Route Selection Process Demistified

A router can learn the routes to remote networks and subnets using manual instructions or by means of configuring routing protocols. This way routers learn how to reach destination networks/subnets dynamically. This post's primary focus is about Interior Gateway Protocols (IGPs) such as: RIP, OSPF, EIGRP. The only Exterior Gateway Protocol (EGP) we use nowadays is called BGP. 

First things first. There are three terms I need to define in order to explain the process of selecting the best route. These are:

1. The Longest Match Rule
2. Administrative Distance
3. Metric

Router , Router-function , Traffic Flow , Routing Table Components

What is a router?
A router is a specialized computer which can connect multiple networks to allow exchange of packets between them. Since a router uses IP header information (layer 3 protocol data unit), to transmit the packets between networks, this ability makes it a layer 3 device. Like switches, routers build a special database which serves as the source of information on what to do with incoming packets. This database is formally called Routing Information Base(RIB). But most often people call it a routing table. How a router constructs a routing table and maintains information in it, will be the topics of quite a few upcoming posts.

Address Resolution Protocol (ARP)

OVERVIEW

The Address Resolution Protocol (ARP) was developed to enable communications on an internetwork and is defined by RFC 826. Layer 3 devices need ARP to map IP network addresses to MAC hardware addresses so that IP packets can be sent across networks. Before a device sends a datagram to another device, it looks in its ARP cache to see if there is a MAC address and corresponding IP address for the destination device. If there is no entry, the source device sends a broadcast message to every device on the network. Each device compares the IP address to its own. Only the device with the matching IP address replies to the sending device with a packet containing the MAC address for the device (except in the case of "proxy ARP"). The source device adds the destination device MAC address to its ARP table for future reference, creates a data-link header and trailer that encapsulates the packet, and proceeds to transfer the data. The figure below illustrates the ARP broadcast and response process.