EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol (often called "hybrid") that was originally Cisco-proprietary but is now an open standard. It is designed for rapid convergence, scalability, and efficient bandwidth usage. EIGRP uses the DUAL (Diffusing Update Algorithm) to calculate the shortest path and ensure a loop-free topology.
Key Features
- Fast Convergence: EIGRP pre-calculates backup routes (Feasible Successors) and switches immediately if the primary route fails.
- Protocol Dependent Modules (PDM): Supports multiple network layer protocols like IPv4 and IPv6.
- Partial & Bounded Updates: Sends updates only when there is a change, and only to routers that need the information, saving bandwidth.
- Equal & Unequal Cost Load Balancing: The only protocol that supports load balancing across links with different metrics (using the
variancecommand).
EIGRP Terminology
Successor
The best path to a destination network, stored in the routing table.
Feasible Successor
A loop-free backup path stored in the topology table. It is used immediately if the Successor fails.
Feasible Distance (FD)
The total metric to reach the destination via the Successor.
EIGRP Tables
| Table Name | Description | Command |
|---|---|---|
| Neighbor Table | Lists directly connected EIGRP neighbors. | show ip eigrp neighbors |
| Topology Table | Contains all learned routes (Successors & Feasible Successors). | show ip eigrp topology |
| Routing Table | Contains the best routes used to forward traffic. | show ip route eigrp |
EIGRP Packet Types
- Hello: Used to discover and maintain neighbors. Sent periodically.
- Update: Contains routing information. Sent only when necessary.
- Query: Sent when a route fails and no Feasible Successor is available.
- Reply: Response to a Query packet.
- ACK: Acknowledges receipt of Update, Query, or Reply packets.
Metric Calculation (K-Values)
EIGRP uses a composite metric based on Bandwidth and Delay by default.
Formula: Metric = 256 * ((10^7 / Min Bandwidth) + (Total Delay / 10))
Basic Configuration
Router(config)# router eigrp 100 ! 100 is the Autonomous System (AS) number Router(config-router)# network 192.168.1.0 ! Classful network Router(config-router)# network 10.0.0.0 0.255.255.255 ! With Wildcard Mask Router(config-router)# no auto-summary ! Disable automatic summarization