Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol developed by Cisco Systems. It is proprietary to Cisco, but in 2013, Cisco announced their intention to release a basic version of EIGRP as an open standard.

Features and Characteristics of EIGRP:

  1. Dual Algorithm: EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate the shortest path to a destination. This ensures loop-free paths and quick convergence times.
  2. Metric: Unlike RIP which uses only hop count, EIGRP uses a composite metric that can consider bandwidth, delay, reliability, and load. However, by default, only bandwidth and delay are used.
  3. VLSM and CIDR Support: EIGRP supports Variable Length Subnet Masking (VLSM) and Classless Inter-Domain Routing (CIDR), which enables more efficient use of IP addresses.
  4. Updates: EIGRP doesn’t send the entire routing table periodically like RIP. Instead, it sends partial updates only when network topology changes occur.
  5. Scalability: EIGRP is suitable for both small and large networks due to its rapid convergence and scalability features.
  6. Multiple Network Layer Protocols: EIGRP can be used for routing different layer 3 protocols including IP, IPv6, and even IPX (a now largely obsolete protocol).
  7. Neighbor Discovery: EIGRP routers periodically send hello packets to discover and maintain neighbor relationships.
  8. Fast Convergence: EIGRP keeps backup routes in its topology table, allowing for faster convergence if a primary route fails.

Key Concepts in EIGRP:

  1. Neighbor Table: Contains a list of neighboring routers with which EIGRP is directly exchanging routing information.
  2. Topology Table: Holds all routes advertised by neighboring routers, including backup paths to destinations.
  3. Route States: Routes in EIGRP’s topology table can be in one of two states: Passive (stable) or Active (when EIGRP is actively trying to compute a backup path).
  4. Feasible Distance (FD): This is the lowest calculated metric to reach the destination.
  5. Advertised Distance (AD) or Reported Distance: This is the metric that a neighboring router is using for a path to a network.
  6. Feasible Successor: A backup route in the topology table that has an AD less than the FD for the current primary route. This route can be immediately used if the primary route fails.
  7. Route Summarization: EIGRP supports manual and automatic summarization, allowing for a reduction in the size of routing tables.

Limitations:

  • Proprietary: Though Cisco has opened up basic EIGRP, many advanced features remain proprietary, limiting interoperability with non-Cisco devices.

Despite the emergence of newer protocols like OSPF and IS-IS, EIGRP remains popular in Cisco-dominated environments because of its efficiency, fast convergence, and ease of configuration.