The Cisco IP routing table is structured in a classful manner, which means that it uses to the default, classful addresses, to organize the route entries. The source of a routing entry can be a directly connected network, static route, or a route learned dynamically from a routing protocol.
In this chapter, you learned that there are level 1 and level 2 routes. A level 1 route can be either an ultimate route or a parent route. A level 1 ultimate route is a route with a subnet mask equal to, or less than the default classful mask of the network; and either a next hop address or an exit interface. For example, a route learned through RIP with the network address of 192.168.1.0 and a /24 network mask is a level 1 ultimate route. These routes are displayed in the routing table as a single route entry, such as:
R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:25, Serial0/0/0
Another type of level 1 route is a parent route. A level 1 parent route is automatically created when a subnet route is added to the routing table. The subnet route is known as a level 2 child route. The parent route is a header for level 2 child routes. Here is an example of a level 1 parent route and a level 2 child route:
172.16.0.0/24 is subnetted, 1 subnets
R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:07, Serial0/0/0
The subnet mask of the child routes are displayed in the parent route unless VLSM is used. With VLSM, the parent route displays the default classful mask and the subnet mask is included with the individual VLSM route entries.
When a packet is received by the router, it looks for the longest match with one of the routes in the routing table. The longest match is the route with the largest number of left-most bits that match between the destination IP address of the packet and the network address of the route in the routing table. The subnet mask associated with the network address in the routing table defines the minimum number of bits that must match for that route to be a match.
A level 1 or 2 route is an ultimate route when the route also contains a next hop IP address or exit interface
A subnet mask is displayed with the child route and not the parent route when you use VLSM or a classless protocol
The parent route is created when the subnet (level 2 child) route is added to the routing table.
A parent route cannot exist without any child routes
The default routing behavior on cisco routing is classless, this can be modified by using the no ip classless command
