Routing protocol:
A routing protocol uses software and routing algorithms to determine optimal network data transfer and communication paths between network nodes. Routing protocols facilitate router communication and overall network topology understanding.
A routing protocol is also known as a routing policy.

Classifying Routing Protocols:
Routing protocols can be classified into different groups according to their characteristics. Specifically, routing protocols can be classified by their:
Purpose: Interior Gateway Protocol (IGP) or Exterior Gateway Protocol (EGP)
Operation: Distance vector protocol, link-state protocol, or path-vector protocol
Behavior: Classful (legacy) or classless protocol
For example, IPv4 routing protocols are classified as follows:
RIPv1 (legacy): IGP, distance vector, classful protocol
IGRP (legacy): IGP, distance vector, classful protocol developed by Cisco (deprecated from 12.2 IOS and later)
RIPv2: IGP, distance vector, classless protocol
EIGRP: IGP, distance vector, classless protocol developed by Cisco
OSPF: IGP, link-state, classless protocol
IS-IS: IGP, link-state, classless protocol
BGP: EGP, path-vector, classless protocol

The Internet is based on the AS concept; therefore, two types of routing protocols are required:
Interior Gateway Protocols (IGP): Used for routing within an AS. It is also referred to as intra-AS routing. Companies, organizations, and even service providers use an IGP on their internal networks. IGPs include RIP, EIGRP, OSPF, and IS-IS.
Exterior Gateway Protocols (EGP): Used for routing between autonomous systems. It is also referred to as inter-AS routing. Service providers and large companies may interconnect using an EGP. The Border Gateway Protocol (BGP) is the only currently viable EGP and is the official routing protocol used by the Internet.

Distance Vector Routing Protocols:
Distance vector means that routes are advertised by providing two characteristics:
Distance: Identifies how far it is to the destination network and is based on a metric such as the hop count, cost, bandwidth, delay, and more
Vector: Specifies the direction of the next-hop router or exit interface to reach the destination.

RIP:
Researchers developed Routing Information Protocol in the 1980s for use on small- or medium-sized internal networks that connected to the early Internet. RIP is capable of routing messages across networks up to a maximum of 15 hops.

RIP-enabled routers discover the network by first sending a message requesting router tables from neighboring devices. Neighbor routers running RIP respond by sending the full routing tables back to the requestor, whereupon the requestor follows an algorithm to merge these updates into its own table. At scheduled intervals, RIP routers then periodically send out their router tables to their neighbors so that any changes can be propagated across the network.

OSPF
Open Shortest Path First was created to overcome some of its limitations of RIP including:

15 hop count restriction
Inability to organize networks into a routing hierarchy, important for manageability and performance on large internal networks
Significant spikes of network traffic generated by repeatedly re-sending full router tables at scheduled intervals.

As the name suggests, OSPF is an open public standard with widespread adoption across many industry vendors. OSPF-enabled routers discover the network by sending identification messages to each other followed by messages that capture specific routing items rather than the entire routing table. It is the only link-state routing protocol listed in this category.

EIGRP and IGRP
Cisco developed Internet Gateway Routing Protocol as another alternative to RIP. The newer Enhanced IGRP (EIGRP) made IGRP obsolete starting in the 1990s. EIGRP supports classless IP subnets and improves the efficiency of the routing algorithms compared to older IGRP. It does not support routing hierarchies, like RIP. Originally created as a proprietary protocol runnable only on Cisco family devices, EIGRP was designed with the goals of easier configuration and better performance than OSPF.

IS-IS
The Intermediate System to Intermediate System protocol functions similarly to OSPF. While OSPF became the more popular choice overall, IS-IS remains in widespread use by service providers who have benefitted from the protocol being more easily adaptable to their specialized environments. Unlike the other protocols in this category, IS-IS does not run over Internet Protocol (IP) and uses its own addressing scheme.

BGP and EGP
The Border Gateway Protocol is the Internet standard External Gateway Protocol (EGP). BGP detects modifications to routing tables and selectively communicates those changes to other routers over TCP/IP.

Internet providers commonly use BGP to join their networks together. Additionally, larger business sometimes also use BGP to connect multiple internal networks. Professionals consider BGP the most challenging of all routing protocols to master due to its configuration complexity.