Types of EIGRP Packet in Computer Network

EIGRP (Enhanced Interior Gateway Routing Protocol) is a distance-vector routing protocol that is used to automate routing decisions and configuration on a computer network. It’s a network protocol that allows routers to exchange data more quickly than prior protocols like the Interior Gateway Routing Protocol (IGRP) or the Border Gateway Protocol (BGP).

EIGRP has port No. 88 that runs on top of IP and is thus defined as a Transport Layer Protocol. BGP, Telnet, FTP, and other application-layer protocols lack a built-in reliability mechanism and rely on TCP (Protocol No. 6) at the Transport layer to ensure reliability. Hello, Update, Query, Reply, and ACK are the five types of EIGRP packets.

IP packets are used by EIGRP to convey routing information. IP packets with the Protocol Number field value 88 (0x58) in the IP header enclose EIGRP packets. EIGRP Packets which are holding the routing control information i.e. Reply, Update & Query are not delivered on regular basis, each trustworthy packet is allocated a sequence number, and an explicit acknowledgment is needed for that sequence number.

EIGRP has its dependability mechanism for acknowledging the receipt of its various sorts of packets, and it uses RTP to deliver or exchange packets between neighbors in a guaranteed and orderly manner.

RTP makes sure that communication between neighboring routers remains constant. As a result, each neighbor has its own retransmission list. This list shows packets that have not yet been acknowledged by a neighbor in the RTO (Round-Trip Timed Out). It’s used to keep track of all the trustworthy packets that haven’t been acknowledged.

Types of EIGRP Packet:

When connecting with other EIGRP routers, EIGRP will use five different packets, which are as follows:

1. Hello
2. Update
3. Query
4. Reply
5. ACK (Acknowledgement)

1. Hello

Before establishing adjacency, this method is used to find a neighbor. EIGRP Hellos are multicast messages with a 0 acknowledgment number. Before sharing EIGRP updates, EIGRP routers must establish neighbor associations.

The purpose of the hello message is to:

1. The “Hello” message is meant to find out who your neighbors are.
2. On whatever sort of link, it is always multicast.
3. The timer is set to 5 seconds and is transmitted regularly.
4. 15 seconds of hold-down timer is there.

2. Update

Used to transmit the converged routes used by a certain router. When a new route is identified or while convergence is complete (the route becomes passive), EIGRP Updates are transmitted as multicasts; when syncing topology tables with neighbors during EIGRP startup, they are sent as unicasts. They are reliably sent across EIGRP routers.

The topology table and routing table are built by sending update messages between neighbors. The OSPF neighbor router uses Update Message to share their prefix with their neighbor. Multicast and Unicast update messages are both possible. The update message on the serial connection is Unicast. 

The Ethernet link, on the other hand, can be both Multicast and Unicast. Update Message’s opcode is 1. It also has an Autonomous Number in it. RTP (Reliable Transport Protocol) was utilized in update messages to ensure packet reliability. Update Message makes use of the Sequence and Acknowledgement Numbers.

3. Query

When DUAL is re-computing a route in which the router does not have a viable successor, it will ask additional EIGRP neighbors for a possible successor. EIGRP Queries are reliably sent as multicasts. Query Packets are delivered when a successor route fails and there is no viable successor in the EIGRP topology database, as the name implies. The Router that has lost the route sends a query message to their neighbor to see if the route is still present in their topology table. In Query Message, you can also set the maximum delay period. When using a serial link, it is always Unicast and when using an Ethernet link, it is always Multicast.

Properties of Query Message:

1. Having Opcode 3
2. It can be both Unicast or Multicast
3. It uses Reliable Transport Protocol (RTP)
4. Sequence and Acknowledgement numbers are present.

4. Reply

In response to Query packets, EIGRP Reply packets are issued. To reliably respond to a Query packet, Reply packets are utilized. The originator of the Query receives reply packets in a Unicast format. An OPCode of 4 is allocated to EIGRP Reply packets.

Properties of Reply message:

1. Having Opcode 4
2. It uses Reliable Transport Protocol (RTP)
3. It is a Unicast message

5. Acknowledge

An EIGRP Acknowledgment (ACK) packet is nothing more than an empty EIGRP Hello packet. EIGRP uses acknowledgment packets to ensure that EIGRP packets are delivered reliably. ACKs are always transmitted to a Unicast address, not the EIGRP Multicast group address, which is the source address of the sender of the reliable packet. Acknowledgment packets will also always have a non-zero acknowledgment number. Because the ACK is effectively simply a Hello with no information, it utilizes the same OPCode as the Hello Packet. The OPCode for this item is 5.

Working of EIGRP Packet Format:

All EIGRP-enabled interfaces receive Hello packets from an EIGRP router. 224.0.0.10 is the EIGRP multicast address. Only other routers in the same autonomous system can create neighbor connections (adjacencies) with an EIGRP router.

On LANs, EIGRP hello packets are transmitted every 5 seconds as well as point-to-point links and multipoint circuits having a bandwidth larger than T1 also these hello packets are to be sent every 60 seconds on low-speed interfaces or T1. The HELLO interval is been configured by an interface subcommand ip hello-interval eigrp {as-num} {sec} for an EIGRP routing procedure on a network interface.

