Messages

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EIGRP uses 5 different Messages

1. Hello

                 • Hello is used to perform dynamic neighbor discovery

                 • Using hello packet routers identify other neighbors

                 • After getting very first hello packet from neighbor, router sends a null update to check whether it is real EIGRP neighbor or that is just a dummy hello packet received from neighbor.

                 • When router gets null update they give ACK to neighbor.

2. Update

                    • Is used to indicate a change in metric or an addition of a new destination

                    • We can categorize update packet in different ways

                    • Null update: is used to check whether neighbor can give ACK of real update or not.

                    • Full update: after Null update router sends the Full update if they have any.

                • Partial update: in future whenever routers gets new update they just send the latest update to neighbor, they never send full update every time.

3. Ack

                    • Routers use ACK packet to give Acknowledge to routers of last packet.

4. Query

                  • Whenever routers do not find any backup path in Topology table and best path goes down then routers send Query packets to other active neighbors

                    • Router also use SIA Query & SIA Reply but in different condition

                    • About Query packet and Topology table we will discuss more*

5. Reply

                    • Reply is used to send in the response of Query or SIA Query.

                    • Reply can by Positive or can be Negative

                    • If neighbor knows about the information asked they send new metric values and that’s a Positive reply.

                    • If neighbor don’t know about the information asked they send infinite metric and that reply is considered as Negative reply.

                    

               Check out our Complete EIGRP Video on Youtube