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CS2D General Joining Red ServersJoining Red Servers
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Or one of the million other threads regarding that topic?
The red marked servers are hosted on another version of the game, making it unable to join. The only possibility would be to update/downgrade to the specific version the server is running on. If your game is up to date, in 99,9% of the time it will definetly be an older version.
@ Bowlinghead: I couldn't find anything related to mismatched version or servers that are shown in red inside the FAQ. It isn't stated there why they're red.
The search function probably should've done the trick but it's always easier to just ask questions. At least it's not done frequently, so there's gonna be an answer for that in the future ready to be found.
Here's an older answer to the question
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