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The "HOSTFLAPPING: Host 00:00:00:00:00:00 in vlan xyz is flapping between port xx/xx and port xx/xx" error message appears on Cisco Catalyst switch when connected with WLC


Core Issue:-

This is an example of the exact message that can appear:

month date time %C4K_EBM-4-HOSTFLAPPING: Host 00:00:00:00:00:00 in vlan xx is flapping between port xx/xx and port xx/xx.

This happens if the controller ports are set up as Link Aggregation (LAG) and the WLC is connected between two different switches.


LAG cannot be set up between two different switches, especially with Catalyst 4500 series switches, because the switches do not support port channels that span over multiple switches. Disable LAG in order to resolve this issue.

For more information, Please Check details of "Enabling Link Aggregation":-

Link aggregation (LAG) is a partial implementation of the 802.3ad port aggregation standard. It bundles all of the controller's distribution system ports into a single 802.3ad port channel, thereby reducing the number of IP addresses needed to configure the ports on your controller. When LAG is enabled, the system dynamically manages port redundancy and load balances access points transparently to the user.

Cisco 4400 series controllers support LAG in software release 3.2 and higher, and LAG is enabled automatically on the controllers within the Cisco WiSMand the Catalyst 3750G Integrated Wireless LAN Controller Switch. Without LAG, each distribution system port on the controller supports up to 48 access points. With LAG enabled, a 4402 controller's logical port supports up to 50 access points, a 4404 controller's logical port supports up to 100 access points, and the logical port on each Cisco WiSM controller supports up to 150 access points.

You can bundle all four ports on a 4404 controller (or two on a 4402 controller) into a single link.

LAG simplifies controller configuration because you no longer need to configure primary and secondary ports for each interface. If any of the controller ports fail, traffic is automatically migrated to one of the other ports. As long as at least one controller port is functioning, the system continues to operate, access points remain connected to the network, and wireless clients continue to send and receive data.

When configuring bundled ports on the controller, you may want to consider terminating on two different modules within a modular switch such as the Catalyst 6500; however, we do not recommend connecting LAG ports of a 4400 controller to multiple Catalyst 6500 or 3750G switches.

Terminating on two different modules within a single Catalyst 6500 switch provides redundancy and ensures that connectivity between the switch and the controller is maintained when one module fails. above Figure illustrates this use of redundant modules. A 4402-50 controller is connected to two different gigabit modules (slots 2 and 3) within the Catalyst 6500. The controller's port 1 is connected to gigabit interface 3/1, and the controller's port 2 is connected to gigabit interface 2/1 on the Catalyst 6500. Both switch ports are assigned to the same channel group

For more detailed information, refer to Enabling Link Aggregation on WLC:-

Problem Type

Error message


Catalyst switches

Wireless LAN Controllers

Lightweight Access Points

Reference Links:-

Configuring Ports and Interfaces on WLAN release 4.0:-

For Configuring Ports and Interfaces release 5.0:-

Please check another CSC document for similar kind of issue on Catalyst 4500 and 4000 series switch:-


I'm experiencing this right now and suspect that this is the case since I'm seeing it flap between Po between my standby core and WLC.

Thank you for this.


000233: Jan 26 07:54:15.008 cst: %C4K_EBM-4-HOSTFLAPPING: Host F0:CB:A1:2B:B4:B9 in vlan 888 is flapping between port Po1 and port Po44

Group  Port-channel  Protocol    Ports
1      Po1(SU)          -        Gi2/45(P)   Gi2/46(P)   Gi2/47(P)     ----> 0 CCM-MDC
44     Po44(SU)         -        Gi2/11(P)   Gi2/12(P)   Gi2/27(P)     ----->  wlc-mcc-02

Vlan 888 is Guest Wireless

001682: Jan 27 10:48:22.455 CST: %C4K_EBM-4-HOSTFLAPPING: Host D0:C1:B1:86:E4:09 in vlan 888 is flapping between port Po46 and port


46     Po46(SU)         -        Gi2/37(P)   Gi2/38(P)      -----> WLC-MIS-01

50     Po50(SU)         -        Gi2/39(P)   Gi2/40(P)      -----> WLC-BHC-01

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