208 BigIron RX Series Configuration Guide53-1002253-01Configuring forwarding parameters7Configuring a default network routeThe device enables you to specify a candidate default route without the need to specify the nexthop gateway. If the IP route table does not contain an explicit default route (for example, 0.0.0.0/0)or propagate an explicit default route through routing protocols, the software can use the defaultnetwork route as a default route instead.When the software uses the default network route, it also uses the default network route's next hopgateway as the gateway of last resort.This feature is especially useful in environments where network topology changes can make thenext hop gateway unreachable. This feature allows the device to perform default routing even if thedefault network route's default gateway changes.The feature thus differs from standard default routes. When you configure a standard defaultroute, you also specify the next hop gateway. If a topology change makes the gateway unreachable,the default route becomes unusable.For example, if you configure 10.10.10.0/24 as a candidate default network route, if the IP routetable does not contain an explicit default route (0.0.0.0/0), the software uses the default networkroute and automatically uses that route's next hop gateway as the default gateway. If a topologychange occurs and as a result the default network route's next hop gateway changes, the softwarecan still use the default network route.If you configure more than one default network route, the device uses the following algorithm toselect one of the routes.1. Use the route with the lowest administrative distance.2. If the administrative distances are equal:• Are the routes from different routing protocols (RIP, OSPF, or BGP4)? If so, use the routewith the lowest IP address.• If the routes are from the same routing protocol, use the route with the best metric. Themeaning of “best” metric depends on the routing protocol:• RIP – The metric is the number of hops (additional routers) to the destination. The bestroute is the route with the fewest hops.• OSPF – The metric is the path cost associated with the route. The path cost does notindicate the number of hops but is instead a numeric value associated with each route.The best route is the route with the lowest path cost.• BGP4 – The metric is the Multi-exit Discriminator (MED) associated with the route. TheMED applies to routes that have multiple paths through the same AS. The best route is theroute with the lowest MED.Configuring a default network routeYou can configure up to four default network routes. To configure a default network route, entercommands such as the following:BigIron RX(config)# ip default-network 209.157.22.0BigIron RX(config)# write memorySyntax: ip default-network The parameter specifies the network address.To verify that the route is in the route table, enter the following command at any level of the CLI.
