A grey route is one of three categories of Internet routes defined by the Policy Analysis of Internet Routing (PAIR) project, an initiative dedicated to the development of tools that ISPs (Internet service providers), network operators, and end users can use to troubleshoot Internet routing and policy problems. As defined by PAIR, a grey route is one that has been received by a route server, but is not configured to be proxied in any view. The other two categories are green and red routes: a green route is one that is registered with the IRR, complies with policy, and is being proxied by the route servers, and a red route is one that is registered with the Internet Routing Registry (IRR), is configured to be proxied by the route servers, but is not currently announced in a view.
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The number of route states and corresponding colors may be expanded in the future to define states with greater specificity, as some route states aren't sufficiently well distinguished. A route may be designated as red for a number of reasons that are not clear from knowing its category. For example, a route may not be available for proxying because it has never been received by the route server, or may have flapped (repeatedly become unavailable) and been removed from the view, or may have been supplanted in the view by a route with higher preference, despite meeting criteria for designation as a green route.