EIGRP initial route discovery process

Unanswered Question
Oct 18th, 2007

Hello,

In the Cisco Press book "Authorized Self-Study Guide BSCI, 3rd Ed.", on page 78, we can see the figure 3-4 showing the initial neighbor discovery process. This process is described step by step on page 79 and it seems the text description does not match the figure.

On the figure, we can see that Router A sends a Hello message to Router B and then Router B sends a Hello message to Router A back. Router B immediately sends the Update messages with its routes.

In the text, point 2 is very unclear to me. It is mentionned that

"Router B receiving the hello packet on one interface reply with update packets that contain all the routes it has in its routing table, except those learned through that interface (because of the split horizon rule). Router B sends an update packet to Router A, but a neighbor relationship is not established until Router B sends a hello packet to Router A. The update packet contains information about the routes that Router B is aware of, including the metric that the neighbor is advertising for each destination."

All this point is very very confusing to me. How does exactly Router B behaves when he receives a Hello packet from a router he has not formed an adjacency with ? Does it first send a Hello packet back (like in the figure, this would be a reasonable answer for me) or does it send an Update packet back without worrying about sending back an Hello ? In that case, why does it stupidly send an Update while it has not fully established the neighborship ?

Thank you very much for your answers.

Yann

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LordFlasheart Thu, 10/18/2007 - 11:35

Hi Yann,

I've just up a test network on GNS3 (ree Cisco router simulator, which is excellent) with two EIGRP neighbors peering. I ran a debug eigrp packet on R0 and saw the following:

*Mar 1 00:03:52.887: EIGRP: Received HELLO on FastEthernet0/0 nbr 10.128.100.2

*Mar 1 00:03:52.891: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0

*Mar 1 00:03:52.891: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.128.100.2

(FastEthernet0/0) is up: new adjacency

*Mar 1 00:03:52.891: EIGRP: Enqueueing UPDATE on FastEthernet0/0 nbr 10.128.100

.2 iidbQ un/rely 0/1 peerQ un/rely 0/0

*Mar 1 00:03:52.895: EIGRP: Requeued unicast on FastEthernet0/0

*Mar 1 00:03:52.899: EIGRP: Sending HELLO on FastEthernet0/0

*Mar 1 00:03:52.899: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0

*Mar 1 00:03:52.903: EIGRP: Sending UPDATE on FastEthernet0/0 nbr 10.128.100.2

As you can see it appears to create an Update packet then sends Hello packet first then sends the queued Update packet afterwards.

Definitely is a strange description in the book but would say it is Hello, Hello, Update.

HTH,

Chris

yann.gauteron Thu, 10/18/2007 - 11:43

Thanks Chris for this simulation.

At least I'm in agreement with your simulated result.

Yann

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