There are three main types of static routes that can be configured:
- Directly attached static routes
- Recursive static routes
- Fully specified static routes
Directly Attached Static Routes
Point-to-point serial interfaces do not need to maintain an adjacency table and do not use address resolution protocol, when used in a static route the configuration can directly reference the outbound of the router.
A directly attached static route is configured with this command syntax example:
ip route 192.168.1.0 255.255.255.0 Serial1/0
Configuring a directly attached static route on an interface that utilises ARP is not recommended and may cause problems. One problem being the router will need to repeat the ARP process for every destination that matches the static route. Depending on the size of the prefix, this can cause the number of lookups and entries for ARP to grow very large.
Recursive Static Routes
A Cisco router requires that it knows the interface an outbound packet should use.
A configured recursive static route specifies the IP address rather than an interface as a next-hop destination.
The recursive part of a recursive static route begins when the router recursively looks up the routing information base (RIB) table for the next hop IP address and then cross-references it with the adjacency table.
A recursive static route is configured as:
ip route 192.168.1.0 255.255.255.0 192.168.6.254
Recursive static routes require the next-hop IP address to exist in the routing table to install the static route. If the static route route fails to meet this requirement it will not be inserted into the routing information base (RIB).
A recursive static route also can not use the default route (0.0.0.0/0) as a next-hop destination.
Fully Specified Static Routes
Static route recursion can simplify a topology if a link fails, as it may allow a static route to remain in the routing information base if a different interface is part of the same network when its original interface fails. Problems can arise with this and there may scenarios where you want to force traffic out of a particular interface.
ip route 192.168.1.0 255.255.255.0 Ethernet0/0 192.168.7.254
A configured static route can have both an outbound interface and a next-hop IP address, known as a fully specified static route. If the interface configured as part of the statement is not an up state, the route is removed from the routing information base. Specifying the IP address along with the physical address also removes the possibility of the ARP lookup problems that can be brought by only specifying the interface.