3.5.d [i] General operations
The following are the most common causes of problems with EIGRP neighbor relationships:
● Unidirectional link
● Uncommon subnet, primary, and secondary address mismatch
● Mismatched masks
● K value mismatches
● Mismatched AS numbers
● Stuck in active
● Layer 2 problem
● Access list denying multicast packets
● Manual change (summary router, metric change, route filter)
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3143-3154). . Kindle Edition.
3.5.d [ii] Topology table, update, query, active, passive
The show ip eigrp topology command displays the EIGRP router ID. The EIGRP router ID comes from the highest IP address assigned to a loopback interface. If no loopback interfaces are configured, the highest IP address assigned to any other active interface is chosen as the router ID. No two EIGRP routers can have the same EIGRP router ID. If they do, you will experience problems exchanging routes between the two routers with equal router IDs.
Route filtering enables routes to be filtered from an EIGRP routing advertisement as they come in from a neighbor or as they are sent out to a neighbor. These filters can cause routes to be missing from the routing table. The show ip protocols command shows whether any filter lists are applied to EIGRP.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3162-3163). . Kindle Edition.
3.5.d [iii] Stuck in active
Stuck in active means that an EIGRP router is involved in a diffusing computation (a process of asking the neighboring routers to assist in locating a replacement path to a network for which the usable path has been lost) for a new path to some network, and this computation seems to be stalled because some expected replies have not arrived in a reasonable time.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3167-3170). . Kindle Edition.
3.5.d [iv] Graceful shutdown
With graceful shutdown, a goodbye message is broadcast when an EIGRP routing process is shutdown, to inform adjacent peers about the impending topology change. This feature allows supporting EIGRP peers to synchronize and recalculate neighbour relationships more efficiently than would occur if the peers discovered the topology change after the hold time expired.
EIGRP sends an interface goodbye message with all K values set to 255 towards neighbors connected to its different interfaces. If the IOS neighbor router supports this graceful shutdown then it says “interface goodbye received”. If not, it says K values mismatch.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3172-3178). . Kindle Edition.
3.5.d [i] General operations
The lowest cost route is calculated by adding the cost between the next hop router and the destination (Reported Distance, RD), and the cost between the local router and the next hop. This sum is the Feasible Distance (FD).
The successor is the next hop router to the destination that the local router has chosen. Multiple successors may exist if they provide equal cost paths.
If a backup path to the destination exists it may become a feasible successor. To qualify, this next hop router must report an RD that is less than the FD of the successor.