Basics of IP Routing

IP Routing is a process that forward packets from one network to another network. A device called router is used to perform the IP routing functions. Whenever a router receives a packet, it checks the source and destination addresses mentioned in the packet. The router matches the packet with its routing database and decides whether the packet needs to be forwarded or dropped. If the destination address matches in the routing database, the router forwards it to gateway address for the appropriate network. If the destination network address does match in the routing table, the router discards the packet and sends the Destination host unreachable message to the source device. However, one can set a gateway of last resort to forward packets for the destinations that are not listed in the routing table. This is done by setting the default route.

What is Routing Table?

A Routing table contains information needed for a router to forward packets to a next hop relay device. It contains the destination network, destination prefix, and the gateway address for the remote networks. The following figure shows a sample of a routing table.Sample of IP Routing Table

In the preceding figure, the first highlighted box contains the various codes used to describe the routing methods. The second highlighted box displays a sample of a route entry.

Let’s have a look at the following example of a route entry.

S   [1/0]   via

The following table explains the different parts of the routing table entries shown in the preceding example.Syntax of IP routing table

What is Gateway of Last Resort?

In the routing table, there is an entry called Gateway of last resort. It is used to send packets to a next hop relay device if the destination address is not found in the routing table. In the following figure, you can see that the Gateway of last resort is set as If a router receives a packet going to a remote network that is not listed in its routing table, the router will forward that packet to gateway address.what is gateway of last resort in IP routing

What is Administrative Distance?

Before configuring a routing, you need to know what the Administrative Distances (AD) value is. An AD value is used to rate the trustworthiness of routing information received on a router from a neighbor router. It is an integer value from 0 to 255. The lower value means higher trustworthiness. 0 means the most trusted, and 255 means the worst trusted and no traffic will be forwarded via this route.

The following table lists the Administrative Distance (AD) values of different routing algorithms.

If there is more than one IP routing algorithms have been configured for a single destination network, the IP routing algorithm having the lower AD value will be preferred. For example, an administrator has configured EIGRP and RIP routing protocols for /8 destination network. The EIGRP routing protocol will be preferred over the RIP routing protocol because EIGRP has lower AD value (90) than the AD value (120) of the RIP routing protocol.

What is Route Convergence?

In the IP routing, Route Convergence is the time taken by routers participating in a routing process to agree on the network topology, after a change occurs in the network. Different routing protocols use different methods and timers to converge a network.

In this post, we have discussed the basics of IP routing and the basic components used with the IP routing. If you have any queries, please mention in the comment box, we will try to solve your queries.

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