EIGRP Data Structure

So far so good. We have learned so many new things about EIGRP. May be you won’t either have heard these concepts. I think this has really helped you. So let’s keep our flow going. Now we will discuss Data Structure of EIGRP Routing protocol in detail.

As we know OSPF, EIGRP relies on different types of packets to maintain its various tables and establish complex relationships with neighbour routers. What types of packets, what they consist of, and their purpose of use? Let’s go through them one by one.

Here are five EIGRP packet types are:

• Hello
• Acknowledgment
• Update
• Query
• Reply

Hello Packets

Hello packets support EIGRP to discover, verify, and rediscover neighbour routers. Rediscovery occurs only if EIGRP routers do not receive hellos from each other for a Hold Time interval but then re-establish communication.

There is a fixed but configurable interval, called the hello interval during which EIGRP send hello packets. The default hello interval depends on the bandwidth of the interface. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10.

Neighbour table is used by EIGRP router to stores information about neighbours. That includes the Sequence Number (Seq No) field to record the number of the last received EIGRP packet sent by each neighbour. it also includes a Hold Time field which records the time the last packet was received. Packets should be received within the Hold Time interval period to maintain a Passive state. This is a reachable and operational status.

EIGRP considers that neighbour down, if a neighbour is not heard from for the duration of the Hold Time, and then DUAL must step in to re-evaluate the routing table. By default, the Hold Time is three times the hello interval, but an administrator can configure or reset these both timers as desired.

OSPF requires neighbour routers to have the same hello and dead intervals to communicate. On the other hand EIGRP has no such restriction. Neighbour routers learn about each of the other respective timers via the exchange of hello packets. Then they use that information to forge a stable relationship regardless of unlike timers.

So that was all about hello packets. We learned what is there significance, why they are used and for what purpose. One more fact about it is that Hello packets are always sent unreliably. This means that no acknowledgment is transmitted.

Acknowledgment Packets

So recently we discussed hello packets. Hold on to your pens and copies. We are continuing from hello packets onwards. This time It’s acknowledgement packets turn.

So let’s go through them.

These packets are used During a reliable exchange, an EIGRP router uses acknowledgment packets to indicate receipt of any EIGRP packet. Communication between EIGRP hosts is provided by RTP. To be reliable, a sender's message must be acknowledged by the recipient. Acknowledgment packets are basically simple hello packets. Yes, this is true. But the difference is that acknowledgement packets are without data, are used for this purpose. Hello packets are multicast, where as acknowledgment packets are unicast. Acknowledgments can be made by attaching them to other kinds of EIGRP packets, such as reply packets.

So that was all about Acknowledgement packets. Let’s move to next session now. That is update packet.

Update Packets

What are Update packets and why they are used, for what purpose? Keep reading and you will get to know about it.

A router discovers a new neighbour with the help Update packets. An EIGRP router sends unicast update packets to that new neighbour so that it can add to its topology table. More than one update packet may be needed to convey all the topology information to the newly discovered neighbour.

Secondly, a router detects a topology change with the help of Update packets. In this case, the EIGRP router sends a multicast update packet to all neighbours, which alerts them to the change. All update packets are sent reliably.

So that was all about update packets. Let’s move to next session. Keep reading  few left.

Query Packets

Query packets are used by EIGRP router whenever it needs specific information from one or all of its neighbours. A reply packet is used to respond to a query.

Reply Packets

Last type of packet used by EIGRP is Reply packet. If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the Active state. A query is then multicasted to all neighbours in an attempt to locate a successor to the destination network. Neighbours must send replies that either provide information on successors or indicate that no information is available. Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.

So that’s it. We did it. We are done with EIGRP data structure. We learned many new concepts related to it. Hope you enjoyed the Topic.