The hold time – the length of time a router assumes a neighbor is up without receiving a Hello or any EIGRP packet from that neighbor – is likewise kept in the EIGRP neighbor table. The IP hold-time eigrp {as-num} {sec} interface subcommand configures the EIGRP routing process’s hold time interval. The hold time interval should be at least three times that of the Hello interval. In reality, the default hold time period is three times the Hello interval. When the Hello interval is changed, the hold time interval is not immediately modified. After changing the Hello interval, the hold time interval must be manually adjusted to match the new Hello interval.

Here is one thing we should note that Instead of affecting the local router, the newly specified hold time value impacts nearby routers! On neighbor routers, use the show ip eigrp neighbors EXEC command to verify the newly specified hold time interval. A neighbor router that receives the Hello packet will utilize the hold time value because it is a parameter in the Hello packet.

The neighbor adjacency is terminated if an EIGRP packet is not received before the hold time interval ends, the topology table entries learned from the neighbor are erased, and an update stating that the routes are unreachable is sent out. If for any destination network, the neighbor is a successor then from the routing table those networks will be deleted and DUAL is used to recalculate alternate paths.

When the router is not doing re-computation for that route, it is termed passive; after the router is performing re-computation to find a new successor when the present successor has become invalid, it is considered active.

EIGRP Packet Format

Terminologies in EIGRP Packet Format:

The fields in the EIGRP header are as follows :

Version: The EIGRP process version is identified by this value. Version 2 of the EIGRP is currently in use.

Opcode: Update (0x01), Query (0x03), Reply (0x04), and Hello (0x05) are the different types of EIGRP packets (0x05). The TLVs that follow the EIGRP header is determined by it. Hello, packets with a non-zero ACK number are known as ACKs. Opcode specifies one of the following EIGRP packet types:

update(1)
query(3)
reply(4)
hello/ack(5)
SIA query(10)
SIA reply(11)

Checksum: The full EIGRP packet’s checksum, except the IP header.

Flags: The first LSB bit (0x00000001) is used to signal the initial batch of routing updates when a new neighbor relationship is established. Conditional Receive bit (0x00000002), utilized in Cisco’s proprietary reliable multicast technology – Reliable Transport Protocol (RTP). Other components aren’t being utilized.

SEQ & ACK: RTP uses this to ensure that EIGRP messages are reliably exchanged.

AS number: An EIGRP packet’s autonomous system is identified. Only EIGRP packets inside an EIGRP domain are processed by an EIGRP process (same AS number)

Type/Length/Value (TLV): contains the following route entries and DUAL information :

IPv4 Internal routes = 0x0102 
IPv4 External routes = 0x0103 
IPv6 Internal routes = 0x0402 
IPv6 External routes = 0x0403
EIGRP parameters =0x0001
Authentication Type = 0x0001
Sequence = 0x0003 
Software version = 0x0004 
Next Multicast Sequence = 0x0005 
Multi Protocol Internal Routes (AFI) = 0x0602 
Multi Protocol External Routes (AFI) = 0x0603 

Type: In the Value field, it specifies the type of information.

Length: specifies the overall length (This is the whole TLV in EIGRP).

Value: It contains the original information.

A destination network within an EIGRP domain is an internal route, whereas a destination network outside an EIGRP domain is an external route, as from other routing processes, redistributed routes into an EIGRP domain. The Type field of EIGRP IP internal routes is 0x0102. The metric information of EIGRP is similar to that of IGRP, with the addition of two new fields: Next Hop and Prefix Length (for support of VLSM).

Internal EIGRP routes are those that originate from the same EIGRP autonomous system as the receiving router.

EIGRP IP Internal Route Packet Format:

The following information is contained in the EIGRP IP Internal Route TLV :

EIGRP IP Internal Route Packet Format

• Next Hop: The next hop’s IP address, to which packets should be sent.
• Delay: The route metric’s delay parameter. The total of all delay parameters on the interface over the path to the destination network is the delay value.
• Bandwidth: The route metric’s bandwidth parameter. The interface provides the bandwidth, which is the lowest bandwidth on the interface over the path to the target network.
• MTU: The route metric’s interface MTU parameter
• Hop Count: The number of hops required to reach the destination network.
• Reliability: Out of a potential range of 1 to 255, the interface’s dependability. Reliability of 1 means the interface is 1/255 reliable, but the reliability of 255 means the interface is 100% trustworthy.
• Load: Out of a potential range of 1 to 255, the interface’s load. A load number of 1 represents a relatively little load, whereas a load value of 255 indicates a fully saturated interface.
• Prefix Length: The destination network’s subnet mask.

EIGRP IP External Route Packet Format:

External EIGRP routes are routes that are redistributed across EIGRP autonomous systems. In addition to  EIGRP IP Internal Route TLV fields, EIGRP IP External Route TLV contains some extra field information as below:

EIGRP IP External Route Packet Format

• Originating router: The router ID of the router from which the external EIGRP routes originate.
• Originating autonomous system number: Before being redistributed into this EIGRP autonomous number, the EIGRP autonomous system number routes.
• External protocol metric: The route’s metric before it’s redistributed into EIGRP.
• External protocol ID: The type of routing protocol that generates the redistributed routes in EIGRP. RIP, IGRP, BGP, OSPF, and other routing protocols are examples.

The information regarding the source (router and routing protocol) from which an external route is formed is specified in the Originating Router, Originating AS, External Protocol Metric, and External Protocol ID fields. Here we need to know that in OSI-based contexts, the Inter-Domain Routing Protocol (IDRP) is equivalent to the Border Gateway Protocol (BGP) in TCP/IP environments. IDRP is not supported by Cisco IOS software.