Regarding EIGRP neighborship adjacency Process

[surendrasinghtanwar667]

Hey everyone, Actually i was learning one of the topic and i was totally confused in EIGRP neighborship adjacency process that what exactly is Update Packet (Unicast) with Init set and its true that first update packet does not contain the routing table, so anyone who can help me out to resolve this doubt just let me know.

Thank you

[MHM Cisco World]

https://www.ciscopress.com/articles/article.asp?p=2999383&seqNum=2

Did you check it 

MHM

[Gopinath_Pigili]

Hello surendrasinghtanwar667,

https://community.cisco.com/t5/routing/eigrp-startup-process-unicast-and-multicast-updates-containing/td-p/2782761

Best regards
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[David Ruess]

Hello,

From the EIGRP RFC it holds the K-values for the neighbor to make sure they are compatible to form an adjacency. It also contains the Hold timers which don't have to match but should as best practice.

Initial update packet implies it would have the "Init" bit set.

rfc-editor.org/rfc/rfc7868.txt

Hope that helps

-David

[M02@rt37 · Accepted solution]

Hello @surendrasinghtanwar667 

In EIGRP, when a router sends an initial Update packet with the Init bit set, it indicates that it is beginning the process of forming a neighbor adjacency with another router. This Update packet serves as a notification that the sending router is ready to exchange routing information; however, it does not contain the complete routing table. Instead, it includes essential parameters, such as the K-values, which are crucial for compatibility between neighbors. The K-values must match for the routers to agree on how to calculate metrics for routes; if they differ, adjacency will not be established. The initial Update packet may also include Hold timers, which, while they do not need to match, are best kept similar to avoid potential issues. Therefore, the first Update packet with the Init bit set is vital for initiating the adjacency process, ensuring routers can exchange necessary routing information while adhering to compatibility requirements like K-values and optimal Hold timers.

[surendrasinghtanwar667]

So basically first update is to just compare the k values for forming the Neighborship adjacency

[Joseph W. Doherty]

To summarize the great information provided in the other replies, you might think of EIGRP going through 3 states, (my suggested terms) neighbor discovery => neighbor establishment agreement => neighbor routing information maintenance.

The 2nd state, although it's using an update packet, denotes its special status by the init flag.  I.e. it's initializing the routing information maintainance.

The conceptional states, above, I believe make sense, but one might question why use an update packet with a special flag rather than a different packet structure for that purpose?  Only the designers could answer that question, but although it might create questions/confusions such as in your OP, I strongly suspect the perceived benefits of this approach were better, although such benefits, if you were designing EIGRP, today, from scratch, may no longer apply.  I mention this because when you look at many technologies, including networking, without understanding environments decades ago, you can easily think what-the-heck.  (My personal favorite example of WTH networking technology is Classful IP.)

Oh, and as strictly a guess, one possibly reason of also using an update packet structure for the initial setup, might have been save even under a hundred bytes of the memory requirement of EIGRP.

[MHM Cisco World]

 I run this lab for you 
it show you eigrp use both multicast and unicast 

first the R1 send Hello multicast check if there are any eigrp neighbor in same subnet of link same is done by R2 and R3
Now each router knows neighbor and it IP 

each router then send update as unicast and neighbor must reply by ACK 

if there is no ACK then router start to
1- retransmit the unicast packet, sure it not for infinite but until the retransmit count reach 16 
2- increase the RTO, and max is 5000