EIGRP, or Enhanced Interior Gateway Routing Protocol, is an advanced distance-vector routing protocol developed by Cisco Systems. While traditionally Cisco-proprietary, it has since been released as an open standard (RFC 7868), allowing other vendors to implement it, though its strongest implementation and adoption remain in Cisco environments.

EIGRP is often referred to as a hybrid routing protocol because it combines the best features of both distance-vector and link-state protocols. This hybrid nature contributes to its reputation for rapid convergence, scalability, and efficient use of network resources.

How EIGRP Works: The Core Components

EIGRP operates using several key components:

1. Neighbor Discovery:

   • EIGRP routers discover neighbors by sending Hello packets to the multicast address 224.0.0.10.

      

   • Hello packets are small, do not require acknowledgment, and are used to establish and maintain neighbor adjacencies.

   • Once an adjacency is formed, routers can begin exchanging routing information.

2. Reliable Transport Protocol (RTP):

   • Unlike many other routing protocols that rely on TCP or UDP, EIGRP uses its own transport protocol called RTP.

      

   • RTP provides reliable, ordered delivery of EIGRP packets (Updates, Queries, Replies) to ensure that routing information is consistently exchanged. Hello packets do not require reliability.

      
      

   • It supports both unicast and multicast delivery.

      

3. Diffusing Update Algorithm (DUAL):

   • DUAL is the heart of EIGRP’s loop-free path calculation and fast convergence.

      

   • It enables EIGRP to quickly find the best path (called a Successor) to a destination and, crucially, to maintain pre-calculated backup paths (called Feasible Successors).

      

   • Successor: The primary best path to a destination, stored in the routing table.

      

   • Feasible Successor (FS): A backup path that is guaranteed to be loop-free. A route qualifies as a Feasible Successor if its Reported Distance (RD) (the metric reported by the neighbor) is less than the Feasible Distance (FD) (the metric of the current best path) to that destination. This is known as the Feasibility Condition.

      
      

   • When the primary Successor path fails, EIGRP can immediately switch to a Feasible Successor from its topology table without going through a full re-convergence process, leading to extremely fast failover.

      

   • If no Feasible Successor is available, DUAL enters an “Active” state and sends Query packets to its neighbors to find an alternative path. Neighbors respond with Reply packets, and once a new path is found, the route transitions back to “Passive.”

      

4. Protocol Dependent Modules (PDMs):

   • EIGRP is designed to be protocol-independent, meaning it can route different network layer protocols. While primarily used for IPv4 and IPv6 today, historically it also supported IPX and AppleTalk through these modules.

      
      

EIGRP Tables:

EIGRP maintains three distinct tables:

1. Neighbor Table: Stores information about directly connected EIGRP neighbors, including their IP address, interface, and hold time.

    

2. Topology Table: This is the most crucial table. It stores all learned routes to a destination, including the Successor (best path) and any Feasible Successors (loop-free backup paths). It contains the Advertised Distance (metric from the neighbor) and the Feasible Distance (total calculated metric to the destination).

    
    

3. Routing Table: Contains only the best routes (Successors) selected from the topology table, which are then used to forward actual data traffic.

    

EIGRP Metrics:

EIGRP uses a composite metric to determine the best path, calculated from a combination of:

• Bandwidth (K1): The slowest link in the path. (Default: used)

• Delay (K3): The cumulative interface delay along the path. (Default: used)

   

• Load (K2): The current load on the interfaces. (Default: not used)

• Reliability (K4/K5): The reliability of the interfaces. (Default: not used)

   

• MTU (Minimum Transmission Unit): The smallest MTU in the path. (Default: not used in metric calculation, but exchanged)

By default, EIGRP only uses bandwidth and delay in its metric calculation, but these “K values” can be adjusted to include load and reliability if desired.

Key Features and Benefits:

• Fast Convergence: Achieved through DUAL and the use of Feasible Successors, allowing for near-instantaneous failover in many cases.

   

• Partial and Bounded Updates: Unlike traditional distance-vector protocols (like RIP) that send periodic full routing table updates, EIGRP only sends updates when a change occurs and only to the affected routers. This significantly reduces bandwidth usage and CPU overhead.

   

• Unequal-Cost Load Balancing: A unique and powerful feature. While most routing protocols only allow load balancing over paths with equal costs, EIGRP can distribute traffic across paths with unequal costs, provided the metric of the poorer path is within a configurable “variance” of the best path. This maximizes bandwidth utilization.

   

• Scalability: Suitable for large and complex networks due to its efficient convergence and update mechanisms.

   

• Support for VLSM and Route Summarization: EIGRP is a classless routing protocol, supporting Variable Length Subnet Masks and allowing for both automatic and manual route summarization, which helps reduce routing table size and improve efficiency.

   

• Authentication: Supports MD5 authentication to secure routing updates between neighbors.

   

• Administrative Distance (AD): EIGRP routes have a default AD of 90 for internal routes and 170 for external routes (learned via redistribution).