects populating that parent field but doesn’t have any other downstream effects. In general the parent field is helpful but should not be treated as 100% accurate because the parent can change without a zbjoin event. Improving the accuracy of this information is something we are working on.