Overview
This lecture covers Enhanced Interior Gateway Routing Protocol (EIGRP), a Cisco-developed dynamic routing protocol emphasizing key features, operation, metrics, tables, and configuration.
EIGRP Basics and History
- EIGRP is an advanced distance-vector routing protocol for automating network routing decisions.
- Developed by Cisco and initially proprietary, with limited open-version for other vendors since 2013.
- Superseded IGRP in 1993 due to support for classless IPv4 addressing.
Operation and Core Features
- Used within an autonomous system to share route information among routers.
- Sends only incremental routing updates, reducing network and CPU load.
- Does not periodically send the whole routing table, only changes.
- Considers a hybrid protocol due to some link-state-like behaviors.
EIGRP Tables
- Neighbor Table: Lists directly connected eigrp routers.
- Topology Table: Maintains EIGRP-learned routes and their metrics, classifying them as passive or active.
- Routing Table: Only usable, loop-free routes from the topology table are inserted here.
Fundamental Concepts
- Adjacency formed when two EIGRP routers exchange their full routing tables.
- Uses five packet types: Hello, Update, Query, Reply, and Acknowledgement.
- Routes can be in passive (stable) or active (searching for a new path) states.
Metrics and Calculation
- Path value determined by bandwidth, load, delay, reliability, and MTU (latter not used in metric).
- Combined via a weighted formula using K-values; by default, only delay and bandwidth are used.
- Metric scaling uses a factor (256) for backward compatibility with IGRP.
Loop Prevention and Path Selection
- Successor: best path for a destination.
- Feasible Successor: backup, loop-free path.
- Feasibility condition: a neighbor's reported distance is less than router's own feasible distance.
- Successors and feasible successors are kept as possible paths; their total number can be configured.
Protocol Technicals
- Default administrative distance is 90 (internal), 170 (external).
- Runs directly over IP using Cisco's Reliable Transport Protocol (RTP), not TCP/UDP.
- Multicast address for EIGRP is 224.0.0.10, protocol number 88.
Advanced Features
- Supports unequal-cost load balancing, allowing variance setting up to 128.
- Offers authentication (MD5, SHA-2), protocol-dependent modules for IP, IPv6, IPX, AppleTalk.
- Periodically checks route availability and propagates changes.
Compatibility and Standardization
- Cisco published EIGRP details in RFC 7868, but omitted some features, limiting full multi-vendor interoperability.
- Partial support in FRRouting; generally unsupported outside Cisco environments.
Key Terms & Definitions
- EIGRP — Enhanced Interior Gateway Routing Protocol, a dynamic Cisco routing protocol.
- DUAL — Diffusing Update Algorithm, used by EIGRP for loop-free path calculation.
- Successor — Primary next-hop route for a destination.
- Feasible Successor — Backup route, guaranteed loop-free.
- K-values — Modifiers/factors in EIGRP metric calculation.
- Variance — Multiplier for including unequal cost paths in load balancing.
- Autonomous System (AS) — Group of routers managed under a single administrative domain.
Action Items / Next Steps
- Review configuration commands for EIGRP setup.
- Practice calculating EIGRP metrics using K-values, bandwidth, and delay.
- Study EIGRP adjacency and topology table behavior in lab simulations.