Chapter 9 High Availability for FortiOS 5.0 : HA and load balancing : Transparent mode active-active cluster packet flow : Packet flow from client to mail server
  
Packet flow from client to mail server
1. The client computer requests a connection from 10.11.101.10 to 10.11.101.200.
2. The client computer issues an ARP request to 10.11.101.200.
3. The primary unit forwards the ARP request to the mail server.
4. The mail server responds with its MAC address (MAC_Server) which corresponds to its IP address of 10.11.101.200. The primary unit returns the ARP response to the client computer.
5. The client’s request packet reaches the primary unit internal interface.
 
IP address
MAC address
Source
10.11.101.10
MAC_Client
Destination
10.11.101.200
MAC_Server
6. The primary unit decides that the subordinate unit should handle this packet, and forwards it to the subordinate unit internal interface. The source MAC address of the forwarded packet is changed to the actual MAC address of the primary unit internal interface.
 
IP address
MAC address
Source
10.11.101.10
MAC_P_int
Destination
10.11.101.200
MAC_S_int
7. The subordinate unit recognizes that packet has been forwarded from the primary unit and processes it.
8. The subordinate unit forwards the packet from its external interface to the mail server.
 
IP address
MAC address
Source
10.11.101.10
MAC_S_ext
Destination
10.11.101.200
MAC_Server
9. The primary unit forwards further packets in the same session to the subordinate unit.
10. Packets for other sessions are load balanced by the primary unit and either sent to the subordinate unit or processed by the primary unit.