How do robots know to return to their primary hub?

Document ID : KB000034303
Last Modified Date : 14/02/2018
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Introduction: 

Robots sometimes failover to another hub due to environment or network issues. Once the condition is corrected and the original hub comes back online, how is the robot made aware that it can move back?

Question: 

How do robots know when to return to their primary hub?
 

Environment:

General information applicable to most UIM versions
 

Answer:

When a hub comes up it will send broadcasts (real UDP) on port 48000 and 48002. Robots that have been previously registered with a primary hub, and receiving this broadcast from it's primary hub should then move back to this hub.
 
In many environments broadcasts are not allowed - the hub probe has a back-up functionality to account for this:
It has an internal list of all the robots which have been connected to itself, and it will start querying for these robots once it is back online. The hub will then send a 'checkin' command to all the robots in this list, and these robots will then connect to its primary hub again (this of course requires that the robot was registered to that hub prior to the hub going down).
 
If the spooler is not able to send alarms (or QoS messages) to its primary hub, it will switch over to the secondary immediately. There is no setting to delay this.

If the spooler has nothing to send, there may be a long delay until it switches over.

When the primary is back up it will send a broadcast to all robots. All robots which receive this broadcast will switch back immediately.
 
Then the hub starts to poll the rest of the robots one by one. This may take some time.