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A routing protocol specifies how routers communicate with each other for distributing routing information between routers on a network. It helps each router to determine the best path to deliver packets by knowing about all other routers connected to the network.   

There are two categories of routing protocols, namely Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (EGP).

An Interior Gateway Protocol (IGP) refers to a routing protocol that exchanges routing information between gateways within a single autonomous system. The routing information is used by the Internet Protocol (IP) or other network protocols to specify how to route transmissions. Following are the Interior Gateway Protocols (IGPs):

• IS-IS
• OSPF
• IGRP
• EIGRP
• RIP (RIPv1 and RIPv2)

Intermediate System-to-Intermediate System (IS-IS) is a link-state routing protocol standardized by the ISO. It supports variable length subnet masks (VLSM), uses multicast to discover neighboring routers using hello packets, and supports authentication of routing updates.

Open Shortest Path First (OSPF) is a routing protocol that is used in large networks. Internet Engineering Task Force (IETF) designates OSPF as one of the Interior Gateway Protocols. A host uses OSPF to obtain a change in the routing table and to immediately multicast updated information to all the other hosts in the network.

Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary distance vector Interior Gateway Protocol (IGP). It used by Cisco routers to exchange routing data within an autonomous system (AS). This is a classful routing protocol and does not support variable length subnet masks (VLSM). IGRP supports multiple metrics for each route, including bandwidth, delay, load, MTU, and reliability.

EIGRP is a Cisco proprietary protocol. It is an enhanced version on IGRP. It has faster convergence due to use of triggered update and saving neighbor’s routing table locally. It supports VLSM and routing summarization. As EIGRP is a distance vector protocol, it automatically summarizes routes across Class A, B and C networks. It also supports multicast and incremental updates and provides routing for three routed protocols i.e. IP, IPX, and AppleTalk.

RIPv1 was designed to fulfill dynamic routing needs of a network. It uses MAC-level broadcasts, which are not desirable in the current scenario because all nodes must process all broadcasts. It does not support VLSM and route summarization.

RIPv2 is a classless protocol. It improves upon RIPv1 with the ability to use VLSM and route summarization. It supports route authentication and multicasting of route updates. It is used in small to medium size networks. RIPv2 uses triggered updates to speed up convergence. It supports variable length subnet masks (VLSM).

Variable Length Subnet Masks (VLSMs) allow administrators to divide a given IP address space into more than one subnet. It helps in efficient use of IP addressing. A classless routing protocol is required to deploy VLSM. Classful protocols such as RIP v1 do not support VLSM.

An Autonomous system (AS) is a group of networks under a single administration and with single routing policies. Each AS is assigned a unique number in order to differentiate it from other autonomous systems. The assigned unique number can be from 1 to 65,535. The Internet Assigned Numbers Authority (IANA) is responsible for assigning these numbers. There are public and private AS numbers like private and public IP addresses.

Administrative distance defines the reliability of a routing protocol. The router uses administrative distance in order to choose the best path when there are two or more different routes on a router to the same destination using different routing protocols. Each routing protocol’s administrative distance value decides which routing protocol is more reliable. The lower the administrative distance value, the more reliable the routing protocol.

Protocol	Administrative distance value
IGRP	100
OSPF	110
IS-IS	115
RIP	120

Note: The administrative distance value for static route is 0.

Exterior Gateway Protocol (EGP) is a protocol that exchanges routing information between different autonomous systems. It is commonly used between hosts on the Internet to exchange routing table information. Border Gateway Protocol (BGP) is the only active EGP.

BGP stands for Border Gateway Protocol. It is an interautonomous system routing protocol and is a form of Exterior Gateway Protocol (EGP). This protocol is defined in RFC-1267 and RFC-1268. It is used for exchanging network reachability information with other BGP systems. This information includes a complete list of intermediate autonomous systems that the network traffic has to cover in order to reach a particular network. This information is used for figuring out loop-free interdomain routing between autonomous systems. BGP-4 is the latest version of BGP.

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