EIGRP Header Format

EIGRP Protocol Structure

EIGRP uses IP protocol 88 and includes a common header format for all packet types. Understanding the header structure is essential for packet analysis and troubleshooting.

EIGRP Header Structure

Every EIGRP packet begins with a 20-byte header that contains common fields for all packet types:

EIGRP Header Format (20 bytes)


 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Version    |    Opcode     |          Checksum             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Flags                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Acknowledgment Number                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Virtual Router ID         |       Autonomous System       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field Lengths:
- Version: 8 bits
- Opcode: 8 bits  
- Checksum: 16 bits
- Flags: 32 bits
- Sequence Number: 32 bits
- Acknowledgment Number: 32 bits
- Virtual Router ID: 16 bits
- Autonomous System: 16 bits

Header Field Descriptions

Field	Size	Purpose	Values
Version	8 bits	EIGRP version number	2 (current version)
Opcode	8 bits	Packet type identifier	1=Update, 3=Query, 4=Reply, 5=Hello
Checksum	16 bits	Error detection	IP checksum algorithm
Flags	32 bits	Packet flags	Init, Conditional Receive, etc.
Sequence Number	32 bits	Reliable transport	Incremental for reliable packets
Acknowledgment Number	32 bits	Reliable transport	Last received sequence number
Virtual Router ID	16 bits	Multiple instances	0 for single instance
Autonomous System	16 bits	AS number	1-65535

Packet Type Opcodes

Opcode	Packet Type	Reliability	Purpose
1	Update	Reliable	Routing information exchange
3	Query	Reliable	Route recomputation request
4	Reply	Reliable	Response to Query
5	Hello	Unreliable	Neighbor discovery/keepalive
6	IPX SAP	Reliable	IPX Service Advertisement

Reliable Transport Protocol (RTP)

EIGRP Reliability

EIGRP implements its own reliable transport protocol to ensure critical packets are delivered:

Sequence Numbers: Track packet order and detect duplicates
Acknowledgments: Confirm packet reception
Retransmission: Resend unacknowledged packets
Selective Reliability: Only critical packets need acknowledgment

Packet Encapsulation

EIGRP Packet Structure

Ethernet Header
(14 bytes)

IP Header
(20 bytes, Protocol=88)

EIGRP Header
(20 bytes)

EIGRP Data
(Variable length)

Hello Packets

The Foundation of EIGRP

Hello packets are the foundation of EIGRP neighbor relationships. They handle neighbor discovery, maintain relationships, and exchange essential parameters.

Hello Packet Purpose

Neighbor Discovery

Find other EIGRP routers on the network segment

Keepalive

Maintain existing neighbor relationships

Parameter Exchange

Share timers, K-values, and capabilities

Failure Detection

Detect neighbor failures through timeouts

Hello Timers

Interface Type	Hello Interval	Hold Time	Reasoning
High-bandwidth	5 seconds	15 seconds	Fast failure detection
Low-bandwidth	60 seconds	180 seconds	Bandwidth conservation
Multipoint	60 seconds	180 seconds	Reduce overhead

Update Packets

The Heart of EIGRP

Update packets carry routing information and are the primary mechanism for topology exchange in EIGRP. They use reliable transport and contain route advertisements.

Update Packet Purpose

Route Advertisement

Advertise reachable networks to neighbors

Topology Exchange

Share network topology information

Metric Propagation

Distribute path metrics and components

Route Withdrawal

Remove unreachable networks

Update Packet Characteristics

Characteristic	Value	Description
Opcode	1	Update packet identifier
Reliability	Reliable	Requires acknowledgment
Delivery	Unicast/Multicast	Depends on scenario
Sequence Number	Non-zero	Incremental for reliability
Flags	Init, EOT	Control packet flow

Query Packets

DUAL Query Process

Query packets are sent when a router goes active for a destination and needs to find alternate paths. They're a crucial part of the DUAL algorithm's loop prevention mechanism.

Query Packet Purpose

Route Recomputation

Request alternate paths when primary route fails

DUAL Process

Essential part of diffusing computation

Loop Prevention

Ensure loop-free alternate paths

Network Convergence

Coordinate network-wide route changes

When Queries Are Sent

Scenario	Trigger	Action	Query Scope
Successor Loss	Primary path fails	No feasible successor available	All neighbors except source
Metric Increase	Path cost increases	Exceeds feasible successor threshold	Affected neighbors
Interface Down	Interface failure	Direct connection lost	All neighbors
Neighbor Loss	Neighbor goes down	Routes through neighbor lost	Remaining neighbors

Reply Packets

Query Response

Reply packets are sent in response to Query packets and complete the DUAL diffusing computation process. They provide route information or indicate route unavailability.

Reply Packet Purpose

Query Response

Respond to neighbor queries with route information

Route Information

Provide alternate path metrics or unreachable status

DUAL Completion

Complete the diffusing computation process

Convergence

Enable network convergence after topology changes

Reply Types

Reply Type	Meaning	Metric	Impact
Route Available	Neighbor has alternate path	Actual path metric	Potential new successor
Route Unreachable	Neighbor cannot reach destination	Infinite (0xFFFFFFFF)	No help from this neighbor

ACK Packets

Reliable Transport

ACK packets provide reliable transport for EIGRP by acknowledging receipt of reliable packets. They ensure that critical routing information is delivered successfully.

ACK Packet Purpose

Acknowledgment

Confirm receipt of reliable packets

Reliable Transport

Enable EIGRP's reliable delivery mechanism

Flow Control

Manage packet transmission rate

Error Recovery

Trigger retransmission of lost packets

ACK Packet Characteristics

Characteristic	Value	Description
Opcode	5	Same as Hello packet
Reliability	Unreliable	Does not require acknowledgment
Sequence Number	0	Unreliable packets use sequence 0
Acknowledgment Number	Non-zero	Sequence number being acknowledged
Data	None	No TLV data in ACK packets

Reliable Transport Mechanism

Reliable Packet Flow

Send

Reliable packet
transmitted

Receive

Neighbor receives
packet

ACK

ACK packet
sent back

Confirm

Sender receives
ACK

Previous: Chapter 2 - Fundamentals Next: Chapter 4 - DUAL Algorithm