Routing protocols are the backbone of modern network communication, enabling data to travel efficiently across interconnected networks. Whether it’s a small home network or a large enterprise infrastructure, routing protocols ensure that data packets reach their intended destinations through the most optimal paths.
In simple terms, routing protocols determine how routers communicate with each other to share information about network topology. They allow routers to dynamically adapt to changes, such as link failures or network congestion, ensuring reliable and efficient data delivery. Understanding these protocols is crucial for network engineers and administrators tasked with maintaining seamless connectivity.
This guide provides an in-depth look at the types of routing protocols and their unique roles in network communication. We’ll explore key categories such as Distance Vector, Link State, and Hybrid protocols, highlighting their differences and use cases. Additionally, we’ll cover specific protocols like RIP, OSPF, EIGRP, and BGP, explaining how each operates and when to use them.
By the end of this guide, you’ll have a comprehensive understanding of the various routing protocols and their significance in network design and troubleshooting. Whether you’re new to networking or seeking to refine your skills, this ultimate guide will equip you with the knowledge to optimize and secure your network infrastructure.
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
Distance Vector and Link State Protocols
| Distance Vector | Link State |
|---|---|
| Sends entire routing table during updates | Only provides link state information |
| Sends periodic updates every 30-90 seconds | Uses triggered updates |
| Broadcasts updates | Multi casts updates |
| Vulnerable to routing loops | No risk of routing loops |
| RIP, IGRP | OSPF, IS-IS |
Distance vector routing protocols are protocols that use distance to work out the best routing path for packets within a network.
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
Link state protocols take a different approach to finding the best routing path in that they share information with other routers in proximity. The route is calculated based on the speed of the path to the destination and the cost of resources.
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
Routing protocols can also be categorized as Interior Gateway Protocols (IGPs) or Exterior Gateway Protocols (EGPs).
IGPs
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.
Pros:
- Historical Significance: RIP is one of the oldest and widely recognized routing protocols.
- Operational Simplicity: It’s relatively straightforward to understand and implement.
- Application Layer Operation: Operates on the application layer, making it easy to manage and configure.
- Multicast Capability (RIPv2): RIPv2 can multicast its routing table, providing a more efficient way to communicate with other routers than broadcasting.
- Enhanced Security (RIPv2): RIPv2 offers authentication measures to enhance data security.
Cons:
- Maximum Hop Count: RIP’s maximum hop count of 15 restricts its use in larger networks.
- Lack of Scalability: Due to its hop count limitation, it is not suited for modern expansive networks.
- Broadcaster (RIPv1): RIPv1’s method of broadcasting its entire table can lead to increased traffic and potential inefficiencies.
- Limited Route Metric: RIP uses hop count as its sole metric, which may not always represent the best path in complex networks.
- Slower Convergence: RIP can be slower to adapt to network changes, leading to potential temporary routing loops.
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.
Pros:
- Enhanced Scalability: IGRP addresses the shortcomings of RIP by allowing a maximum hop count of 255, making it suitable for larger networks.
- Multiple Metrics: Uses a combination of metrics (bandwidth, delay, reliability, and load) for improved routing decisions.
- Frequent Updates: Broadcasts updates every 90 seconds, ensuring the network is well-informed and up-to-date.
- Loop Resistance: Built-in features that automatically update routes, making IGRP resistant to routing loops.
- Cisco Legacy: Developed by Cisco, it benefits from being backed by one of the industry leaders in networking.
Cons:
- Proprietary Protocol: Being a Cisco product, IGRP isn’t universally adaptable across all devices from different manufacturers.
- Limited Default Metrics: Even though it has multiple metrics, only bandwidth and delay are considered under default settings, potentially overlooking other valuable information.
- Superseded by EIGRP: IGRP has been replaced by Enhanced IGRP (EIGRP), which offers more advantages, leading to its diminished use in modern networks.
- Larger Overhead: Given its broader capabilities, IGRP can generate more network overhead compared to simpler protocols like RIP.
- Potential Complexity: The multiple metrics and larger hop count can make configuration and troubleshooting more complex than simpler protocols.
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.
Pros:
- Efficient Routing: Utilizes the Shortest Path First (SPF) algorithm to ensure optimal data packet transmission.
- Detailed Network Insight: OSPF routers maintain a database on the network’s topology, offering a detailed perspective on its structure.
