Protocol Compatibility Promise

We expect Consul to run in large clusters of long-running agents. Because safely upgrading agents in this sort of environment relies heavily on backwards compatibility, we have a strong commitment to keeping different Consul versions protocol-compatible with each other.

We promise that every subsequent release of Consul will remain backwards compatible with at least one prior version. Concretely: version 0.5 can speak to 0.4 (and vice versa) but may not be able to speak to 0.1.

Backwards compatibility is automatic unless otherwise noted. Consul agents by default will speak the latest protocol but can understand earlier ones.

Note: If speaking an earlier protocol, new features may not be available.

The ability for an agent to speak an earlier protocol is to ensure that any agent can be upgraded without cluster disruption. Consul agents can be updated one at a time, one version at a time.

For more details on the specifics of upgrading, see the upgrading page.

Protocol Compatibility Table

Consul Version	Protocol Compatibility
0.1 - 0.3	1
0.4	1, 2
0.5	1, 2. 0.5.X servers cannot be mixed with older servers.
0.6	1, 2, 3
>= 0.7	2, 3. Will automatically use protocol > 2 when speaking to compatible agents

Note: Raft Protocol is versioned separately, but maintains compatibility with at least one prior version. See here for details.