Day 25 EIGRP

EIGRP = Enhanced Interior Gateway Routing Protocol

• Advanced distance-vector routing protocol (sometimes called "hybrid")

• Originally Cisco proprietary (now partially open standard - RFC 7868)

• Used to share routing information between routers within an organization

• Successor to IGRP (Interior Gateway Routing Protocol)

Advantages:

✓ Very fast convergence (fastest among IGPs)

✓ Low bandwidth usage (partial updates only)

✓ Supports unequal-cost load balancing

✓ Easy to configure

✓ Supports large networks

✓ Loop-free paths guaranteed by DUAL

✓ Supports multiple network layer protocols

Disadvantages:

✗ Originally Cisco proprietary (limited multi-vendor support)

✗ More complex than RIP

✗ Can be difficult to troubleshoot

Step 1: Discover Neighbors

Router A Router B

| |

|-------- Hello Packet --------->|

|<------- Hello Packet ----------|

| |

| Neighbors Established |

Step 2: Exchange Routes

Router A Router B

| |

|-------- Update Packet -------->|

|<------- Update Packet ---------|

| |

| Routes Exchanged |

Step 3: Choose Best Routes

DUAL Algorithm calculates:

- Successor (best route)

- Feasible Successor (backup route)

Step 4: Maintain Relationships

- Hello packets sent every 5 seconds (LAN)

- Hello packets sent every 60 seconds (WAN)

- If no Hello received within Hold Time, neighbor is dead

EIGRP maintains THREE tables:

Table 1: Neighbor Table

- Lists all directly connected EIGRP neighbors

- Built using Hello packets

- Contains: Neighbor IP, Interface, Hold Time, Uptime, Queue Count

View command:

Router# show ip eigrp neighbors

Example output:

IP-EIGRP neighbors for process 100

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 192.168.1.2 Gi0/0 12 00:05:32 10 100 0 15

1 192.168.2.2 Gi0/1 14 00:03:21 15 100 0 12

Table 2: Topology Table

- Contains ALL routes learned from neighbors

- Includes Successor and Feasible Successor routes

- Used by DUAL algorithm to calculate best paths

View command:

Router# show ip eigrp topology

Example output:

IP-EIGRP Topology Table for AS(100)

P 10.1.1.0/24, 1 successors, FD is 28160

via 192.168.1.2 (28160/25600), GigabitEthernet0/0

via 192.168.2.2 (30720/28160), GigabitEthernet0/1

P 10.2.2.0/24, 1 successors, FD is 25600

via Connected, GigabitEthernet0/0

Table 3: Routing Table

- Contains only the BEST routes (Successors)

- These routes are actually used for forwarding traffic

- Shared with other routing protocols

View command:

Router# show ip route eigrp

Example output:

D 10.1.1.0/24 [90/28160] via 192.168.1.2, 00:05:32, Gi0/0

D 10.3.3.0/24 [90/30720] via 192.168.2.2, 00:03:21, Gi0/1

D 10.4.4.0/24 [90/33280] via 192.168.1.2, 00:05:32, Gi0/0

Successor

- The BEST route to a destination

- Stored in BOTH Topology Table AND Routing Table

- Used for actual packet forwarding

Feasible Successor (FS)

- BACKUP route to a destination

- Stored ONLY in Topology Table

- Used immediately if Successor fails (instant convergence!)

Feasible Distance (FD)

- Total metric from local router to destination

- FD = Local metric + Neighbor's metric

Reported Distance (RD) / Advertised Distance (AD)

- Metric from neighbor to destination

- What the neighbor reports as its distance

Feasibility Condition

A route becomes a Feasible Successor if:

Reported Distance < Feasible Distance of Successor

This GUARANTEES a loop-free backup path!

