Routing is one of the most fundamental areas of networking that an administrator has to know. Routing protocols determine how your data gets to its destination and helps to make that routing process as smooth as possible. However, there are so many different types of routing protocol that it can be very difficult to keep track of them all!

Router protocols include:

  • Routing Information Protocol (RIP)
  • Interior Gateway Protocol (IGRP)
  • Open Shortest Path First (OSPF)
  • Exterior Gateway Protocol (EGP)
  • Enhanced Interior Gateway Routing Protocol (EIGRP)
  • Border Gateway Protocol (BGP)
  • Intermediate System-to-Intermediate System (IS-IS)

Before we get to looking at the routing protocols themselves, it is important to focus on the categories of protocols.

All routing protocols can be classified into the following:

  • Distance Vector or Link State Protocols
  • Interior Gateway Protocols (IGP) or Exterior Gateway Protocols (EGP)
  • Classful or Classless Protocols

These protocols measure the distance based on how many hops data has to pass to get to its destination. The number of hops is essentially the number of routers it takes to reach the destination.

Generally, distance vector protocols send a routing table full of information to neighboring devices. This approach makes them low investment for administrators as they can be deployed without much need to be managed. The only issue is that they require more bandwidth to send on the routing tables and can run into routing loops as well.

Link state routing protocols use an algorithm to work this out. One of the key differences to a distance vector protocol is that link state protocols don’t send out routing tables; instead, routers notify each other when route changes are detected.

Routers using the link state protocol creates three types of tables; neighbor table, topology table, and routing table. The neighbor table stores details of neighboring routers using the link state routing protocol, the topology table stores the whole network topology, and the routing table stores the most efficient routes.

IGP and EGPs

IPGs

IGPs are routing protocols that exchange routing information with other routers within a single autonomous system (AS). An AS is defined as one network or a collection of networks under the control of one enterprise. The company AS is thus separate from the ISP AS.

Each of the following is classified as an IGP:

  • Open Shortest Path First (OSPF)
  • Routing Information Protocol (RIP)
  • Intermediate System to Intermediate System (IS-IS)
  • Enhanced Interior Gateway Routing Protocol (EIGRP)

EGPs

On the other hand, EGPs are routing protocols that are used to transfer routing information between routers in different autonomous systems. These protocols are more complex and BGP is the only EGP protocol that you’re likely to encounter. However, it is important to note that there is an EGP protocol named EGP.

Examples of EGPs include:

  • Border Gateway Protocol (BGP)
  • Exterior Gateway Protocol (EGP)
  • The ISO’s InterDomain Routing Protocol (IDRP)

Types of Routing Protocol

Routing Protocols Timeline

  • 1982 – EGP
  • 1985 – IGRP
  • 1988 – RIPv1
  • 1990 – IS-IS
  • 1991 – OSPFv2
  • 1992 – EIGRP
  • 1994 – RIPv2
  • 1995 – BGP
  • 1997 – RIPng
  • 1999 – BGPv6 and OSPFv3
  • 2000 – IS-ISv6

Routing Information Protocol (RIP)

Routing Information Protocol or RIP is one of the first routing protocols to be created. RIP is used in both Local Area Networks (LANs) and Wide Area Networks (WANs), and also runs on the Application layer of the OSI model. There are multiple versions of RIP including RIPv1 and RIPv2. The original version or RIPv1 determines network paths based on the IP destination and the hop count of the journey.

RIPv1 interacts with the network by broadcasting its IP table to all routers connected to the network. RIPv2 is a little more sophisticated than this and sends its routing table on to a multicast address. RIPv2 also uses authentication to keep data more secure and chooses a subnet mask and gateway for future traffic. The main limitation of RIP is that it has a maximum hop count of 15 which makes it unsuitable for larger networks.

