What is EIGRP?
Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol developed by Cisco. It utilizes the Diffusing Update Algorithm (DUAL) to calculate loop-free backup routes (Feasible Successors) and enable ultra-fast network convergence. EIGRP operates directly over IP using Protocol Number 88 and communicates using multicast traffic (224.0.0.10).
Core EIGRP Concepts & Terminology
- Autonomous System (AS) Number: A process identifier that MUST match on all neighboring routers for adjacency to form.
- Feasible Distance (FD): The total calculated metric to reach a destination network from the local router.
- Reported Distance / Advertised Distance (RD/AD): The calculated metric to reach a destination network as advertised by an adjacent neighbor.
- Successor: The primary optimal route selected by DUAL and inserted into the Routing Table.
- Feasible Successor (FS): A pre-calculated loop-free backup route kept in the EIGRP Topology Table that meets the Feasibility Condition (RD < FD).
EIGRP Metric Calculation (K-Values)
By default, EIGRP uses composite metrics based primarily on Bandwidth and Delay (K1=1, K3=1). Optional parameters include Load, Reliability, and MTU.
- Formula: Metric = [K1 * Bandwidth + (K2 * Bandwidth)/(256 - Load) + K3 * Delay] * [K5 / (Reliability + K4)] * 256
- Default Composite Formula: Metric = 256 * ((10^7 / Minimum Bandwidth in Kbps) + (Cumulative Delay in tens of microseconds))
EIGRP Packet Types
- Hello Packets: Used to discover and maintain neighbor adjacencies (sent unreliably via multicast).
- Update Packets: Transmits routing information updates reliably (unicast or multicast) using RTP.
- Query Packets: Sent to request routing information when a route fails without a Feasible Successor.
- Reply Packets: Sent in response to Query packets to confirm route availability.
- ACK Packets: Unicast acknowledgments confirming receipt of Update, Query, or Reply packets.
EIGRP Security & Best Practices
- MD5 / HMAC-SHA Authentication: Prevents unauthorized routers from injecting rogue routes by validating MD5 keys on neighbor interfaces.
- Passive-Interface: Prevents sending Hello messages on user-facing LAN subnets while maintaining network advertisements.
- Automatic Summarization: Disabling
no auto-summary is recommended in legacy implementations to avoid classful routing boundary conflicts.
Where to Use EIGRP?
- Enterprise Campus Topologies: Large-scale, rapidly converging networks with Cisco-centric hardware.
- Hub-and-Spoke WAN / DMVPN Networks: Ideal for scalable dynamic routing over multipoint IPsec VPN topologies.