- The passive-interface command should have been issued on serial 0/1/0.
- The hello interval has been altered on serial 0/1/0 and is preventing a neighbor relationship from forming.
- The network statement used for EIGRP 55 does not enable EIGRP on interface serial 0/1/0.
- The IPv4 address configured on the neighbor that is connected to R1 serial 0/1/0 is incorrect.
- The networks that are configured on serial 0/0/0 and serial 0/1/0 of router R1 are overlapping.
Explanation: According to the status message, serial 0/1/0 is receiving EIGRP packets coming from the IPv4 address 192.168.254.9. This IPv4 address is on a different subnet in contrast to the IP address configured on serial 0/1/0 of R1. The passive-interface command would have prevented any neighbor relationship from forming if the command was issued on serial 0/1/0. Changing the hello interval on one router and not another will not cause an EIGRP neighbor relationship to fail.