EIGRP neighborship Packet Exchange

[surendrasinghtanwar667]

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 

[MHM Cisco World · Accepted solution]

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

[surendrasinghtanwar667]

That's the reason I have marked a several time that a router is sending the update to multicast with same internal routes as well same routes introduced in the unicast message too I was confused that what exactly is going on So at the end it clear that initially it will use multicast if no acknowledgment arrives then only it will switch to unicast for reliable.

[surendrasinghtanwar667]

Why does this hello packet contain ack bit inside it ?? do you have any idea

[surendrasinghtanwar667]

This hello packet contains ack bit inside it do you have any idea what exactly it is 

[David Ruess]

This is what EIGRP uses as an ACKnowledgement. Its just an empty hello packet with the ACK bit set.

[surendrasinghtanwar667]

but ack bit for which packet ?? and second thing i have seen when EIGRP packet are sent on multicast the receiver router will send a unicast acknowledgment in these hello ack packet why ???

[David Ruess]

As I mentioned before check the packet details. The ACK should have a sequence # related to the packet it was trying to Acknowledge. 

As far as sending a unicast ACK thats a matter of efficiency and common sense.

Let's say I have 10 routers on the link, so that means each router has 9 neighbors on a multiaccess network. Does any other neighbor besides the one I am trying to acknowledge a packet for need to see or process my acknowledgement. The answer is no, so why would I NOT send it unicast to the only neighbor that can and will process it. If I sent an ACK multicast then every neighbor will have to listen to it and process it just to find out they dont even need it.

[surendrasinghtanwar667]

Do you have any idea about this hello (Ack) packet

[surendrasinghtanwar667]

This is the packet which contains acknowledgment inside the hello packet do you have any idea regarding this hello packet

[MHM Cisco World]

Sorry I was busy 
NOW 
why hello + ACK ?
the IGP send hello periodic in specific time, some time the router send hello with ACK for low link and low CPU utilize
instead of build header for Hello and other for ACK the router make it together in one packet .

each update have Seq No. 
this No. is same you can see in ACK for this update 

check below the Seq 8 and ACK 8 
and
Seq 9 and ACK 9

[surendrasinghtanwar667]

sorry for delay reply , so if we work on real time router it will send the seperate ack packet instead of hello if i am not wrong