nodes disappearing from RPL border router route list

I am running an RPL border router (https://github.com/contiki-os/contiki/tree/master/examples/ipv6/rpl-border-router) on a microcontroller connected to a Linux host, and 5 nodes running the Erbium coap server (https://github.com/contiki-os/contiki/blob/master/examples/er-rest-example/er-example-server.c). I observed, sometimes nodes disappear from route list, is it normal? Nodes are in close proximity, not very far from the RPL border router. Please let me know if any…

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