Interior Gateway Routing Protocol (IGRP) is a dynamic routing protocol developed by Cisco Systems in the mid-1980s to address the limitations of the then-existing Routing Information Protocol (RIP). IGRP is a distance vector routing protocol and was designed to support larger and more complex networks with a greater range of hop counts and more sophisticated metrics to improve route selection and network performance. IGRP is considered an Interior Gateway Protocol (IGP).

Key features of IGRP include:

  1. Enhanced Metrics: Unlike RIP, which uses only hop count as its metric, IGRP uses a combination of metrics such as bandwidth, delay, load, and reliability to calculate the best path to a destination. This allows for more nuanced and efficient routing decisions.

  2. Composite Metric: IGRP combines its metrics into a single composite metric to compare and select routes. This composite metric is calculated using a formula that takes into account the bandwidth, delay, reliability, and load, making the routing decision more comprehensive.

  3. Variable-Length Subnet Masking (VLSM): IGRP supports VLSM, allowing for more efficient use of IPv4 address space and the ability to create subnets of different sizes within the same network.

  4. Routing Updates: IGRP sends periodic routing updates to its neighbors every 90 seconds by default, which includes the entire routing table. This ensures that all routers have consistent and up-to-date routing information.

  5. Hop Count Limit: IGRP supports a maximum hop count of 255, significantly higher than RIP's maximum of 15. This extended hop count support makes IGRP suitable for larger networks.

  6. Route Poisoning and Holddown Mechanisms: To prevent routing loops and count to infinity problems, IGRP implements route poisoning and holddown mechanisms. These features help stabilize the network during topology changes.

Despite its advancements over RIP, IGRP has been largely superseded by Enhanced Interior Gateway Routing Protocol (EIGRP), also developed by Cisco. EIGRP retains the benefits of IGRP while incorporating additional features such as faster convergence and support for multiprotocol environments, making it more efficient and scalable. EIGRP is considered an advanced distance-vector protocol, sometimes referred to as a hybrid routing protocol because it incorporates features of both distance-vector and link-state protocols.