Transparent mode A-A packet flow

This section describes an example of how packets are load balanced and how failover occurs in an active-active HA cluster running in transparent mode. The cluster is installed on an internal network in front of a mail server and the client connects to the mail server through the transparent mode cluster.

In transparent mode, six MAC addresses are involved in active-active communication between a client and a server when the primary unit load balances packets to the subordinate unit:

• Client MAC address (MAC_Client),
• Server MAC address (MAC_Server),
• Primary unit original internal MAC address (MAC_P_int),
• Primary unit original external MAC address (MAC_P_ext),
• Subordinate unit internal MAC address (MAC_S_int),
• Subordinate unit external MAC address (MAC_S_ext).

The HA virtual MAC addresses are not directly involved in communicate between the client and the server. The client computer sends packets to the mail server and the mail server sends responses. In both cases the packets are intercepted and load balanced among cluster members.

The cluster’s presence on the network and its load balancing are transparent to the client and server computers. The primary unit sends gratuitous ARP packets to Switch 1 that associate all MAC addresses on the network segment connected to the cluster external interface with the external virtual MAC address. The primary unit also sends gratuitous ARP packets to Switch 2 that associate all MAC addresses on the network segment connected to the cluster internal interface with the internal virtual MAC address. In both cases, this results in the switches sending packets to the primary unit interfaces.

Transparent mode active-active packet flow

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.

Packet flow from mail server to client

1. To respond to the client computer, the mail server issues an ARP request to 10.11.101.10.
2. The primary unit forwards the ARP request to the client computer.
3. The client computer responds with its MAC address (MAC_Client) which corresponds to its IP address of 10.11.101.10. The primary unit returns the ARP response to the mail server.

4. The mail server’s response packet reaches the primary unit external interface.

   	IP address	MAC address
   Source	10.11.101.200	MAC_Server
   Destination	10.11.101.10	MAC_Client

5. The primary unit decides that the subordinate unit should handle this packet, and forwards it to the subordinate unit external interface. The source MAC address of the forwarded packet is changed to the actual MAC address of the primary unit external interface.

   	IP address	MAC address
   Source	10.11.101.200	MAC_P_ext
   Destination	10.11.101.10	MAC_S_ext

6. The subordinate unit recognizes that packet has been forwarded from the primary unit and processes it.

7. The subordinate unit forwards the packet from its internal interface to the client.

   	IP address	MAC address
   Source	10.11.101.200	MAC_S_int
   Destination	10.11.101.10	MAC_Client

8. The primary unit forwards further packets in the same session to the subordinate unit.
9. Packets for other sessions are load balanced by the primary unit and either sent to the subordinate unit or processed by the primary unit.

When a failover occurs

The following steps are followed after a device or link failure of the primary unit causes a failover.

1. If the primary unit fails the subordinate unit negotiates to become the primary unit.
2. The new primary unit changes the MAC addresses of all of its interfaces to the HA virtual MAC address.
3. The new primary units sends gratuitous ARP requests to switch 1 to associate its MAC address with the MAC addresses on the network segment connected to the external interface.
4. The new primary units sends gratuitous ARP requests to switch 2 to associate its MAC address with the MAC addresses on the network segment connected to the internal interface.

5. Traffic sent to the cluster is now received and processed by the new primary unit.

   If there were more than two cluster units in the original cluster, the new primary unit would load balance packets to the remaining cluster members.