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:
- 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.
- 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.
- 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.
- Updates: EIGRP doesn’t send the entire routing table periodically like RIP. Instead, it sends partial updates only when network topology changes occur.
- Scalability: EIGRP is suitable for both small and large networks due to its rapid convergence and scalability features.
- 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).
- Neighbor Discovery: EIGRP routers periodically send hello packets to discover and maintain neighbor relationships.
- Fast Convergence: EIGRP keeps backup routes in its topology table, allowing for faster convergence if a primary route fails.
Key Concepts in EIGRP:
- Neighbor Table: Contains a list of neighboring routers with which EIGRP is directly exchanging routing information.
- Topology Table: Holds all routes advertised by neighboring routers, including backup paths to destinations.
- 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).
- Feasible Distance (FD): This is the lowest calculated metric to reach the destination.
- Advertised Distance (AD) or Reported Distance: This is the metric that a neighboring router is using for a path to a network.
- 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.
- 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.