Floating Static Route

By | September 7, 2013

Floating static routes are static routes that have an administrative distance greater than the administrative distance of dynamic routes. Administrative distances can be configured on a static route so that the static route is less desirable than a dynamic route. In this manner, the static route is not used when the dynamic route is available. However, if the dynamic route is lost, the static route can take over, and traffic can be sent through this alternate route. If this alternate route is provided using a DDR interface, then that interface can be used as a backup mechanism.

Following is the sequence for floating static routes:

  1. The primary interface learns a primary route to a remote network (using a static route or a dynamic routing protocol). The administrative distance of this learned route is less than the floating static, so the learned route is used.
  2. The primary interface becomes inoperable, although line protocol may remain up. Loss of routing updates eventually removes the learned primary route from the routing table.

    Note: When the primary route is a static route, the primary interface line protocol must go down for the floating static route to be used.

  3. The floating static route is used since it is now the route with the lowest administrative distance.

Advantages

  • This is independent of line protocol status. This is an important consideration on Frame Relay circuits, where the line protocol may not go down if the DLCI is inactive.
  • It is encapsulation independent.
  • It can backup multiple interfaces/networks on a router.

Disadvantages

  • This requires a routing protocol.
  • It is dependent upon the routing protocol convergence times. A flapping route can cause the backup interface to be activated unnecessarily.
  • It can typically only provide backup for a single router.
  • It is dependent upon interesting traffic to trigger the DDR backup call. Therefore, even if the router installs the floating static route in the route table, the router does not actually trigger the backup call unless it receives interesting traffic for that backup interface. In most cases, you must mark the routing protocol as uninteresting to prevent the periodic updates/hellos from keeping the backup link up.

Configuring a Floating Static Route

Routers believe static routes over any dynamically learned route.

  1. To change this default behavior and make a static route appear in the routing table only when the primary route goes away, create a floating static route.
  2. The administrative distance of the static route is configured to be higher than the administrative distance of the primary route and it “floats” above the primary route, until the primary route fails.
  3. To configure a static route use the ip route command with the distance parameter.

Floating static route