EIGRP Protocol

EIGRP Overview and Features

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol that blends features of distance vector and link-state protocols, often called a "hybrid protocol." It's engineered for fast convergence, efficient bandwidth use, and support for multiple network protocols like IPv4 and IPv6.

Key Features:

• Fast Convergence: Uses the Diffusing Update Algorithm (DUAL) for rapid recovery.
• Loop-Free Routing: Ensures no loops via DUAL and feasible successors.
• Bandwidth Efficiency: Sends partial updates only on changes.
• VLSM/CIDR Support: Enables flexible subnetting.
• Authentication: Supports MD5 and SHA for security.

EIGRP excels in enterprise networks due to its scalability and adaptability.

EIGRP Packet Types

EIGRP uses five packet types:

1. Hello: Discovers neighbors, sent every 5 seconds on LANs.
2. Update: Shares routing info on topology changes.
3. Query: Seeks alternate paths when a route fails.
4. Reply: Responds to Queries with route details.
5. ACK: Confirms receipt of Updates, Queries, or Replies.

Example: A router multicasts Hellos to 224.0.0.10, then sends Updates to new neighbors.

EIGRP Metrics and K-values

EIGRP's metric combines bandwidth, delay, reliability, and load, weighted by K-values (K1–K5). Defaults are K1=1 (bandwidth), K3=1 (delay), K2, K4, K5=0.

Adjust K-values:

router eigrp 100
 metric weights 0 1 0 1 0 0

To include reliability:

metric weights 0 1 0 1 1 0

K-values must match for adjacency.

EIGRP Metric Calculation

Default formula: Metric = [(10^7 / min_bandwidth) + (sum_delay / 10)] × 256

Example 1: 10 Mbps, 10 ms delay:

• Bandwidth: 10^7 / 10000 = 1000
• Delay: 1000 / 10 = 100
• Metric: (1000 + 100) × 256 = 281600

Example 2: T1 (1544 kbps), 20 ms delay:

• Bandwidth: 10^7 / 1544 ≈ 6477
• Delay: 2000 / 10 = 200
• Metric: (6477 + 200) × 256 = 1708032

EIGRP Configuration

Basic (IPv4):

router eigrp 100
 network 192.168.1.0 0.0.0.255
 network 10.0.0.0 0.255.255.255

Advanced: Adjust timers:

interface GigabitEthernet0/0
 ip hello-interval eigrp 100 5
 ip hold-time eigrp 100 15

EIGRP for IPv4 and IPv6

Named mode supports both:

router eigrp MY_EIGRP
 address-family ipv4 unicast autonomous-system 100
  network 192.168.1.0 0.0.0.255
 exit-address-family
 address-family ipv6 unicast autonomous-system 100
  network 2001:db8::/64
 exit-address-family

EIGRP Neighbor Relationships

Neighbors form when AS, K-values, and authentication align. Check with:

show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address       Interface  Hold Uptime   SRTT RTO  Q  Seq
0   192.168.1.2   Gi0/0      14   00:05:23 10   200  0  5

DUAL Algorithm

DUAL picks the successor (best path) and feasible successors (backups). A feasible successor's RD < FD.

Example: FD=1000, Neighbor B RD=800 (feasible), Neighbor C RD=1200 (not feasible).

Feasibility Condition

RD < FD ensures loop-free backups. RD is the neighbor's metric; FD is the best metric.

Example: FD=1000, RD=800 (feasible), RD=1100 (not feasible).

EIGRP Topology Table

Stores all routes:

show ip eigrp topology
P 192.168.2.0/24, 1 successors, FD is 30720
 via 192.168.1.2 (30720/28160), Gi0/1
 via 192.168.1.3 (40960/28160), Gi0/2

EIGRP Stub Routing

Limits queries:

router eigrp 100
 eigrp stub connected summary

EIGRP Route Summarization

Manual summarization:

router eigrp 100
 no auto-summary
interface Gi0/0
 ip summary-address eigrp 100 10.0.0.0 255.0.0.0

EIGRP Authentication

MD5:

key chain EIGRP_KEY
 key 1
  key-string mysecret
interface Gi0/0
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp 100 EIGRP_KEY

Passive Interfaces

router eigrp 100
 passive-interface Gi0/1

Stops hellos on Gi0/1.

Load Balancing

Unequal-cost with variance:

router eigrp 100
 variance 2

EIGRP Timers

Default: Hello=5s, Hold=15s. Adjust:

interface Gi0/0
 ip hello-interval eigrp 100 10
 ip hold-time eigrp 100 30

Route Filtering

Prefix-list example:

ip prefix-list ALLOW permit 10.0.0.0/8
router eigrp 100
 distribute-list prefix ALLOW in

EIGRP Over Frame Relay/NBMA

router eigrp 100
 neighbor 192.168.1.2 Serial0/0
interface Serial0/0
 bandwidth 1544

Troubleshooting EIGRP

Commands:

• show ip eigrp neighbors
• show ip eigrp topology
• debug eigrp packets

EIGRP Named Mode

router eigrp MY_EIGRP
 address-family ipv4 unicast autonomous-system 100
  network 192.168.1.0 0.0.0.255
  af-interface Gi0/0
   hello-interval 10
  exit-af-interface
 exit-address-family