Parents and Child Routes


A level 1 parent route is a network route that does not contain a next-hop IP address or exit interface for any network. A parent route is actually a heading that indicates the presence of level 2 routes, also known as child routes. A level 1 parent route is automatically created any time a subnet is added to the routing table. In other words, a parent route is created whenever a route with a mask greater than the classful mask is entered into the routing table. The subnet is the level 2 child route of the parent route. In this case, the level 1 parent route that was automatically created is:
172.16.0.0/24 is subnetted, 1 subnets
A level 2 route is a route that is a subnet of a classful network address. Like a level 1 route, the source of a level 2 route can be a directly connected network, a static route, or a dynamic routing protocol. In this case, the level 2 route is the actual subnet route that was added to the network when we configured the FastEthernet 0/0 interface:


A level 2 route is a route that is a subnet of a classful network address. Like a level 1 route, the source of a level 2 route can be a directly connected network, a static route, or a dynamic routing protocol. In this case, the level 2 route is the actual subnet route that was added to the network when we configured the FastEthernet 0/0 interface:

C 172.16.3.0 is directly connected, FastEthernet0/0
Note: Remember that the routing table hierarchy in Cisco IOS has a classful routing scheme. A level 1 parent route is the classful network address of the subnet route. This is the case even if a classless routing protocol is the source of the subnet route.
Level 1 Parent Route
This parent route contains the following information:
172.16.0.0 - The classful network address for our subnet. Remember, the Cisco IP routing table is structured in a classful manner.
/24 - The subnet mask for all of the child routes. If the child routes have variable length subnet masks (VLSM), the subnet mask will be excluded from the parent route and included with the individual child routes. This will be shown in a later section.
is subnetted, 1 subnet - This part of the route specifies that this is a parent route and in this case has one child route, that is, 1 subnet.
Level 2 Child Route
he second entry, 172.16.3.0, is the actual route for our directly connected network. This is a level 2 route, also known as a child route, and contains the following information:
C - The route code for directly connected network.
172.16.3.0 - The specific route entry.
is directly connected - Along with the route code of C, this specifies that this is a directly connected network with an administrative distance of 0.
FastEthernet0/0 - The exit interface for forwarding packets that match this specific route entry.
The level 2 child route is the specific route entry for the 172,16.3.0/24 subnet. Notice that the subnet mask is not included with the subnet, the level 2 child route. The subnet mask for this child route (subnet) is the /24 mask included in its parent route, 172.16.0.0.
Level 2 child routes contain the route source and the network address of the route. Level 2 child routes are also considered ultimate routes because they will contain the next-hop IP address and/or exit interface.

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