Interior Gateway Protocols Interior Gateway Protocols

What is Interior Gateway Protocols(IGP)?


What is IGP?

IGP stands for Interior Gateway Protocol, which is a type of routing protocol used in computer networks. It is responsible for exchanging routing information within an Autonomous System (AS), which is a network under a single administrative control. In simpler terms, IGP helps in determining the best path for data packets to travel within a network.

IGP, in essence, is the roadmap that routers within a network follow to exchange data efficiently. It is the backbone of routing within a local network, ensuring that data reaches its destination accurately and swiftly.

How Does IGP Work?

IGP works by creating a routing table that contains information about the network’s topology and the best path to reach different destinations. It uses various algorithms and metrics to determine the optimal route for data packets. These metrics can include factors like hop count, bandwidth, delay, and reliability.

Learn What is a Router and What does a Router do?

To understand how it works, we need to explore its key components and operations.

  1. Routing Tables: IGP is responsible for building and maintaining routing tables within routers. These tables are like maps that routers use to decide how to forward data packets. They contain information about the available paths, such as network destinations and the next-hop routers leading to those destinations.
  2. Route Updates: IGP constantly keeps routers informed about the network’s current topology. This means that routers regularly exchange information about the network’s status, including any changes or new routes that have become available. This dynamic updating ensures that routers always have the most up-to-date information about how to reach different parts of the network.
  3. Loop Prevention: One of the critical functions of IGP is to prevent routing loops. A routing loop occurs when data packets keep circulating endlessly between routers, unable to reach their intended destination. IGP algorithms incorporate mechanisms to detect and break these loops, ensuring that data packets eventually find their way to the right place.

Imagine you have a network with multiple routers, and you want to send data from one computer to another within the same network. When you initiate this data transfer, the router at the source computer consults its routing table, which was populated by IGP. This table tells the router which path to take to reach the destination.

The router then forwards the data packet to the next-hop router based on the information in its routing table. This process repeats until the data packet reaches its intended destination.

Throughout this journey, IGP continues to update routing tables and monitor the network for changes. If a new, more efficient path becomes available (perhaps due to a router going offline or a new router joining the network), IGP ensures that routers are promptly informed and can adjust their routing accordingly.

In summary, IGP works by creating and maintaining routing tables, regularly updating routers about the network’s status, and preventing routing loops. This dynamic and adaptive process allows data to flow smoothly and efficiently within a network, making IGP a fundamental component of modern computer networking.


Types of Interior Gateway Protocols

IGPs can be categorized into two main types: Distance Vector Protocols and Link-State Protocols.

1. Distance Vector Protocols

Distance Vector Protocols, such as Routing Information Protocol (RIP), use a simple and intuitive approach to routing. Routers using this type of IGP exchange routing tables with their neighbors at regular intervals, providing information about the distance and direction to reach various destinations. RIP, for instance, uses hop count as a metric to determine the best path.

Examples of distance-vector routing protocols:


2. Link-State Protocols

Link-state protocols, such as Open Shortest Path First (OSPF), take a more sophisticated approach. Instead of simply exchanging routing tables, routers using Link-State Protocols share detailed information about the state and cost of individual links in the network. This results in a more accurate and efficient routing process.

Examples of link-state routing protocols:

What is EIGRP?

EIGRP, or Enhanced Interior Gateway Routing Protocol, is a highly efficient IGP developed by Cisco Systems. It is widely used in large enterprise networks and provides several advantages over other IGPs. EIGRP uses a Diffusing Update Algorithm (DUAL) to dynamically calculate the best path for data packets.



While IGP is a general term for any interior gateway protocol, EIGRP specifically refers to Cisco’s proprietary protocol. The main difference between the two is that IGP is a broader category that includes multiple protocols, while EIGRP is a specific protocol developed by Cisco.

AspectIGP (Interior Gateway Protocol)EIGRP (Enhanced Interior Gateway Routing Protocol)
Protocol CategoryGeneric IGPCisco Proprietary IGP
Routing ApproachDistance Vector and Link-StateAdvanced Hybrid Protocol
Vendor DependenceVendor-agnosticCisco-specific
Convergence SpeedVaries (slower in some cases)Rapid Convergence
Metric CalculationHop Count (in some IGPs)Bandwidth, Delay, Reliability, Load
CompatibilitySupports various IGPsCisco Devices Only
Administrative ControlLimitedExtensive (fine-tuning)
Route SummarizationSupportedSupported
Load BalancingLimitedEffective Load Balancing
Triggered UpdatesYesYes

Advantages and Disadvantages of IGP


Advantages of IGP

  • Simplicity of configuration and management.
  • Compatibility with various network topologies.
  • Support for multiple routing metrics.
  • Adaptability to network changes.

Disadvantages of IGP

  • Limited scalability for large networks.
  • Susceptibility to routing loops in certain situations.
  • Lack of support for advanced routing features found in exterior gateway protocols (EGP).

Interior Gateway Protocols (IGP) are the backbone of efficient and reliable routing within autonomous systems. Understanding the types of IGPs, with a focus on Enhanced Interior Gateway Routing Protocol (EIGRP), and the nuances of IGP versus EIGRP can significantly enhance your network management skills. While IGPs offer simplicity and adaptability, it’s essential to weigh their advantages and disadvantages carefully to make informed networking decisions.

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