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Router configuration rules may contain static routes. A static route minimally has a destination address, a prefix length or subnet mask, and a definition where to send packets for the route. That definition can refer to a local interface on the router, or a next-hop address that could be on the far end of a subnet to which the router is connected. The next-hop address could also be on a subnet that is directly connected, and, before the router can determine if the static route is usable, it must do a recursive lookup of the next hop address in the local routing table. If the next-hop address is reachable, the static route is usable, but if the next-hop is unreachable, the route is ignored.
Static routes also may have preference factors used to select the best static route to the same destination. One application is called a floating static route, where the static route is less preferred than a route from any routing protocol. The static route, which might use a dialup link or other slow medium, activates only when the dynamic routing protocol(s) cannot provide a route to the destination.
Static routes that are more preferred than any dynamic route also can be very useful, especially when using traffic engineering principles to make certain traffic go over a specific path with an engineered quality of service.
Routers use three main methods to add routes to their routing tables: connected routes, static routes, and dynamic routing protocols. Routers always add connected routes when interfaces have IP addresses configured and the interfaces are up and working.
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