Host 0021.5a79.65a6 in vlan 100 is flapping between port Gi2/0/20 and port Gi1/0/20

[jeanSamarani_2]

Hi All,

can anyone explain what does mean the below logs messages? Is something critical and how can we fix it?

Host 0021.5a79.65a6 in vlan 100 is flapping between port Gi2/0/20 and port Gi1/0/20

Host 0021.5a79.64ca in vlan 100 is flapping between port Gi1/0/19 and port Gi2/0/19

Host 0021.5a79.65a5 in vlan 668 is flapping between port Gi2/0/10 and port Gi1/0/10

Host 0021.5a79.65a6 in vlan 100 is flapping between port Gi1/0/20 and port Gi2/0/20

Host 0021.5a79.64c9 in vlan 668 is flapping between port Gi1/0/9 and port Gi2/0/9

Host 0021.5a79.64c9 in vlan 668 is flapping between port Gi2/0/9 and port Gi1/0/9

Host 0021.5a79.65a6 in vlan 100 is flapping between port Gi2/0/20 and port Gi1/0/20

Host 0021.5a79.65a6 in vlan 100 is flapping between port Gi1/0/20 and port Gi2/0/20

Host 0021.5a79.64ca in vlan 100 is flapping between port Gi1/0/19 and port Gi2/0/19

thanks a lot !

[Julio Garcia]

could you tell us what these ports are -- are they trunks ?

sh run int gi 1/0/20

sh run int gi 2/0/20  etc...

I would say is quite critical, normally spanning tree makes sure there are not redundant links to hosts ( switch is seeing host/s on 2 different ports -- when should only be seeing on 1), these messages indicates something is not quite right on network , is it still happening?

[jonathanaxford]

Hi,

This can also happen if you have Wireless APs connected to the ports and a client is roaming between the two Access Points...

Cheers

Jonathan

[Richard Burts]

Jean

The symptom of MAC addresses appearing in VLAN100 and then appearing in VLAN668 can be caused because someone connected something (a small switch or a hub) that connected ports in VLAN 100 and in 668. So you might start by checking the interfaces mentioned in the error messages and verify what is connected on those ports.

I have also seen this symptom in a network that was using VM. The process of cloning VMs can results in two different machines both having the same MAC address. So you might also want to check the interfaces mentioned, check the machinges connected to those ports and verify what MAC address is on the machine interface.

HTH

Rick

[jeanSamarani_2]

Basically let me explain more the setup. We are having two Cisco switches connected to each other via a Stackwise cable. The ports having this issue are access ports and they are connected to the same server using two different NIC cards.

[lalarajaraman1985]

Hi,

We have been facing the same issue in our 4948 switch while connecting a new server with redundant link.The cisco document was saying the reason for host flapping as improper STP configuration, But upon correcting the config on both server side and switch side, the issue was fixed. So i recommend you to check if the fail over configuration is correct on the server side.

Cheers

Raja