we are using one cisco 4507 in our network as core. some vlans are created on that and EIGRP is running on that. now I am getting one error message that says " IP-EIGRP(Default-IP-Routing-Table:1): Neighbor 10.2.4.251 not on common subnet for Vlan20 " and " IP-EIGRP(Default-IP-Routing-Table:1): Neighbor 10.2.20.251 not on common subnet for Vlan4".
while 10.2.4.251 is ip address on interface vlan 4 and 10.2.20.251 is interface vlan ip for vlan 20
Can anyone please suggest any resolution for this.
someone may have joined broadcast domains Vlan 20 and Vlan 4.
EIGRP messages have a multicast destination both at Layer3 and at Layer2 so the switch is receiving on vlan 20 its own hellos sent in vlan 4 and viceversa.
if CDP is enabled looks for CDP messages in log messages specially for native vlan-id mismatch also on access layer switches to find the unwanted connection.
Or from an host on vlan4 make an ARP request and look for host's MAC address in Vlan 20 CAM table if you find it you have confirmation of above and looking for the port where the MAC address is learned you can find the unwanted connection.
Or it could be sign of bridging loop but it would cause network performance to drop down.
Hope to help