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EIGRP neighborship Packet Exchange

surendrasinghtanwar667_0-1728650701091.png

During EIGRP neighborship form process both router exchange the update packet and first packet is init and second update packets contain the routes. But my doubt is first R1 has send a update packet on multicast and then again it send unicast in the same way R2 has followed the same process?? Can someone please clear my doubt 

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When implementing the EIGRP routing protocol, EIGRP enabled routers will create neighbor adjacencies by exchanging EIGRP messages. Initially, they will exchange hello messages and then will start sending updates containing network information.

The way these initial updates are exchanged depends on several factors.

EIGRP operates both using obsidian/notes/Multicast and unicast. Initially, a prefix will be advertised using multicast destination addresses so that it can be advertised to all neighbors in a single go. Unicast updates are sent in response to queries or to provide specific updates to a particular neighbor.

There may be cases where updates are resent but they do not contain any new routes. This may occur because they might be part of a convergence process where no new routes are available yet, or they could be part of the EIGRP Reliable Transport Protocol (RTP) acknowledging receipt of an update. <<- this for my case, and I even capture more traffic I see same behave as you check below more comment 

RTP (not to be confused with Real-time Transport Protocol) is a Cisco proprietary transport layer protocol that is used exclusively for the exchange of EIGRP packets.

In addition, we may see multiple Update packets with the same routes due to the Reliable Multicast feature of EIGRP. If the EIGRP router doesn’t get a reply from a neighbor via the multicast, it will use unicasts to resend the same data. If it does not get a reply from the neighbor after 16 unicast attempts, it declares the router dead. This process is known as reliable multicast and may be behind some of the repeated updates.<<- This what you and I see multicast then unicast because the neighbor not send ACK for multicast update, this will return to your previous post about RTO.

 

ask more if you have Q, it really nice conversation.

MHM

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M02@rt37
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Hello @surendrasinghtanwar667 

When Router 1 (R1) and Router 2 (R2) establish their neighborship, the first Update packet from R1 will be multicast, advertising its routing information to all potential neighbors. After this, R1 sends unicast Updates directly to R2 to finalize the exchange. R2 follows a similar process: it multicasts its initial routing information and then unicasts to R1 for further synchronization.

M02rt37_0-1728652054659.png

 

Best regards
.ı|ı.ı|ı. If This Helps, Please Rate .ı|ı.ı|ı.

Hello,

 

Basically the device is advertising itself on the link. It uses multicast first to be efficient (as in talk to everyone listening on this link at the same time). Then once it hears responses it can just end the unicast updates to the neighbor. I believe in some instances it will send multicast updates as well.

As an example, say you're in a room full of people. You shout (multicast) "Hey I want to talk EIGRP protocol with people. Anyone listening for that keyword can come over and you can talk to individuals (unicast) about EIGRP.

 

 

Hope that helps

-David

but see R1 has send the update packet twice time because each and everytime i get different outcome sometime i am able to see single multicast update and sometime more then 1, Idk why 

Thats because EIGRP Update packets are sent reliably. If it's not Acknowledged by the neighbor, then it will send it again until it is up to a certain amount. When you dive deeper into the packet you should see a sequence number of what's being sent vs what's being acknowledged. It's likely your device hadn't fully initialized to ACK the update packet or maybe it got lost on the link. Sometimes it happens, which is why devices retransmit packet. AS you mentioned sometimes you only see one. That means it made it successfully the first time.

Hello
The rtrs will send mc to the eigrp group address 224.0.0.10  if no answer is received the rtr it will resend numerous times (16)  via a unicast before it declares the peer dead/unreachable, this is known as Reliable Multicast.


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Paul

the devil in detail 

open the update message unicast one and compare to multicast 
you can see that update unicast is empty it only for checking the real update that contain prefix is send in multicast.

MHM

Screenshot (846).png

Hello
FYI -With that lab setup , you dont even require mc, you could make the adjacency's all unicast and remove the BW  K value so you just use delay much easier to path manipulate.



Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

I build general lab' when I have issue with eigrp I retrun to it.

My concern here is  packet capture of eigrp message.

Thanks 

MHM

"Could you please specify the image used in your topology?"

Sorry I don't get it, 

I share  packet capture detail for both 

Multicast and unicast update, you can see the destination to know which capture it is.

MHM

which router image you are using in the gns3 

Thanks @MHM Cisco World for that sharing.

Best regards
.ı|ı.ı|ı. If This Helps, Please Rate .ı|ı.ı|ı.

You are welcome friend

MHM

surendrasinghtanwar667_0-1728721346905.png

As you mentioned, multicast updates are used to share routes; however, this isn't entirely accurate. You can observe that when R1 sends updates via multicast, it still shares internal routes. Additionally, during unicast communication, it continues sharing routes as well.

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