Network Routing Protocols

 

Hundreds of different network protocols have been created for supporting communication between computers and other types of electronic devices. So-called routing protocols are the family of network protocols that enable computer routers to communicate with each other and in turn to intelligently forward traffic between their respective networks. The protocols described below each enable this critical function of routers and computer networking.

The specific characteristics of routing protocols include the manner in which they avoid routing loops, the manner in which they select preferred routes, using information about hop costs, the time they require to reach routing convergence, their scalability, and other factors.Many software implementations exist for most of the common routing protocols. Examples of open-source applications are Bird Internet routing daemon, Quagga, GNU Zebra, OpenBGPD, OpenOSPFD, and XORP.

Every network routing protocol performs three basic functions:

Discovery – identify other routers on the network

Route management – keep track of all the possible destinations (for network messages) along with some data describing the pathway of each

Path determination – make dynamic decisions for where to send each network message

A few routing protocols (called link-state protocols) enable a router to build and track a full map of all network links in a region while others (called distance-vector protocols) allow routers to work with less information about the network area.

Although there are many types of routing protocols, three major classes are in widespread use on IP networks:

 

Interior gateway protocols type 1, link-state routing protocols, such as OSPF and IS-IS

Interior gateway protocols type 2, distance-vector routing protocols, such as Routing Information Protocol, RIPv2, IGRP.

Exterior gateway protocols are routing protocols used on the Internet for exchanging routing information between Autonomous Systems, such as Border Gateway Protocol (BGP), Path Vector Routing Protocol. Exterior gateway protocols should not be confused with Exterior Gateway Protocol (EGP), an obsolete routing protocol.

OSI layer designation

 

Routing protocols, according to the OSI routing framework, are layer management protocols for the network layer, regardless of their transport mechanism:

IS-IS runs on the data link layer (Layer 2)

Open Shortest Path First (OSPF) is encapsulated in IP, but runs only on the IPv4 subnet, while the IPv6 version runs on the link using only link-local addressing.

IGRP, and EIGRP are directly encapsulated in IP. EIGRP uses its own reliable transmission mechanism, while IGRP assumed an unreliable transport.

Routing Information Protocol (RIP) runs over the User Datagram Protocol (UDP). Version 1 operates in broadcast mode, while version 2 uses multicast addressing.

BGP runs over the Transmission Control Protocol (TCP).

Interior gateway protocols

 

Interior gateway protocols (IGPs) exchange routing information within a single routing domain. Examples of IGPs include:

Open Shortest Path First (OSPF)

Routing Information Protocol (RIP)

Intermediate System to Intermediate System (IS-IS)

Enhanced Interior Gateway Routing Protocol (EIGRP)[a]

Exterior gateway protocols

 

Exterior gateway protocols exchange routing information between autonomous systems. Examples include:

Exterior Gateway Protocol (EGP)

Border Gateway Protocol (BGP)

Five Most Popular Routing Protocols

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.