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The "%C4K_EBM-4-HOSTFLAPPING" error message displays on the Cisco Catalyst 4500 and 4000 series switches running Cisco IOS System Software


Core issue

The %C4K_EBM-4-HOSTFLAPPING:Host [mac-addr] in vlan [dec] is flapping between port [char] and port [char] error message signifies that the specified host is detected as a source address on multiple ports. Typically, a host is supposed to be learned on only one port.

These are common causes for this error message:

  • There is an spanning-tree loop.
    All traffic from the specified host is temporarily dropped. After 15 seconds, forwarding is re-enabled. If the problem persists, the traffic from the problem host is dropped indefinitely. (However, in this case, expect the switch to see a few more MAC addresses on different VLANs.)

  • The host is connected to ports x/y, and x/y runs a type of clustering or redundancy and has an issue with this mechanism.
    If the MAC address specified in the error message can be traced, this is likely the issue. For an example, refer to this error message:

    %C4K_EBM-4-HOSTFLAPPING: Host 02:01:00:00:00:00 in vlan x is flapping between port Gi x/y and port Gi x/y

    The MAC address 02:01:00:00:00:00 appears to be associated to Microsoft's Network Load Balancing (NLB) Service Heartbeat. It is suggested that the connected device (Microsoft host) be checked for causing these messages.

  • A hub is connected to two different ports, or some other bridging device (for example, an IP phone, hub, switch, or wireless access device.)
    Note: Wireless devices that roam the network can cause the %C4K_EBM-4-HOSTFLAPPING error message.


To resolve this problem, make sure that spanning tree is enabled. This prevents spanning tree loops. If spanning tree is disabled, make sure there are no spanning tree loops in the network.

To display spanning-tree state information for the VLAN ID mentioned in the error message, issue the show spanning-tree vlan vlan_id detail command and look for the last topology change.

If the MAC address in the error message is related to Microsoft Network Load Balancing (NLB), then you need to disable or clear the Network Load Balancing check box.

For additional information, refer to the Error Message when the Host Address is a Source Address on Multiple Ports section of Common Issues with Catalyst 4500 Switches that Run Cisco IOS Software.