When ClusterXL member in High Availability mode / in Load Sharing Multicast with Sticky Decision Function forwards a packet for processing to a peer member, the packet is forwarded over Synchronization Network.
The Source MAC Address of a forwarded packet is set to 00-00-00-00-XX-ZZ, where:

• XX - value of kernel parameter 'fwha_mac_forward_magic'
• ZZ - ID of destination cluster member, to which packet is being forwarded

If several ClusterXL members (e.g., Member_B and Member_C) forward their packets to the same peer member (e.g., Member_A), then the Source MAC Address of each forwarded packet would be identical.

This causes the switch, to which the Synchronization interfaces are connected, to "think" that the same Host is connect to different ports.
Example:

1. Forwarded packet from Member_B enters via Port_1 (and is forwarded to a Port, to which the Member_A is connected)
2. Switch updates its CAM table to indicate that the Host with MAC Address 00-00-00-00-XX-ZZ is connected to Port_1
3. Forwarded packet from Member_C enters via Port_2 (and is forwarded to a Port, to which the Member_A is connected)
4. Switch updates its CAM table to indicate that the Host with MAC Address 00-00-00-00-XX-ZZ is connected to Port_2

Since there are multiple CCP packets, CAM table is frequently updated for the same MAC addresses. As a result, switch declares port flapping.