EIGRP Packet Format

In this article we are going to discuss EIGRP Packet Format in detail. Enhanced Interior Gateway Routing Protocol (EIGRP) incorporates five packet types that are Hello, Update, Query, Reply, ACK for its operation. Each of these five EIGRP Packets has a common header fields and therefore it is important to know the EIGRP Packet format and its related fields in order to understand EIGRP operation. EIGRP Packet Header and its fields are shown in the picture below.

EIGRP Packet Header Fields

Version: This field specifies different versions of EIGRP. Version 2 of EIGRP was implemented beginning with Cisco IOS Software Releases 10.3(11), 11.0(8), and 11.1(3). EIGRP Version 2 is the most recent version that contains many enhancements to improve the stability and scalability of EIGRP.
OPCode: This field specifies the types of EIGRP packet contained. OPCode 1 is the Update packet, OPCode 3 is the Query packet, OPCode 4 is the Reply Packet, and OPCode 5 is the EIGRP Hello packet and EIGRP ACK Packet.
Checksum: This field is used as the regular IP checksum, calculated based on the entire EIGRP packet, excluding the IP header.
Flags: This field involves only two flags now. The flag indicates either an init for new neighbor relationship or the conditional receive for EIGRP RTP (Reliable Transport Protocol).
Sequence: Specifies the sequence number used by the EIGRP RTP.
Acknowledgment: Used to acknowledge the receipt of an EIGRP reliable packet.
Autonomous System Number: AS Number field specifies the number for the identification of EIGRP domain.

EIGRP packets might also contain some TLVs (Type, Length, Value). These TLVs are placed after the AS number Field in EIGRP header. The TLV Triplets carry route entries, as well as provide the fields for DUAL process management. Some of common TLVs are listed below:

a TLV to carry EIGRP K parameters or EIGRP K Values (seen in Hello packet)
a TLV to carry IP internal routes (seen in Update packet)
a TLV to carry IP External routes (seen in Update packet)

EIGRP K Parameters TLV

EIGRP K Parameters TLV - Wireshark Capture

EIGRP IP Internal Route TLV

EIGRP IP Internal Route TLV Fields

Next hop: This field specifies the IP address of the next hop to which packets should be forwarded.
Delay: This field specifies the Delay parameter of the route metric. The delay value is the sum of all the delay parameters on the interface across the path to the destination network. Delay is one of main parameters used to calculated EIGRP Metric.
Bandwidth: This field specifies the Bandwidth parameter of the route metric. Bandwidth is the another main parameters used to calculated EIGRP Metric. The bandwidth is obtained from the interface, and it is the lowest bandwidth on the interface across the path to the destination network.
MTU: This field specifies the interface MTU parameter of the route metric.
Hop count: This field specifies the number of hops to the destination network.
Reliability: This field specifies the reliability of the interface, out of a possible range of 1 to 255. A reliability of 1 indicates that the reliability is 1/255, whereas a reliability of 255 indicates that the interface is 100 percent reliable.
Load: This field specifies the load of the interface, out of a possible range of 1 to 255. A load value of 1 indicates that the interface has a very light load, while a load value of 255 indicates that the interface is highly saturated.
Prefix length: This field specifies the subnet mask of the destination network.
Originating router: This field specifies the router ID of the router that originates the external EIGRP routes.
Originating autonomous system number: This field specifies the EIGRP autonomous system number of the routes before getting redistributed into this EIGRP autonomous number.
External protocol metric: This field specifies the metric of the routes before getting redistributed into EIGRP.
External protocol ID: This field specifies the type of routing protocol that originates the routes that were redistributed into EIGRP. The routing protocol type can be BGP, OSPF, RIP, IGRP, and so forth.

EIGRP IP Internal Route TLV - Wireshark Capture

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I hope you have found this article informative and useful and now have a fair understanding of EIGRP Packet Format, EIGRP Packet Header, related fields and EIGRP TLVs. For any of the related queries or feedback, kindly write to us at networkurge@gmail.com

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