Router behaviors when join a thread network

[Linxi]

Hello Nordic：

I wanna to know what actions a thread have done from reset to joined a thread network.

I observed the air packets by using Wireshark. I found a router(rx_on_when_idle) send Link Request/Parent Request/Child ID Request. It makes me confused. Why does a router need to do Parent Request/Child ID Request actions. Is there a guideline to detail the router(with known mesh parameters) behaviors when it starts up and then join an existed thread network.

[Edvin]

Hello,

I suggest you read about the thread roles here.

I don't know exactly what you saw on Wireshark, but Thread nodes will always join the network as a child. If suitable, then it will be promoted to a router, depending on how many other routers that are present in the network, and some other factors. Particularly, if a new node wants to join, and the only node within range is a child, it will promote itself to a router in order to be able to attach the new node to the network. 

The Thread network will always try to keep a number of routers so that there is room for more routers, in case of the scenario above (a child only in reach of another child wants to join). Keep in mind that there is a maximum number of routers in a thread network, which is why every REED (Router Eligible End Device) is not automatically promoted to routers. 

Best regards,

Edvin