An agent can run in both client and server mode. Server nodes are responsible for running the consensus protocol and storing the cluster state. Before a Consul cluster can begin to service requests, a server node must be elected leader. Thus, the first nodes that are started are generally the server nodes. Bootstrapping is the process of joining these server nodes into a cluster.
To trigger leader election, we must join these machines together and create a cluster. There are two options for joining; you can use [Atlas by HashiCorp](https://atlas.hashicorp.com?utm_source=oss&utm_medium=guide-bootstrapping&utm_campaign=consul) to auto-join or you can access the machine and manually run the join.
For auto-join using Atlas, you must set three configurations — the name of your Atlas infrastructure with the [`-atlas` flag](/docs/agent/options.html#_atlas), your Atlas token with the [`-atlas-token` CLI flag](/docs/agent/options.html#_atlas_token), and the [`-atlas-join` flag](/docs/agent/options.html#_atlas_join). Below is an example configuration:
```text
$ consul agent -server -data-dir="/tmp/consul" -bootstrap-expect 3 \
-atlas=ATLAS_USERNAME/infrastructure \
-atlas-join \
-atlas-token="YOUR_ATLAS_TOKEN"
```
To get an Atlas username and token, [create an account here](https://atlas.hashicorp.com/account/new?utm_source=oss&utm_medium=guide-bootstrapping&utm_campaign=consul) and replace the respective values in your Consul configuration with your credentials.
To manually create a cluster, access one of the machines and run the following:
Since a join operation is symmetric, it does not matter which node initiates it. Once the join is successful, one of the nodes will output something like:
As a sanity check, the [`consul info`](/docs/commands/info.html) command
is a useful tool. It can be used to verify `raft.num_peers` is now 2,
and you can view the latest log index under `raft.last_log_index`. When
running [`consul info`](/docs/commands/info.html) on the followers, you
should see `raft.last_log_index` converge to the same value once the
leader begins replication. That value represents the last log entry that
has been stored on disk.
Now that the servers are all started and replicating to each other, all
the remaining clients can be joined. Clients are much easier as they can
join against any existing node. All nodes participate in a gossip
protocol to perform basic discovery, so once joined to any member of the
cluster, new clients will automatically find the servers and register
themselves.
-> **Note:** It is not strictly necessary to start the server nodes before the clients; however, most operations will fail until the servers are available.
