What

Advanced version of IGRP

Why

Faster convergence by using Diffusing Update Algorithm (DUAL)
Also creates loop-free networks
Only sends updates on topology changes -> more efficient
Supports VLSM and CIDR
Multicast to discover neighbours
ACKs for reliability

Concepts

• Store ALL routes, not only the best ones
  • Allows for failover
• Neighbour table tracks adjacent EIGRP routes and state (similar to NDP)

Election

No election, all routers are equal peers

How

Neighbour Discovery and Adjacency Formation

1. Router boots up

• Router R1 starts EIGRP process for AS 100
• Enabled on interfaces: Gi0/0 and Serial0/0
• Begins sending Hello packets

2. Send Hello packets

R1 sends Hello on Gi0/0:
  Source: 192.168.1.1
  Destination: 224.0.0.10 (EIGRP multicast)
  AS Number: 100
  K-values: K1=1, K2=0, K3=1, K4=0, K5=0
  Hold time: 15 seconds
  
R1 sends Hello on Serial0/0:
  Source: 10.0.1.1
  Destination: 224.0.0.10
  AS Number: 100
  K-values: K1=1, K2=0, K3=1, K4=0, K5=0
  Hold time: 180 seconds (WAN link)

3. R2 receives Hello and responds

R2 receives Hello from R1 on 10.0.1.0/24
Checks parameters:
  - AS number matches? ✅ (both AS 100)
  - K-values match? ✅
  - Authentication matches? ✅ (if configured)
  
R2 sends Hello back to R1:
  Destination: 224.0.0.10
  Contains: R2's parameters

4. Adjacency formation

R1 Neighbor Table:
Neighbor: 10.0.1.2 (R2)
  Interface: Serial0/0
  Hold time: 180 seconds (resets with each Hello)
  Uptime: 00:00:05
  Sequence number: 0
  State: Up
  
R2 Neighbor Table:
Neighbor: 10.0.1.1 (R1)
  Interface: Serial0/0
  Hold time: 180 seconds
  Uptime: 00:00:05
  Sequence number: 0
  State: Up

5. Exchange topology information

• Once adjacency formed, routers exchange full topology
• This is the ONLY time full updates are sent
• Uses reliable delivery (requires ACK)

Route exchange

1. Routers build their own initial tables by calculating metrics for directly connected networks
2. Send update to adjacent peers
3. Send ACK for received updates
4. Process update & calculate metrics for received routes
5. Add routes to routing table

DUAL Algorithm

Out of scope for this course
Meant to create a loop-free, performant network