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Old Salty, that picture above is the vent, it's susposed to draw air in to allow the drain to work properly, then seal shut.
In you cap, or plug it, On such a short run, the worst that happen if you cap the pipe is the drain will 'burp', I think you can get by without one in a pinch, but it's best to have it. If it fails, just replace it with another... or have someone do it for you, a simple repair.
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