faq Unknown protocol CP2 exceptions in the Hazelcast member log files when I run a client

I'm getting "Unknown protocol: CP2" exceptions in the Hazelcast member log files when I run a client.

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Nov 04, 2022 4:46:05 PM com.hazelcast.nio.tcp.TcpIpConnection
WARNING: [10.217.0.79]:5701 [dev] [3.12.3] Connection[id=34, /10.217.0.79:5701->/10.217.0.83:42983, qualifier=null, endpoint=null, alive=false, type=NONE] closed. Reason: Exception in Connection[id=34, /10.217.0.79:5701->/10.217.0.83:42983, qualifier=null, endpoint=null, alive=true, type=NONE], thread=hz._hzInstance_1_dev.IO.thread-in-0
java.lang.IllegalStateException: Unknown protocol: CP2
at com.hazelcast.nio.tcp.UnifiedProtocolDecoder.onRead(UnifiedProtocolDecoder.java:107)
at com.hazelcast.internal.networking.nio.NioInboundPipeline.process(NioInboundPipeline.java:135)
at com.hazelcast.internal.networking.nio.NioThread.processSelectionKey(NioThread.java:369)
at com.hazelcast.internal.networking.nio.NioThread.processSelectionKeys(NioThread.java:354)
at com.hazelcast.internal.networking.nio.NioThread.selectLoop(NioThread.java:280)
at com.hazelcast.internal.networking.nio.NioThread.run(NioThread.java:235)
...

The above exception occurs when you have a client using an incompatible version of Hazelcast. Make sure the client is running with at least the same major version as the members. For example, the exception shown above indicates that the Hazelcast member (server) is running Hazelcast v3.12.3 but the client is running v4.x or v5.x. For this example, change the client to v3.x.