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EIGRP Adjacency


When your neighbor's router is on fire, your internet is not at stake! Wish the same were true in the EIGRP universe as well (unless you have a well designed redundant network, which unfortunately is not very often the case!)





EIGRP works if and only if your router establishes an 'amicable' EIGRP neighborship with the adjacent EIGRP router. Well, this is not an issue in most cases. Of importance, here, is how do you quantify the 'amicability' of the EIGRP adjacency.





Let's inspect various fields in the "show ip eigrp neighbor" output





show ip eigrp neighbor




Autonomous System (AS) number - Here, visible as "Process 100" - EIGRP uses the concept of autonomous systems. An autonomous system is simply a group of EIGRP-enabled routers that should become EIGRP neighbors and exchange routes.





Address: IP address of the EIGRP neighbor





Interface: Which interface is this EIGRP adjacency is formed upon





Hold timer: Hold down timer is the number of seconds EIGRP router would wait to learn a hello packet from its neighbor till the time it decides that the neighbor is no longer 'alive'





Uptime: The total time this particular neighbor has been UP and stable





SRTT: Smooth Round Trip Time - The time it takes to send an EIGRP packet and receive an acknowledgment from the neighbor





RTO: Retransmission Time Out - This is the time that the router would wait before retransmitting a packet from the retransmission queue to a neighbor





Q Cnt – the number of EIGRP packets (Update, Query or Reply) in the queue that are awaiting transmission. Should be 0





Seq Num – the sequence number of the last EIGRP packet which can be update, query, or reply packet that was received from the EIGRP neighbor





End result: The Uptime, SRTT, RTO, Q Cnt can be used to determine the 'amicability' of two EIGRP routers forming adjacency. High uptime, low SRTT and RTO and zero 'Q cnt' should indicate that the EIGRP adjacency is stable.


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