EIGRP Configuration and Advanced Concepts

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco proprietary advanced distance vector routing protocol. It combines the best features of link-state and distance vector protocols, offering rapid convergence, flexibility, and robust scalability for enterprise networks.

Introduction to EIGRP

Fast convergence using Diffusing Update Algorithm (DUAL)
Supports VLSM and CIDR
Equal and unequal-cost load balancing
Efficient, scalable, and suitable for Cisco-only environments

Use Case: EIGRP is ideal where rapid convergence and advanced routing features are needed in all-Cisco deployments.

Basic EIGRP Configuration

Enable EIGRP on the router (specify AS number):
Router(config)# router eigrp 100
100 is the Autonomous System (AS) number.
Configure Network Statements:
Router(config-router)# network 10.0.0.0 0.0.0.255
Network statements use wildcard masks.
Set Router IDs (recommended for stability):
Router(config-router)# eigrp router-id 1.1.1.1

EIGRP Metrics and Calculation

Composite metric uses bandwidth, delay, load, and reliability (by default, only bandwidth and delay are used).

  Formula:

  Metric = [(107 / lowest bandwidth) + cumulative delay] × 256

K-values influence the metric calculation. By default, K1 and K3 are 1 (bandwidth and delay); others are 0.
Exam Tip: K-values must match between neighbors for adjacency!

EIGRP Neighbor Discovery and Adjacency

Hello Timer: How often routers send hello packets (default: 5s LAN, 60s WAN)
Hold Timer: Time to wait before declaring neighbor down (default: 15s LAN, 180s WAN)
Neighbor Relationship Requirements:
- Same AS number
- Matching K-values
- Authentication (if enabled)
- Common network/subnet

Router# show ip eigrp neighbors

Example: John configures EIGRP AS 100 on two routers; both become neighbors after matching key settings.

EIGRP Topology Table

Successor: Best route to a destination
Feasible Successor: Backup route for fast failover
DUAL: Loop-free, efficient path computation

Router# show ip eigrp topology

Exam Tip: EIGRP supports fast failover using feasible successors!

Route Summarization in EIGRP

Automatic Summarization: Enabled by default in older IOS, disables with:
Router(config-router)# no auto-summary
Manual Summarization:
Router(config-if)# ip summary-address eigrp 100 192.168.0.0 255.255.252.0
Use to reduce routing table size and advertisements at network boundaries.

EIGRP Authentication

Purpose: Prevents unauthorized routers from joining the EIGRP domain.
How: Use MD5 authentication and key chains.

  Router(config-if)# ip authentication mode eigrp 100 md5

  Router(config-if)# ip authentication key-chain eigrp 100 MYCHAIN

Create the key chain:

  Router(config)# key chain MYCHAIN

  Router(config-keychain)# key 1

  Router(config-keychain-key)# key-string MySecret

Exam Tip: Authentication settings must match for neighbors to form relationships.

Load Balancing in EIGRP

Equal-cost Load Balancing: Default, traffic split over same-metric routes.
Unequal-cost Load Balancing: Use variance to allow traffic over different-metric routes.

Router(config-router)# variance 2

Example: EIGRP uses both WAN links if their metrics are within the set variance.

EIGRP Stub Routing

Purpose: Limits query scope for edge/branch routers, reducing traffic and improving convergence in hub-and-spoke topologies.

Router(config-router)# eigrp stub

Advanced Timers and Tuning

Adjusting Hello/Hold Timers: Enables faster failure detection, at the cost of higher CPU use.