Example:

FD = 30

Router A ---------> Destination

|

| RD = 20 (from Router B)

v

Router B ---------> Destination

Since RD (20) < FD (30), Router B's path is a Feasible Successor

Default Metric Components:

EIGRP uses K values to calculate metrics:

Default Formula (K1=1, K2=0, K3=1, K4=0, K5=0):

Metric = 256 × [(10^7 / Minimum Bandwidth) + (Sum of Delays / 10)]

Simplified:

Metric = 256 × (10,000,000 / Bandwidth in Kbps) + (Delay in microseconds / 10)

Example Calculation:

Link: FastEthernet (100 Mbps, Delay = 100 microseconds)

Bandwidth component = 10,000,000 / 100,000 = 100

Delay component = 100 / 10 = 10

Total = 256 × (100 + 10) = 256 × 110 = 28,160

Hello Packets:

- Sent periodically to discover and maintain neighbors

- Default intervals:

- LAN (Fast links): Every 5 seconds

- WAN (Slow links): Every 60 seconds

- Hold Time = 3 × Hello Interval

- LAN: 15 seconds

- WAN: 180 seconds

Update Packets:

- Contain routing information

- Sent when:

- New neighbor discovered (full update)

- Route changes (partial update)

- Require acknowledgment (reliable)

Query Packets:

- Sent when Successor is lost and no Feasible Successor exists

- Asks neighbors: "Do you have a route to this network?"

- Route goes into ACTIVE state

Reply Packets:

- Response to Query packets

- Contains route information or indicates no route available

For two routers to become EIGRP neighbors, they must match:

✓ Same Autonomous System (AS) number

✓ Same subnet

✓ Hello packets must be received

✓ Same K values (metric weights)

✓ Authentication must match (if configured)

Basic Configuration:

Topology:

10.0.0.0/24 20.0.0.0/24

[PC1]-------[R1]------------------[R2]-------[PC2]

Gi0/0 Gi0/1 Gi0/0 Gi0/1

10.0.0.1 192.168.1.1 192.168.1.2 20.0.0.1

| |

|192.168.1.0/24|

Router R1 Configuration:

R1> enable

R1# configure terminal

! Step 1: Enable EIGRP with AS number

R1(config)# router eigrp 100

! Step 2: Disable auto-summary (recommended)

R1(config-router)# no auto-summary

! Step 3: Specify networks to advertise

R1(config-router)# network 10.0.0.0

R1(config-router)# network 192.168.1.0

! Step 4: (Optional) Set Router ID

R1(config-router)# eigrp router-id 1.1.1.1

R1(config-router)# exit

R1(config)# exit

R1# write

Router R2 Configuration:

R2> enable

R2# configure terminal

R2(config)# router eigrp 100

R2(config-router)# no auto-summary

R2(config-router)# network 20.0.0.0

R2(config-router)# network 192.168.1.0

R2(config-router)# eigrp router-id 2.2.2.2

R2(config-router)# exit

R2(config)# exit

R2# write

Using Wildcard Masks (More Specific):

R1(config)# router eigrp 100

R1(config-router)# no auto-summary

R1(config-router)# network 10.0.0.0 0.0.0.255

R1(config-router)# network 192.168.1.0 0.0.0.255

Wildcard Mask Explanation:

Wildcard Mask = Inverse of Subnet Mask

Subnet Mask: 255.255.255.0

Wildcard Mask: 0.0.0.255

0 = Must match exactly

255 = Can be any value

Check EIGRP Neighbors:

R1# show ip eigrp neighbors

IP-EIGRP neighbors for process 100

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 192.168.1.2 Gi0/1 13 00:10:25 5 100 0 8

Column Explanations:

Check EIGRP Topology:

R1# show ip eigrp topology

EIGRP-IPv4 Topology Table for AS(100)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - reply Status, s - sia Status

P 10.0.0.0/24, 1 successors, FD is 2816

via Connected, GigabitEthernet0/0

P 20.0.0.0/24, 1 successors, FD is 3072

via 192.168.1.2 (3072/2816), GigabitEthernet0/1

P 192.168.1.0/24, 1 successors, FD is 2816

via Connected, GigabitEthernet0/1