Routers must form a Connected Dominating Set (CDS), which means:
A distributed algorithm maintains the CDS, which ensures a minimum level of redundancy. Every device initially attaches to the network as an End Device (Child). As the state of the Thread network changes, the algorithm adds or removes Routers to maintain the CDS.
Thread adds Routers to:
Thread removes Routers to:
After attaching to a Thread network, the Child device may elect to become a Router. Before initiating the MLE Link Request process, the Child sends an Address Solicit message to the Leader, asking for a Router ID. If the Leader accepts, it responds with a Router ID and the Child upgrades itself to a Router.
The MLE Link Request process is then used to establish bi-directional Router-Router links with neighboring Routers.
A Link Request is a request from the Router to all other Routers in the Thread network. When first becoming a Router, the device sends a multicast Link Request to ff02::2
. Later, after discovering the other Routers via MLE Advertisements, the devices send unicast Link Requests.
A Link Accept and Request is a combination of the Link Accept and Link Request messages. Thread uses this optimization in the MLE Link Request process to reduce the number of messages from four to three.
A Link Accept is a unicast response to a Link Request from a neighboring Router that provides information about itself and accepts the link to the neighboring Router.
When a Router downgrades to a REED, its Router-Router links are disconnected, and the device initiates the MLE Attach process to establish a Child-Parent link.
See Join an existing network for more information on the MLE Attach process.
In some scenarios, it may be necessary to establish a one-way receive link.
After a Router reset, neighboring Routers may still have a valid receive link with the reset Router. In this case, the reset Router sends a Link Request message to re-establish the Router-Router link.
An End Device may also wish to establish a receive link with neighboring non-Parent Routers to improve multicast reliability. We'll learn more about this when we get to Multicast Routing.
What you've learned: