static route

networking

A route entered by hand rather than learned from a protocol: absolute, silent, and exactly as correct as the person who typed it.

Scales beautifully to about a dozen; beyond that, dynamic routing exists for a reason.

RFC 1812 says a router SHOULD let an operator define a static route to a destination given as a network prefix, and RFC 1122 lets hosts keep a table of static routes as an optional extra, each possibly flagged to say whether ICMP Redirects may override it. RFC 1122 also names the cost: static routing information becomes a source of failure as configurations change or equipment fails.

A static route's next hop must be reachable first, normally through a connected prefix, and FRRouting will not resolve a next hop through the default route unless told to. On Linux a route added by an administrator carries the protocol tag static, which routing daemons respect and may even advertise; FRRouting shows static routes with the code S and distance 1.

Because nothing tells a static route that its next hop has died, RFC 5882 says a system that knows the neighbour runs BFD should take action, such as withdrawing the route, when the session is not up. FRRouting implements it with a bfd keyword on ip route, installing the route only while the session is up. Static routes to Null0 or blackhole are also how a summary gets its discard route.

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