Sample quetsions-

EIGRP neighbor not coming up -

1.AS number mismatch between neighbors.
2.K value mismatch between routers.
3.IP is on different subnets
4.Primary and secondary address on interface mismatch between routers.
5.layer 2 switch/hub connecting different subnet devices in the same VLAN. For example, VLAN leaking from vlan 1 to vlan2 by using back to back cable.
6. Access-list blocking EIGRP multicast packets on interface.

CLI Command - Show ip eigrp neighbors

Solution of the Above question --

1- Make sure same AS number is in use.
2- Make sure same K value is in use.
3- Make sure same IP subnets on both side.
4- Make sure Primary to Primary address match on both side and secondary to secondary.
5- Make sure there is no leaking

Scenario 2 -

EIGRP neighbor is established but not exchanging routes and getting reset -

1. Mismatch mask between the neighbor.
2. Hold time getting expired
3. Retry limit exceeded.
4. Vlan leak is happening or cross VLAN domain due to some redundant link or wrong cabling

Cli command - Show ip eigrp neighbors.