Category Archives: Route

Building the Routing Table

Building the Routing Table
The main considerations while building the routing table are:

Administrative distance – This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance. In other words, it is the believability of the source of the route.

Metrics – This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.

Prefix length

As each routing process receives updates and other information, it chooses the best path to any given destination and attempts to install this path into the routing table. For instance, if EIGRP learns of a path toward, and decides this particular path is the best EIGRP path to this destination, it tries to install the path it has learned into the routing table.

The router decides whether or not to install the routes presented by the routing processes based on the administrative distance of the route in question. If this path has the lowest administrative distance to this destination (when compared to the other routes in the table), it’s installed in the routing table. If this route isn’t the route with the best administrative distance, then the route is rejected.

To understand this better, let’s look at an example. Assume a router has four routing processes running: EIGRP, OSPF, RIP, and IGRP. Now, all four of these processes have learned of various routes to the network, and each has chosen its best path to that network through its internal metrics and processes.

Each of these four processes attempts to install their route toward into the routing table. The routing processes are each assigned an administrative distance, which is used to decide which route to install.

Ultimate route, Level 1 route, Level 1 parent & child route

The routing table hierarchy in Cisco IOS was originally implemented with the classful routing scheme. Although the routing table incorporates both classful and classless addressing, the overall structure is still built around this classful scheme.

Routes are discussed in terms of:
Ultimate route
Level 1 route
Level 1 parent route
highlights the level 1 parent routes in the routing table of R1. The routing table basically provides a heading for the specific subnets it contains. Each entry displays the classful network address, the number of subnets, and the number of different subnet masks that the classful address has been subdivided into.

Level 2 child routes
A level 2 child route is a route that is a subnet of a classful network address. As illustrated in Figure 3-61, a level 1 parent route is a level 1 network route that is subnetted.

A level 1 parent route contains level 2 child routes, as shown in Figure 3-62.

Like a level 1 route, 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.


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.