The Cisco IP routing table is a hierarchical structure that is used to speed up the lookup process when locating routes and forwarding packets. Within this structure, the hierarchy includes:
- Ultimate route
- Level1 route
- Level1 parent route
- Level2 child routes
This route contains either a next-hop IPv4 address or an exit interface. Directly connected routes dynamically learned routes, and local routes are also ultimate routes. The figure below illustrates the ultimate routes.
Level 1 Route
The route that is equal to a subnet mask or less than the classful mask of the network address. The source of the level 1 route can be a directly connected network, static route, and also a dynamic routing protocol. Therefore, a level 1 route is also the ultimate routes. The type of level 1 route included:
- Network route– A route that has subnet mask equal to the classful network is called network route.
- Default route– This is a static route for the packet whose destination is not known to the router. The address of the default static route is 0.0.0.0/0.
- Supernet route– This route with network address with a maskless than the classful mask.
Level 1 Parent Route
Level 1 parent route is a level 1 network route that is subnetted. A parent route can never be an ultimate route. The figure below illustrates the level 1 parent route highlighted.
The level 1 parent route is the heading for the specific subnets. Each entry of the level 1 parent route displays the classful network address, the number of subnets and the number of different subnet masks.
Level 2 Child Route
This is a subnet of a classful network address. As illustrated in Figure 1, a level 1 parent route is a level 1 network route that is subnetted. A level 1 parent routes contain level 2 child routes, as shown in the Figure below.
There are two levels 2 child route for level 1 parent route 10.0.0.0 and three levels 2 child route for level 1 parent route 172.16.0.0. The source of a level 2 route can be a directly connected network, a static route, or a dynamically learned route. Level 2 child routes are also ultimate routes.