- Dynamic Adaptability: Employs the Dijkstra algorithm to dynamically adjust to network topology changes, ensuring continuity in data transmission.
- Security Features: Offers protocol change authentication to maintain data security, ensuring that only authorized updates are made.
- Highly Scalable: Suitable for both small and large-scale network environments, making it versatile for various organizational sizes.
Cons:
- Complex Configuration: Given its many features, OSPF can be complex to set up and maintain.
- Higher Overhead: Maintaining detailed databases and frequently recalculating routes can generate more network overhead.
- Sensitive to Topology Changes: While OSPF can adapt to changes, frequent topology alterations can cause performance dips as it recalculates routes.
- Resource Intensive: OSPF routers require more memory and CPU resources due to their database maintenance and route recalculations.
- Potential for Large LSDB: In very large networks, the Link State Database (LSDB) can grow significantly, necessitating careful design and segmenting.
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.
Pros:
- Data Exchange Between Autonomous Systems: Allows gateway hosts to share information across distinct network domains, effectively acting as a bridge.
- Foundation of Early Internet: Served as a precursor and essential component to the modern internet’s formation.
- Routing Database: Contains comprehensive information, including known routers, route costs, and the addresses of neighboring devices.
- Path Information Sharing: Sends route data to neighboring routers, helping them update their tables and make better routing decisions.
Cons:
- Lack of Multipath Support: EGP isn’t suitable for modern multipath networking environments, limiting its adaptability.
- Obsolete: Has been largely phased out in favor of more advanced protocols, notably BGP.
- Limited Scalability: As networks grew, EGP struggled with handling larger and more intricate systems.
- Static Path Determination: While EGP keeps a database of nearby networks, its path determinations are more static, making it less flexible than newer protocols.
- Potential for Redundancy: EGP’s method of sharing all route data with neighboring routers can lead to redundant data transmission and larger routing tables.
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.
Pros:
- Versatility: Supports multiple network protocols, including IP, AppleTalk, and NetWare.
- Advanced Design: A successor to the original IGRP, EIGRP incorporates more modern features for routing.
- Neighbor Information Exchange: By collecting data from neighbors’ routing tables, EIGRP maintains a real-time understanding of the network environment.
- Efficient Notification System: Routers promptly inform neighboring routers of any route changes, fostering a responsive network environment.
- Reliable Transport Protocol (RTP): Ensures the reliability of packet transmissions and acknowledges receipt of routing updates.
- Diffusing Update Algorithm (DUAL): Enhances route calculations and accelerates network convergence, reducing the time the network takes to stabilize after a change.
Cons:
- Proprietary Protocol: EIGRP is Cisco-specific, which can limit interoperability with equipment from other manufacturers.
- Overhead: The frequent exchange of routing updates and queries, especially in larger networks, can consume bandwidth and processing resources.
- Complex Configuration: While powerful, EIGRP’s array of features might pose a steeper learning curve for those unfamiliar with its intricacies.
- Potential for Routing Loops: As with many distance-vector protocols, there’s a risk of routing loops, although measures like split horizon and route poisoning help mitigate this.
- Lack of Wide Adoption: Being proprietary means EIGRP isn’t as universally adopted as open standard protocols.
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.
Pros:
- Internet Backbone: As the primary routing protocol of the internet, BGP plays a pivotal role in global data exchanges.
- Decentralized Design: Unlike its predecessor EGP, BGP’s decentralized nature ensures more robust and adaptable network operations.
- Customizable Path Selection: BGP’s Best Path Selection Algorithm can be tailored to meet unique network demands by adjusting attributes.
- Efficient Updates: Only transmitting updates when there’s a change, BGP reduces unnecessary network traffic.
- Granular Routing Decisions: Administrators have a plethora of factors like weight, AS_Path length, and IGP metric to inform routing decisions, allowing for a high degree of routing precision.
- Authentication: BGP provides security measures allowing only authorized routers to participate in data exchanges, enhancing the security of routing updates.
Cons:
- Complex Configuration: BGP requires meticulous manual configuration since it doesn’t auto-discover topology changes.
- Potential Instability: Mistakes or malicious actions in BGP configurations can inadvertently or intentionally divert internet traffic, potentially leading to large-scale outages.
