Enhanced Interior Gateway Routing Protocol (EIGRP)

2.1 Explain characteristics of routing technologies

Dynamic routing

📘CompTIA Network+ (N10-009)

EIGRP is a dynamic routing protocol used inside a network (an Interior Gateway Protocol – IGP) that helps routers automatically share routing information to efficiently move data across a network. It is an enhanced version of the older IGRP developed by Cisco.

Key Features of EIGRP

Protocol Type
- EIGRP is a hybrid routing protocol.
  - It’s called “hybrid” because it has features of both distance-vector and link-state protocols.
  - Distance-vector features: Routers share information about their distance to networks.
  - Link-state features: Routers also know the state of links (bandwidth, delay) to make smarter routing decisions.
Metric Used
- EIGRP uses a composite metric to determine the best path to a network.
  - Metrics considered include:
    - Bandwidth: The speed of the link.
    - Delay: Time it takes for a packet to traverse the path.
    - Optional: Load and Reliability (rarely used in modern EIGRP).
- Formula simplifies decision-making, so routers pick the path that is fastest and most reliable.
Routing Updates
- EIGRP does not flood the network constantly like some older protocols.
- Instead, it uses incremental updates:
  - Routers only send updates when there is a change in the network topology.
  - This reduces unnecessary network traffic and improves efficiency.
Neighbor Discovery
- EIGRP routers establish neighbor relationships using Hello packets.
- They maintain a neighbor table that tracks connected routers running EIGRP.
Topology Table
- Each router maintains a topology table with all possible routes to a destination, not just the best one.
- From this, the router selects the best path for the routing table.
DUAL Algorithm
- EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate:
  - The best route to a network.
  - A backup route (feasible successor) in case the primary route fails.
- This allows fast convergence, meaning the network quickly adapts to changes without downtime.
Supports IPv4 and IPv6
- EIGRP can work with IPv4 networks and IPv6 networks, making it versatile for modern IT environments.
Classless Routing
- EIGRP is classless, meaning it supports CIDR (Classless Inter-Domain Routing) and can handle subnets of varying sizes.

Advantages of EIGRP

Fast convergence: Quickly adapts when a link goes down.
Efficient updates: Only sends changes instead of full routing tables.
Loop-free: Uses DUAL to prevent routing loops.
Scalable: Works well in both small and large networks.
Flexible: Can route multiple protocol types (IP, IPX, AppleTalk – though IP is most common today).

How EIGRP Works in IT Networks

Neighbor Discovery:
Routers on the same network send Hello packets to find other EIGRP routers.
Building the Topology Table:
Each router stores all known routes from its neighbors.
Choosing the Best Path:
The DUAL algorithm calculates which route is fastest and most reliable based on bandwidth and delay.
Updating Routing Tables:
Only when there’s a change in the network, EIGRP sends incremental updates to neighbors, keeping routing tables current.
Failover:
If the primary path fails, the router uses a feasible successor from the topology table, minimizing downtime.

EIGRP Packet Types

EIGRP uses five types of packets to communicate:

Hello – Discover and maintain neighbors.
Update – Share routing information.
Query – Ask neighbors for a route to a network.
Reply – Respond to a query.
Acknowledgment – Confirm receipt of updates.

EIGRP Metrics Example in IT Terms

Suppose a network has multiple routers connecting a server to different branches.
EIGRP considers the link speed (bandwidth) and delay to decide the best path for traffic.
If a high-speed link goes down, EIGRP quickly switches to the next best path without manual intervention.

EIGRP vs Other Routing Protocols

Feature	EIGRP	RIP	OSPF
Type	Hybrid	Distance-vector	Link-state
Metric	Bandwidth + Delay	Hop count	Cost (bandwidth)
Convergence	Fast	Slow	Fast
Updates	Incremental	Full table every 30s	Incremental, LSA-based
Loop Prevention	DUAL	Split horizon	SPF algorithm
Protocol Support	IPv4 & IPv6	IPv4 only	IPv4 & IPv6

Exam Tips for Network+

Know that EIGRP is Cisco proprietary, but some non-Cisco devices support it in modern networks.
Remember DUAL ensures loop-free, fast convergence.
Incremental updates make it efficient compared to RIP.
It’s classless, supports IPv4 and IPv6, and uses composite metrics (bandwidth + delay).

✅ Summary for Students:
EIGRP is a fast, efficient, and reliable internal routing protocol. It discovers neighbors, calculates best paths using DUAL, maintains backup routes, and only shares updates when the network changes. It’s designed for scalable, modern IP networks.