- The passive-interface command is preventing neighbor relationships on interface GigabitEthernet 0/0.
- The hello timer has been modified on interface GigabitEthernet 0/1 of R3 and not on the neighbor, causing a neighbor adjacency not to form.
- The GigabitEthernet 0/1 interface is not participating in the EIGRP process.
- The GigabitEthernet interfaces are not limiting the flow of EIGRP message information and are being flooded with EIGRP traffic.
Explanation: When enabling EIGRP, the network command must be applied to the classful network address of the interface or to a subnet with the appropriate wildcard mask. The network 172.20.0.0 will only activate interfaces in that network. The wildcard mask 0.1.255.255 must be issued to support both 172.20.0.0 and 172.21.0.0 in a single network statement.