Query-Reply Mechanism

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Whenever a route get’s unreachable from a EIGRP router it sends a Query packet to other active neighbors with some metric changes, router send’s Query packet with some modified details, whenever a router send’s a update packet it contains Valid metric values but whenever a route gets unreachable the EIGRP router sends Query packet with infinite metric Both EIGRP Update packet and Query packet parameters are same but the values are different The Query packet also goes on multicast address 224.0.0.10 and whenever other routers gets the multicast query packet they give a Unicast reply it can be positive or can be negative When router’s knows about the destination they provide valid metric against infinite metric and that reply is considered as positive reply
And when routers don’t know about the destination they reply with the same infinite values to that router and that type of reply considered as negative reply.

Let’s start one by one.

1. Route 5.5.5.5 gets unreachable on R3
2. Router R3 will send a Query Packet to all active EIGRP neighbor on 224.0.0.10
        a. With Seq number 27
        b. Hop count 255
        c. Higher Bandwidth
        d. Infinite delay

3. Whenever Router R1 gets the Query packet it first send a Ack to Router R3 wit ACK 27

4. Then Router R1 will send a Reply packet and in our case Router R1 does not have any valid metric to reach 5.5.5.5 prefix so it will send Reply with same values
        a. With Seq number 28
        b. Hop Count 255
        c. Higher Bandwidth
        d. Infinite Delay

5. Router R3 will get the Query and will reply with a Acknowledgement 28 of previous Reply Seq number

6. And it consider that reply as negative and will not use that prefix anymore
7. And the same process will be done by Router R1 and other routers who received the Query from Router R3

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