When you run docker swarm join and pass the manager token, the Docker Engine switches into swarm mode the same as for workers. Manager nodes also participate in the raft consensus. The new nodes should be Reachable, but the existing manager remains the swarm Leader.
Docker recommends three or five manager nodes per cluster to implement high availability. Because swarm mode manager nodes share data using Raft, there must be an odd number of managers. The swarm can continue to function after as long as a quorum of more than half of the manager nodes are available.
For more detail about swarm managers and administering a swarm, see Administer and maintain a swarm of Docker Engines.
To retrieve the join command including the join token for manager nodes, run the following command on a manager node:
$ docker swarm join-token manager
To add a manager to this swarm, run the following command:
docker swarm join \
--token SWMTKN-1-61ztec5kyafptydic6jfc1i33t37flcl4nuipzcusor96k7kby-5vy9t8u35tuqm7vh67lrz9xp6 \
Run the command from the output on the new manager node to join it to the swarm:
$ docker swarm join
This node joined a swarm as a manager.