Mechanism for Border Router to upgrade to Router even if max router num(32) reached

[fuxiaoming-lumi]

Regarding the vision of matter multi admin and share thread network. I want to know if there is a plan to implement this mechanism, because if a BR(Matter Controller) from an ecosystem happens to become a child role, its performance will be greatly limited, and if the performance of the parent node is poor.

[abtink]

@fuxiaoming-lumi , an existing mechanism, which was implemented in PR [netdata] add feature for BR to request router role upgrade #7597 and is also defined in the Thread specification, allows a BR to request to become a router. You can read the PR commit message for more details or Thread spec.

Note that this is applicable when the absolute maximum router limit has not yet been reached. The overall role promotion process aims to keep the number of routers at around half of the maximum, which is typically 16. If we are at the maximum limit of 32, there is no easy way for any new device to become a router; however, this situation should not be sustained, as existing routers should try to go back to being a REED.

Another aspect to consider is a relatively new mechanism for periodic parent searches ([mle] implement periodic parent search mechanism on FED/REED ), where an FTD child periodically searches to find and switch to a potentially better parent (one with a better link). This was available for MTD devices earlier, and the more recent change added this support for FTD children. So, if a BR stays as a child, it can use this mechanism to select a better parent.

[fuxiaoming-lumi]

@abtink Thank you. Our BR enable OPENTHREAD_CONFIG_BORDER_ROUTER_REQUEST_ROUTER_ROLE but still as REED. Can we manually trigger this upgrade through the application layer？

[fuxiaoming-lumi]

@jwhui @abtink In scenarios where multiple Vendor Border Routers may exist in the same home network (e.g., Nest Hub, Apple HomePod, Aqara Hub, etc.), we believe this parameter may no longer fully meet the requirements. We would like to clarify whether this parameter is intended to be fixed, or if it is allowed to be adjusted by vendors to accommodate such multi-vendor environments.
https://github.com/openthread/openthread/blob/main/src/core/thread/mle.hpp#L1324

[jwhui]

Thread Specification defines this behavior. We would need to propose a change to the Thread Specification via the Thread Group to change this behavior.

That said, Thread Border Router functionality is supposed to be vendor agnostic. As a result, while it is good to have multiple border routers providing external connectivity, it should not matter which ones.

Can you provide an argument for increasing the threshold for when a border router can request to upgrade from a REED to Router?

[fuxiaoming-lumi]

Most Thread Border Routers (TBRs) also function as Matter Controllers/Hubs. When multiple TBRs share a Thread network, we've observed a significant increase in control latency when the Matter Controller/Hub switches from a Router to a REED.

[jwhui]

As this requires a change to the Thread Specification, can you submit a enhancement request within the Thread Group?