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---
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layout: docs
page_title: Consul Protocol Compatibility Promise
description: >-
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.
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---
# Protocol Compatibility Promise
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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.
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We promise that every subsequent release of Consul will remain backwards
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compatible with _at least_ one prior version. Concretely: version 0.5 can
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speak to 0.4 (and vice versa) but may not be able to speak to 0.1.
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Backwards compatibility is automatic unless otherwise noted. Consul agents by
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default will speak the latest protocol but can understand earlier ones.
-> **Note:** If speaking an earlier protocol, _new features may not be available_.
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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.
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For more details on the specifics of upgrading, see the [upgrading page](/consul/docs/upgrading).
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## Protocol Compatibility Table
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| 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 |
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-> **Note:** Raft Protocol is versioned separately, but maintains compatibility with at least one prior version. See [here](/consul/docs/upgrading/upgrade-specific#raft-protocol-version-compatibility) for details.
