EIGRP OVERVIEW

🔸 What is EIGRP?

EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector or hybrid routing protocol developed by Cisco. It combines the simplicity of distance-vector protocols like RIP with link-state features like OSPF for faster convergence and better scalability.

Note: EIGRP is Cisco proprietary, though an open-standard version was later released.

🔸 EIGRP as a Hybrid Protocol

  • Uses metrics like delay and bandwidth
  • Supports DUAL (Diffusing Update Algorithm) for loop-free convergence
  • Maintains neighbor and topology tables (like OSPF)

🔸 EIGRP Features

Feature Description
Classless Supports VLSM and CIDR
Fast Convergence Thanks to DUAL
Load Balancing Supports equal and unequal-cost paths
Scalability Suitable for large networks
Multicast Uses 224.0.0.10 to communicate

🔸 Key Terminology

Term Description
AS (Autonomous System) Logical grouping of routers with same EIGRP process
Neighbor Router discovered via Hello packets
Successor Best route to a destination
Feasible Successor Backup route satisfying FC
Feasibility Condition RD < FD
RD (Reported Distance) Metric from neighbor
FD (Feasible Distance) Best metric to destination

🔸 EIGRP Metric Calculation

Metric = 256 × [(10^7 / bandwidth) + delay]
  • Bandwidth: Minimum on path (in kbps)
  • Delay: Sum across path (in tens of microseconds)
  • K values: K1=1, K3=1, all others=0

Example: Bandwidth=1000kbps, Delay=1000us → Metric = 2,585,600

🔸 Packet Types

Packet Type Purpose
Hello Neighbor discovery
Update Advertise routes
ACK Confirm receipt
Query Lost route
Reply Respond to query

🔸 Neighbor Discovery

  • Hello every 5s (default LAN)
  • Hold Time = 15s
  • Uses RTP for reliable delivery

🔸 Routing and Topology Tables

  • Routing Table: Stores successors
  • Topology Table: All known paths

🔸 Basic Configuration

Router(config)# router eigrp 100
Router(config-router)# network 192.168.1.0
Router(config-router)# router-id 1.1.1.1

AS numbers must match to form neighbors.

🔸 Verification Commands

Command Description
show ip protocols Shows running protocols
show ip eigrp neighbors Neighbor list
show ip eigrp topology Topology table
show ip route eigrp EIGRP routes

🔸 Timers

Hello: 5s, Hold: 15s — must match for adjacency

🔸 Administrative Distance

Route Type AD
Internal 90
External 170

🔸 Troubleshooting

Issue Cause
No neighbor AS/timer mismatch, passive interface
No route Network not advertised
Stuck in Active No feasible successor
Metric error Bandwidth/delay mismatch

🔸 Optional: Authentication

Router(config-if)# ip authentication mode eigrp 100 md5
Router(config-if)# ip authentication key-chain eigrp 100 EIGRP-KEYS

🔸 EIGRP vs OSPF vs RIP

Feature EIGRP OSPF RIP
Type Hybrid Link-state Distance-vector
Metric BW + Delay Cost Hop count
Convergence Fast Fast Slow
AD 90 110 120
Load Balancing Equal/Unequal Equal Equal

🔸 Optional: EIGRP for IPv6

ipv6 router eigrp 100
interface g0/0
 ipv6 eigrp 100

🔸 When to Use EIGRP

  • Cisco-only networks
  • Enterprises needing fast convergence
  • Unequal-cost load balancing required
  • Classless routing needed

Example: In a campus with mixed-bandwidth paths, EIGRP optimizes utilization using unequal-cost paths.

EIGRP Basic Awareness Quiz

1. What type of routing protocol is EIGRP?

Correct answer is C. EIGRP is a hybrid protocol combining distance-vector and link-state features.

2. Which metric components does EIGRP use by default?

Correct answer is A. EIGRP metric calculation uses bandwidth and delay by default (K1 and K3).

3. What multicast address does EIGRP use for neighbor communication?

Correct answer is B. EIGRP uses multicast 224.0.0.10 to send Hello packets and updates.

4. What is a 'successor' route in EIGRP terminology?

Correct answer is D. The successor is the primary best route installed into the routing table.

5. What condition must a feasible successor meet in EIGRP?

Correct answer is C. The feasibility condition requires RD to be less than FD for backup routes.

6. Which EIGRP packet type is sent to confirm receipt of updates?

Correct answer is A. ACK packets confirm the receipt of EIGRP updates.

7. What is the default hello timer for EIGRP on LAN interfaces?

Correct answer is D. EIGRP Hello packets are sent every 5 seconds on LANs by default.

8. What is the default administrative distance (AD) of an internal EIGRP route?

Correct answer is B. Internal EIGRP routes have an AD of 90.

9. Which command is used to start EIGRP with AS number 100?

Correct answer is C. The command router eigrp 100 starts EIGRP with AS 100.

10. What must match between two EIGRP routers to form neighbors?

Correct answer is A. Both routers must have the same AS number to become EIGRP neighbors.

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