See also: LAN Monitoring tools

Interior Gateway Protocol (IGRP)

Interior Gateway Protocol or IGRP is a distance vector routing protocol produced by Cisco. IGRP was designed to build on the foundations laid down on RIP to function more effectively within larger connected networks and removed the 15 hop cap that was placed on RIP. IGRP uses metrics such as bandwidth, delay, reliability, and load to compare the viability of routes within the network. However, only bandwidth and delay are used under IGRP’s default settings.

IGRP is ideal for larger networks because it broadcasts updates every 90 seconds and has a maximum hop count of 255. This allows it to sustain larger networks than a protocol like RIP. IGRP is also widely used because it is resistant to routing loops because it updates itself automatically when route changes occur within the network.

Open Shortest Path First (OSPF)

Open Shortest Path First or OSPF protocol is a link-state IGP that was tailor-made for IP networks using the Shortest Path First (SPF) algorithm. The SPF routing algorithm is used to calculate the shortest path spanning-tree to ensure efficient data transmission of packets. OSPF routers maintain databases detailing information about the surrounding topology of the network. This database is filled with data taken from Link State Advertisements (LSAs) sent by other routers. LSAs are packets that detail information about how many resources a given path would take.

OSPF also uses the Dijkstra algorithm to recalculate network paths when the topology changes. This protocol is also relatively secure as it can authenticate protocol changes to keep data secure. It is used by many organizations because it’s scalable to large environments. Topology changes are tracked and OSPF can recalculate compromised packet routes if a previously-used route has been blocked.

Exterior Gateway Protocol (EGP)

Exterior Gateway Protocol or EGP is a protocol that is used to exchange data between gateway hosts that neighbor each other within autonomous systems. In other words, EGP provides a forum for routers to share information across different domains. The most high profile example of an EGP is the internet itself. The routing table of the EGP protocol includes known routers, route costs, and network addresses of neighboring devices. EGP was widely-used by larger organizations but has since been replaced by BGP.

The reason why this protocol has fallen out of favor is that it doesn’t support multipath networking environments. The EGP protocol works by keeping a database of nearby networks and the routing paths it could take to reach them. This route information is sent on to connected routers. Once it arrives, the devices can update their routing tables and undertake more informed path selection throughout the network.

Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol or EIGRP is a distance vector routing protocol that is used for IP, AppleTalk, and NetWare networks. EIGRP is a Cisco proprietary protocol that was designed to follow on from the original IGRP protocol. When using EIGRP, a router takes information from its neighbors’ routing tables and records them. Neighbors are queried for a route and when a change occurs the router notifies its neighbors about the change. This has the end result of making neighboring routers aware of what is going on in nearby devices.

EIGRP is equipped with a number of features to maximize efficiency, including Reliable Transport Protocol (RTP) and a Diffusing Update Algorithm (DUAL). Packet transmissions are made more effective because routes are recalculated to speed up the convergence process.

Border Gateway Protocol (BGP)

Border Gateway Protocol or BGP is the routing protocol of the internet that is classified as a distance path vector protocol. BGP was designed to replace EGP with a decentralized approach to routing. The BGP Best Path Selection Algorithm is used to select the best routes for data packet transfers. If you don’t have any custom settings then BGP will select routes with the shortest path to the destination.

However many administrators choose to change routing decisions to criteria in line with their needs. The best routing path selection algorithm can be customized by changing the BGP cost community attribute. BGP can make routing decisions based Factors such as weight, local preference, locally generated, AS_Path length, origin type, multi-exit discriminator, eBGP over iBGP, IGP metric, router ID, cluster list and neighbor IP address.

BGP only sends updated router table data when something changes. As a result, there is no auto-discovery of topology changes which means that the user has to configure BGP manually. In terms of security, BGP protocol can be authenticated so that only approved routers can exchange data with each other.

Intermediate System-to-Intermediate System (IS-IS)

Intermediate System-to-Intermediate System (IS-IS) is a link-state, IP routing protocol and IGPP protocol used on the internet to send IP routing information. IS-IS uses a modified version of the Dijkstra algorithm. An IS-IS network consists of a range of components including end systems, (user devices), intermediate systems (routers), areas, and domains.