  Router(config-if)# ip hello-interval eigrp 100 1

  Router(config-if)# ip hold-time eigrp 100 3

EIGRP for IPv6

Uses ipv6 router eigrp command
Must enable EIGRP on each interface
Router ID must be set manually

  Router(config)# ipv6 unicast-routing

  Router(config)# ipv6 router eigrp 100

  Router(config-rtr)# router-id 2.2.2.2

  Router(config)# interface GigabitEthernet0/0

  Router(config-if)# ipv6 eigrp 100

Troubleshooting EIGRP

Mismatched AS numbers
K-value mismatches
Authentication/key-chain issues
Missing network statements

Useful Commands:

  show ip eigrp neighbors

  show ip eigrp topology

  show ip route eigrp

  debug eigrp packets

Redistribution with EIGRP

Purpose: Share routes between EIGRP and other routing protocols.
How: Use redistribute with optional metrics or route-maps.

Router(config-router)# redistribute ospf 1 metric 10000 100 255 1 1500

EIGRP Features and Enhancements

QoS Support: EIGRP can factor bandwidth/delay for QoS-aware routing.
Multiprotocol EIGRP: Supports legacy protocols (rarely used now).

Summary Table: Key EIGRP Commands

Purpose	Command Example
Start EIGRP (AS 100)	router eigrp 100
Network statement	network 10.0.0.0 0.0.0.255
Set router ID	eigrp router-id 1.1.1.1
Disable auto-summary	no auto-summary
Manual summary	ip summary-address eigrp 100 10.1.0.0 255.255.0.0
Authentication	ip authentication mode eigrp 100 md5
Stub configuration	eigrp stub
Load balancing (variance)	variance 2
Show neighbors	show ip eigrp neighbors

Exam Tips

Understand EIGRP metric calculation and DUAL basics.
Practice neighbor relationship setup and troubleshooting.
Be familiar with stub routing, authentication, and summarization.
Remember the main show/debug commands for EIGRP troubleshooting.

EIGRP Configuration and Advanced Concepts Quiz

1. What type of routing protocol is EIGRP?

A. Link-state only B. Cisco proprietary advanced distance vector (hybrid) C. Path-vector D. Open standard distance vector

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

2. What is the default Autonomous System (AS) number used in the example to enable EIGRP?

A. 1 B. 10 C. 100 D. 200

Correct answer is C. The AS number 100 is used as an example for EIGRP configuration.

3. Which two metric components does EIGRP use by default for route calculation?

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

Correct answer is A. EIGRP metric formula uses bandwidth and delay by default (K1 and K3 set to 1).

4. What is the purpose of the EIGRP 'variance' command?

A. Enable authentication B. Configure the hello interval C. Set the Autonomous System number D. Allow unequal-cost load balancing

Correct answer is D. The 'variance' command enables traffic to be load-balanced over routes with unequal metrics.

5. Which EIGRP packet type is used to discover neighbors?

A. Update B. Hello C. Query D. Acknowledgment

Correct answer is B. Hello packets are used by EIGRP to discover and maintain neighbor relationships.

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

A. 5 seconds B. 10 seconds C. 15 seconds D. 60 seconds

Correct answer is A. By default, EIGRP sends hello packets every 5 seconds on LANs.

7. Why would you disable automatic summarization in EIGRP?

A. To enable authentication B. To increase convergence time C. To control route summarization and advertise specific subnets D. To enable unequal-cost load balancing

Correct answer is C. Disabling auto-summary allows manual control over summary routes, avoiding unintended aggregation.

8. What is the purpose of EIGRP stub routing?

A. To redistribute static routes B. To increase hello interval C. To disable neighbor relationships D. To limit query scope and reduce traffic on branch routers

Correct answer is D. Stub routing reduces query traffic and speeds convergence on edge routers.

9. Which command enables MD5 authentication for EIGRP on an interface?

A. ip authentication key-chain eigrp 100 MYKEY B. ip authentication mode eigrp 100 md5 C. router eigrp 100 authentication md5 D. ip eigrp authentication md5

Correct answer is B. The command configures MD5 authentication mode for EIGRP on the interface.

10. What must match between EIGRP neighbors for adjacency to form?

A. Router ID B. IP address C. Autonomous System number and K-values D. Interface MTU size only

Correct answer is C. EIGRP neighbors must share the same AS number and matching K-values to form adjacency.

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