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EIGRP

EIGRP

Enhanced Interior Gateway Routing Protocol

1. What is EIGRP?2. How EIGRP Works3. EIGRP Components4. Neighbor States5. Packet Types6. Route Types7. Metrics (K Values)8. Timers (Default)9. DUAL Algorithm10. Configuration11. Important Commands12. Authentication13. Advanced Concepts

1. What is EIGRP?

EIGRP is a Cisco proprietary Interior Gateway Protocol (IGP) that uses a hybrid of Distance Vector and Link-State concepts. It uses DUAL (Diffusing Update Algorithm) for loop-free routes and fast convergence. IP protocol number 88.

2. How EIGRP Works

  • •Routers discover neighbors using Hello packets
  • •Maintains a topology table with all routes
  • •DUAL algorithm computes loop-free routes
  • •Only incremental updates are sent (efficient)
  • •Best path installed in the routing table

3. EIGRP Components

ComponentDescription
NeighborsDiscovered by Hello packets
Topology TableContains all learned routes
Routing TableBest routes installed
DUALDiffusing Update Algorithm — loop-free path calculation
Feasible SuccessorBackup path (loop-free)
Reported Distance (RD)Distance to destination as reported by neighbor
Feasible Distance (FD)RD + Link Cost to the neighbor

4. Neighbor States

StateDescription
DownNo Hello received
InitHello received, adjacency not yet established
2-WayBi-directional communication established
UpFull adjacency formed

5. Packet Types

PacketPurpose
HelloDiscover and maintain neighbors
UpdateSend routing updates
QuerySend when route is lost
ReplyReply to a Query
ACKAcknowledge packets

6. Route Types

Route TypeADDescription
Internal90Within the same AS
External170Redistributed from another protocol
Summary—Manually or automatically summarized

7. Metrics (K Values)

Metric = [K1*BW + (K2*BW)/(256-Load) + K3*Delay] * [K5 / (Reliability + K4)]
K ValueDefaultDescription
K11Bandwidth (in kbps)
K20Load (in 1/255ths)
K31Delay (in tens of microseconds)
K40Reliability (percentage)
K50Reliability weight

Default

Bandwidth + Delay (K1=1, K3=1, others=0). Lower metric = preferred.

8. Timers (Default)

TimerDefault
Hello Interval5 sec (LAN), 15 sec (WAN)
Hold Time15 sec (LAN), 45 sec (WAN)
Update Interval (Min)0.5 sec
Update Interval (Max)3 min
Active Timer3 min

9. DUAL Algorithm

  • •Core of EIGRP
  • •Computes loop-free, best path to destination
  • •Provides fast convergence
  • •Uses feasible successors for backup paths
  • •Successor = Best path, Feasible Successor = Backup

10. Configuration

Router R1
R1(config)# router eigrp 100
R1(config-router)# network 192.168.1.0 0.0.0.255
R1(config-router)# network 192.168.2.0 0.0.0.255
R1(config-router)# no auto-summary
Router R2
R2(config)# router eigrp 100
R2(config-router)# network 192.168.2.0 0.0.0.255
R2(config-router)# network 192.168.3.0 0.0.0.255
R2(config-router)# no auto-summary

11. Important Commands

CommandPurpose
show ip eigrp neighborsShow neighbors
show ip eigrp topologyShow Topology Table
show ip route eigrpShow EIGRP routes
show ip protocolsShow EIGRP process info
show ip eigrp interfacesShow interfaces
show ip eigrp trafficShow traffic statistics
debug ip eigrp packetsDebug EIGRP packets
clear ip eigrp neighborsClear neighbor adjacency

12. Authentication

EIGRP supports MD5 authentication. Configured under interface mode.

interface g0/0
 ip authentication mode eigrp 100 md5
 ip authentication key-chain eigrp EIGRP-KEY
 key chain EIGRP-KEY
  key-string cisco123

13. Advanced Concepts

ConceptDescription
VarianceAllows multiple paths (up to 6) for load balancing
Stub RouterReceives only a default route to reduce updates
Passive InterfaceDoes not form neighbor adjacency (reduces overhead)
Route RedistributionEIGRP can redistribute routes from other protocols
EIGRP for IPv6Named mode eigrp <AS> is used (same concepts)
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