Under IS-IS routers are organized into groups called areas and multiple areas are grouped together to make up a domain. Routers within the area are placed with Layer 1 and routers that connect segments together are classified as Layer 2. There are two types of network addresses used by IS-IS; Network Service Access Point (NSAP) and Network Entity Title (NET).

Classful and Classless Routing Protocols

Routing protocols can also be categorized as classful and classless routing protocols. The distinction between these two comes down to how they go about executing routing updates. The debate between these two forms of routing is often referred to as classful vs classless routing.

Classful Routing Protocols

Classful routing protocols don’t send subnet mask information during routing updates but classless routing protocols do. RIPv1 and IGRP are considered to be classful protocols. These two are classful protocols because they don’t include subnet mask information in their routing updates. Classful routing protocols have since become outdated by classless routing protocols.

Classless Routing Protocols

As mentioned above, classful routing protocols have been replaced by classless routing protocols. Classless routing protocols send IP subnet mask information during routing updates. RIPv2, EIGRP, OSPF, and IS-IS are all types of class routing protocols that include subnet mask information within updates.

Dynamic Routing Protocols

Dynamic routing protocols are another type of routing protocols that are critical to modern enterprise-grade networks. Dynamic routing protocols allow routers to automatically add information to their routing tables from connected routers. With these protocols, routers send out topology updates whenever the topological structure of the network changes. This means that the user doesn’t have to worry about keeping network paths up-to-date.

One of the main advantages of dynamic routing protocols is that they reduce the need to manage configurations. The downside is that this comes at the cost of allocating resources like CPU and bandwidth to keep them running on an ongoing basis. OSPF, EIGRP, and RIP are considered to be dynamic routing protocols.

Routing Protocols and Metrics

No matter what type of routing protocol is being used, there will be clear metrics that are used to measure which route is the best to take. A routing protocol can identify multiple paths to a destination network but needs to have the ability to work out which is the most efficient. Metrics allow the protocol to determine which routing path should be chosen to provide the network with the best service.

The simplest metric to consider is hop count. The RIP protocol uses hop count to measure the distance it takes for a data packet to reach its destination. The more hops that a packet has to travel through, the farther the packet has to travel. Thus the RIP protocol aims to choose routes while minimizing hops where possible. There are many metrics besides hop count that are used by IP routing protocols. Metrics used include:

  • Hop count – Measures the number of routers that a packet must travel through
  • Bandwidth – Chooses the routing path based on which has the highest bandwidth
  • Delay – Chooses the routing path based on which takes the least time
  • Reliability – Assesses the likelihood that a network link will fail based on error counts and previous failures
  • Cost – A value configured by the administrator or the IOS which is used to measure the cost of a route based on one metric or a range of metrics
  • Load – Chooses the routing path based on the traffic utilization of connected links

Metrics by Protocol Type

Administrative Distance

Administrative distance is one of the most important features within routers. Administrative is the term used to describe a numerical value that is used to prioritize which route should be used when there are two or more available connection routes. When one or more routes are located, the routing protocol with the lower administrative distance is selected as the route. There is a default administrative distance but administrators can also configure their own as well.

The lower the numerical value of the administrative distance, the more the router trusts the route. The closer the numerical value is to zero the better. Routing protocols use administrative distance mainly as a way to assess the trustworthiness of connected devices. You can change the administrative distance of the protocol by using the distance process within the sub-configuration mode.

Closing Words

As you can see, routing protocols can be defined and thought of in a wide array of different ways. The key is to think of routing protocols as distance vector or link state protocols, IGP or EGP protocols, and classful or classless protocols. These are the overarching categories that common routing protocols like RIP, IGRP, OSPF, and BGP fall within.

Of course, within all of these categories, each protocol has its own nuances in how it measures the best routing path, whether that is by hop count, delay, or other factors. Learning everything you can about these protocols that you retain during day-to-day networking will aid you greatly in both an exam and real-world environment.

See also: Tools for traceroute and tracert