- Scalability Concerns: As the internet grows, BGP’s scalability, in its current form, might pose challenges.
- Vulnerabilities: Despite authentication measures, BGP is historically susceptible to certain security issues, like prefix hijacking.
- Learning Curve: Given its complexity and significance, mastering BGP can be challenging for many network administrators.
- Convergence Time: BGP can sometimes take longer to converge after a network change compared to some other protocols.
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).
Pros:
- Hierarchical Design: Organizing routers into areas and domains simplifies management and optimizes routing within large networks.
- Scalability: The division into areas and domains allows for efficient operation in large-scale networks, avoiding unnecessary routing overhead.
- Flexibility: The protocol is not tied exclusively to IP, making it adaptable to various network architectures.
- Efficient Path Selection: Utilizes a modified version of the Dijkstra algorithm for optimal path determination.
- Distinct Addressing Mechanism: With unique addresses like NSAP and NET, IS-IS provides granularity in addressing which can assist in network troubleshooting and management.
- Dual-Level Operation: Layer 1 and Layer 2 classification enables segregation of intra-area routing from inter-area routing, ensuring efficiency and simplifying router roles.
Cons:
- Learning Curve: Given its unique terminology and addressing mechanism, mastering IS-IS might pose a challenge for network engineers unfamiliar with it.
- Address Length: NSAP addresses can be lengthy, which may complicate manual configuration and troubleshooting.
- Lesser Adoption: IS-IS is less commonly used in certain segments of the internet when compared to OSPF, potentially leading to compatibility considerations.
- Complex Configuration: Its hierarchical structure, while providing scalability benefits, might complicate the initial configuration.
- Interoperability: As a protocol with roots in the ISO OSI model, there may be issues when trying to interoperate with purely IP-based protocols.
- Protocol Evolution: While IS-IS has been adapted for IP, its origins in the OSI model mean it might not be as naturally suited to some IP-centric tasks as newer protocols.
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
| Protocol Type | Type of Metric Used |
|---|---|
| RIP | Hop count |
| RIPv2 | Hop count |
| IGRP | Bandwidth, Delay |
| OSPF | Bandwidth |
| BGP | Chosen by administrator |
| EIGRP | Bandwidth, Delay |
| IS-IS | Chosen by administrator |
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.
| Administrative Distance Route Source | Default Distance |
|---|---|
| Connected Interface | 0 |
| Static Route | 1 |
| Enhanced IGRP summary route | 5 |
| External BGP | 20 |
| Internal Enhanced IGRP | 90 |
| IGRP | 100 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| EIGRP external route | 170 |
| Internal BGP | 200 |
| Unknown | 255 |
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.
Routing Protocols FAQs
How do Bellman-Ford Algorithm and Dijkstras algorithm work differently in routing protocols?
The Bellman-Ford and Dijkstra algorithms both include a calculation of the cost (distance) of traversing a link. The main difference between the methodologies is that the cost calculations for Bellman-Ford can be positive or negative, but Dijkstra only operates in the positive. Other differences are that Bellman-Ford only informs neighboring devices but includes calculations of the cost to non-neighbors, while Dijkstra will broadcast to all but only frame its calculations in terms of cost to neighbors.
What is the difference between forwarding and routing?
Forwarding is an internal process for a network device, such as a switch. It just requires the device to transfer data received on one interface out through another interface. Routing involves calculating a path to a destination before deciding which interface to transfer out the incoming data.
Why BGP is preferred over OSPF?
BGP offers more flexibility and more control to the creators and owners of a device than OSPF. BGP processes include options on what routes should be advertised and which notifications will be accepted by the device. It offers more control over route selection. This enables more flexibility to avoid overloading on particular links, which OSPF would automatically assume to provide the fastest route.
See also: Tools for traceroute and tracert
Awesome explanation and one stop/one page for all routing protocols. Great resource for overall understanding of how routing works.
This is great article simple explained everything anyone can understand, cheers, do more like this, best of luck.
Thanks dude
Thank you so much on what you’ve done in this post. I’ve been working in network environment and this is the first time someone summarized routing protocols in a very short and sweet approach. Again thank you so much. May the Lord Jesus blessed the works of your hands and extend His love through you by blessing others with your words. God bless.
This is very useful and thanks you.
Quite informative indeed. I am looking for information about IP addressing and subnet masking.