Routes that are permanent fixtures in the routing and forwarding tables are often configured as static routes. These routes generally do not change, and often include only one or very few paths to the destination.
To create a static route in the routing table, you must, at minimum, define the route as static and associate a next-hop address with it. The static route in the routing table is inserted into the forwarding table when the next-hop address is reachable. All traffic destined for the static route is transmitted to the next-hop address for transit.
This overview contains the following topics:
A static route destination address can have multiple next hops associated with it. In this case, multiple routes are inserted into the routing table, and route selection must occur. Because the primary criterion for route selection is the route preference, you can control the routes that are used as the primary route for a particular destination by setting the route preference associated with a particular next hop. The routes with a higher preference are always used to route traffic. When you do not set a preferred route, traffic is alternated between routes in round-robin fashion.
In general, the default properties assigned to a static route apply to all the next-hop addresses configured for the static route. If, however, you want to configure two possible next-hop addresses for a particular route and have them treated differently, you can define one as a qualified next hop.
Qualified next hops allow you to associate one or more properties with a particular next-hop address. You can set an overall preference for a particular static route and then specify a different preference for the qualified next hop. For example, suppose two next-hop addresses (10.10.10.10 and 10.10.10.7) are associated with the static route 192.168.47.5/32. A general preference is assigned to the entire static route, and then a different preference is assigned to only the qualified next-hop address 10.10.10.7. For example:
- route 192.168.47.5/32 {
- next-hop 10.10.10.10;
-
- qualified-next-hop 10.10.10.7 {
- preference 2;
- }
- preference 6;
- }
In this example, the qualified next hop 10.10.10.7 is assigned the preference 2, and the next-hop 10.10.10.10 is assigned the preference 6.
You can control the importation of static routes into the routing and forwarding tables in a number of ways. Primary ways include assigning one or more of the following attributes to the route:
By default, static routes are not retained in the forwarding table when the routing process shuts down. When the routing process starts up again, any routes configured as static routes must be added to the forwarding table again. To avoid this latency, routes can be flagged as retain, so that they are kept in the forwarding table even after the routing process shuts down. Retention ensures that the routes are always in the forwarding table, even immediately after a system reboot.
Static routes are eligible for readvertisement by other routing protocols by default. In a stub area where you might not want to readvertise these static routes under any circumstances, you can flag the static routes as no-readvertise.
Generally, only active routes are included in the routing and forwarding tables. If a static route's next-hop address is unreachable, the route is marked passive, and it is not included in the routing or forwarding tables. To force a route to be included in the routing tables regardless of next-hop reachability, you can flag the route as passive. If a route is flagged passive and its next-hop address is unreachable, the route is included in the routing table and all traffic destined for the route is rejected.
The basic configuration of static routes defines properties for a particular route. To define a set of properties to be used as defaults on all static routes, set those properties as default values. For example:
- defaults {
- retain;
- no-readvertise;
- passive;
- }
- route 0.0.0.0/0 next-hop 192.168.1.1;
- route 192.168.47.5/32 {
- next-hop 10.10.10.10;
-
- qualified-next-hop 10.10.10.7 {
- preference 6;
- }
- preference 2;
- }
In this example, the retain, no-readvertise, and passive attributes are set as defaults for all static routes. If any local setting for a particular route conflicts with the default values, the local setting supersedes the default.
