Configuring a Default Network Route

A Layer 3 device enables you to specify a candidate default route without the need to specify the next hop 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 default network route as a default route instead.

When the software uses the default network route, it also uses the default network route's next hop gateway as the gateway of last resort. This feature is useful in environments where network topology changes can make the next hop gateway unreachable. This feature allows a Layer 3 device to perform default routing even if the default network route's default gateway changes.

The default network route differs from standard default routes. When you configure a standard default route, 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 route table does not contain an explicit default route (0.0.0.0/0), the software uses the default network route and automatically uses that route's next hop gateway as the default gateway. If a topology change occurs and as a result the default network route's next hop gateway changes, the software can still use the default network route. To configure a default network route, use the following CLI method.

If you configure more than one default network route, the Layer 3 switch uses the following algorithm to select one of the routes.

  1. Use the route with the lowest administrative distance.
  2. If the administrative distances are equal, determine if the routes are from different routing protocols (RIP, OSPF, or BGP). If so, use the route with the lowest IP address.
  3. If the routes are from the same routing protocol, use the route with the best metric. The meaning of "best" metric depends on the routing protocol. Use one of the following choices.
    • RIP: The metric is the number of hops (additional routers) to the destination. The best route is the route with the fewest hops.
    • OSPF: The metric is the path cost associated with the route. The path cost does not indicate 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.
    • BGP: The metric is the Multi-exit Discriminator (MED) associated with the route. The MED applies to routes that have multiple paths through the same Autonomous System. The best route is the route with the lowest MED.
  4. Enter global configuration mode. 
    device# configure terminal
  5. Configure a default network route. 
    device(config)# ip default-network 10.157.22.0
    You can configure up to 4 default network routes.
  6. To verify that the route is in the route table, enter the following command at any level of the CLI. 
    device> show ip route

Total number of IP routes: 2
Start index: 1  B:BGP D:Connected  R:RIP  S:Static  O:OSPF *:Candidate default
    Destination      NetMask           Gateway           Port   Cost   Type
1 		10.157.20.0      255.255.255.0     0.0.0.0           lb1      1      D
2 		10.157.22.0      255.255.255.0     0.0.0.0           1/4/11   1      *D

    This example shows two routes. Both of the routes are directly attached, as indicated in the Type column. However, one of the routes is shown as type "*D", with an asterisk (*). The asterisk indicates that this route is a candidate for the default network route.

Parent topic: IP Addressing