The show IP route command on the Routers

Routers use the routing table entries to make forwarding decisions. The show ip route command displays the routing table's entries. A routing table entry contains various information. Understanding this information helps you know which route the router uses to forward data packets to a destination.

When a router receives a data packet, it compares the packet's destination address with all routes available in the routing table. It selects the route that matches the destination address to forward the packet. If it does not find a matching route, it discards the packet. If it finds multiple matches, it picks the route with the maximum matching network bits.

You can use the show ip route command in Privilege mode to list all available routes in the routing table. The following image shows an output of this command.

The output organizes information into three sections. These are codes, default routes, and routes.

Codes

Each entry in the routing table starts with a letter. The letter shows the route's source. The codes section describes the codes the routing table uses to indicate the source of routes. For example, the letter C shows the directly connected routes in the routing table.

Default Routes

The second section shows the default routes. Routers use default routes to forward a packet when no other routes for the packet's destination address are available.

Routes

The third section shows all routes the router can take to forward the data packets. If two or more routes to the same destination are available, the router compares them based on many parameters and selects the best route. It keeps the best route in the routing table and the remaining routes in their protocol-specific tables.

Let us take an example. A router learns two routes to the same destination. It knows the first route from the RIP protocol and the second route from the EIGRP protocol. It compares both and finds that the second route is better than the first route. It adds the second route to the routing table and keeps the first route in the RIP database table. It uses the routing table to make forwarding decisions. It uses protocol-specific tables for backup purposes. For instance, in the preceding example, if the second route fails, the router automatically adds the first route to the routing table from the RIP database table.

Understanding route entries in the routing table

A route entry contains the following information.

Source

It is the first field of a route entry. It contains a letter that provides information about the route's source. The code section describes the meaning of all the letters this field can have.

Destination

It is the second field. It contains the destination network or host address. The router compares the packet's destination address with the address stored in this field.

AD and Metric value

These are written with the destination field in a square bracket.

The AD (Administrative distance) value is the trustworthiness of the source. Routers use a scale of number range 0 - 255. On this scale, 0 is the most trustworthy source, and 255 is the least reliable source.

Routing protocols assign a metric value to each route. It helps them to identify the best route when multiple routes to the same destination are available.

Next hop IP address

This field saves the IP address of the next hop. If the router selects this route to forward the packet, it forwards it to the IP address specified in this field.

Elapsed time

This field stores the elapsed time since this route was added to the routing table.

Exit interface

It is the last field of a routing entry. It saves the interface name associated with the next hop. If the router selects this route to forward the packet, it forwards it from the interface specified in this field.

Conclusion

Understanding the routing table and its entries is crucial for effectively managing router-based networks. The show ip route command displays routing table entries. A routing table stores information in several fields. This tutorial explained these fields and the data they store.