Why does we use NJ0/NJFF for Coordinator to set End device's parent to nearby R in xbee s2c?

There are a C, R, E. C is nearby R not C. E is nearby R not C.
Sometimes X-CTU’s network side shows E is connected R and E’s MP(parent) is C ,even though the RSSI between is about -90 dbm(lowest value)…

E is 10 sec pin sleep device.
I think that E maybe leave the network because of the low RSSI.
But E will rejoin R because R is nearby E.
But it isn’t…

In your answer, you recommend that E connect R after R join C and C set NJ0. Then E join R.
I think we should don’t be need to do it
In that case the E’s MP is C, low RSSI will make E leave the network… And E should rejoin R nearby E automatically…then it’s okay…
Then we don’t have to set NJ0 for C to set E’s parent R.
But I think it doesn’t work for sometime…It’s a big problem…

In the real situation. it’s difficult to for the installation engineers to set NJ0 and NJFF for C …
I did it.several times…But I forgot to set NJFF for C several times after I set E’s parent R. They complains that it is not proper in the real situation.

You set NJ to a 0 value to tell that specific node not to allow Joining. This way you can control who your device joins. The other option is to just leave NJ alone and move the router so that it is in range of the coordinator but the End device is only in range of the router. Then the end device will have to join the router regardless of what NJ is set to on the Coordinator.