EIGRP

🔸 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?

A. Link-state B. Distance-vector only C. Advanced distance-vector (hybrid) D. Path-vector

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?

A. Bandwidth and delay B. Hop count only C. Cost only D. Load and reliability

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?

A. 224.0.0.9 B. 224.0.0.10 C. 224.0.0.5 D. 224.0.0.6

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?

A. A backup route B. A disconnected route C. A static route D. The best route installed in the routing table

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?

A. RD > FD B. RD = FD C. Reported Distance (RD) < Feasible Distance (FD) D. RD > FD

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?

A. Acknowledgment (ACK) B. Query C. Hello D. Reply

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

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

A. 10 seconds B. 15 seconds C. 30 seconds D. 5 seconds

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?

A. 110 B. 90 C. 120 D. 170

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

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

A. router ospf 100 B. router rip 100 C. router eigrp 100 D. router bgp 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?

A. Autonomous System (AS) number B. Router ID C. Hello timer only D. Interface IP address

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

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