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Types of routing procols

There are two types of routing protocols:

1. Distance vector (RIP, IGRP)
2. Link state (OSPF, IS-IS)

Cisco has created its own routing protocol – EIGRP. EIGRP is considered to be an advanced distance vector protocol, although some materials erroneously state that EIGRP is a hybrid routing protocol, a combination of distance vector and link state.

Administrative distance

A network can use more than one routing protocol, and routers on the network can learn about a route from multiple sources. Routers need to find a way to select a better path. Administrative distance number is used by routers to find out which route is better (lower number is better). For example, if the same route is learned from RIP and EIGRP, a Cisco router chooses EIGRP route and stores the route in the routing table. This is because EIGRP routes have, by default, an administrative distance of 90, while RIP route have a higher administrative distance of 120.

Connected, static & dynamic routes

Connected routes

Subnets directly connected to a router interface are added to the router’s routing table. Interface has to have an IP address configured and both interface status codes must be in the “up and up” state. A router will be able to route all packets destined for all hosts in subnets directly connected to its active interfaces.

What is IP routing? 

IP header

An IP header is a prefix to an IP packet that contains information about the IP version, length of the packet, source and destination IP addresses, etc. It consists of the following fields:

ICMP (Internet Control Message Protocol)

ICMP (Internet Control Message Protocol) is a network layer protocol that reports errors and provides information related to IP packet processing. ICMP is used by network devices to send error messages indicating, for example, that a requested service is not available or that a host isn’t reachable.
ICMP is commonly used by network tools such as ping or traceroute. Consider the following example that illustrates how ping can be used to test the reachability of a host:

APIPA (Automatic Private IP Addressing)

Automatic Private IP Addressing (APIPA) is a feature in Windows operating systems that enables computers to automatically self-configure an IP address and subnet mask when their DHCP server isn’t reachable. The IP address range for APIPA is 169.254.0.1-169.254.255.254, with the subnet mask of 255.255.0.0.

NTP (Network Time Protocol)

Network Time Protocol (NTP) is an application layer protocol used for clock synchronization between hosts on a TCP/IP network. The goal of NTP is to ensure that all computers on a network agree on the time, since even a small difference can create problems. For example, if there is more than 5 minutes difference on your host and the Active Directory domain controller, you will not be able to login into your AD domain.
NTP uses a hierarchical system of time sources. At the top of the structure are highly accurate time sources – typically atomic or GPS clocks. These clocks are known as stratum 0 servers. Stratum 1 servers are directly linked to stratum 0 servers and computers run NTP servers that deliver the time to stratum 2 servers, and so on (image source: Wikipedia):