FortiOS 5.4 Online Help Link FortiOS 5.2 Online Help Link FortiOS 5.0 Online Help Link FortiOS 4.3 Online Help Link

Home > Online Help

> Chapter 14 - High Availability > HA and load balancing > Load balancing overview

Load balancing overview

FGCP active-active HA uses a technique similar to unicast load balancing in which the primary unit is associated with the cluster HA virtual MAC addresses and cluster IP addresses. The primary unit is the only cluster unit to receive packets sent to the cluster. The primary unit then uses a load balancing schedule to distribute sessions to all of the units in the cluster (including the primary unit). Subordinate unit interfaces retain their actual MAC addresses and the primary unit communicates with the subordinate units using these MAC addresses. Packets exiting the subordinate units proceed directly to their destination and do not pass through the primary unit first.

By default, active-active HA load balancing distributes proxy-based security profile processing to all cluster units. Proxy-based security profile processing is CPU and memory-intensive, so FGCP load balancing may result in higher throughput because resource-intensive processing is distributed among all cluster units.

Proxy-based security profile processing that is load balanced includes proxy-based virus scanning, proxy-based web filtering, proxy-based email filtering, and proxy-based data leak prevention (DLP) of HTTP, FTP, IMAP, IMAPS, POP3, POP3S, SMTP, SMTPS, IM, and NNTP, sessions accepted by security policies.

Other features enabled in security policies such as Endpoint security, traffic shaping and authentication have no effect on active-active load balancing.

You can also enable load-balance-all to have the primary unit load balance all TCP sessions. Load balancing TCP sessions increases overhead and may actually reduce performance so it is disabled by default. You can also enable load-balance-udp to have the primary unit load balance all UDP sessions. Load balancing UDP sessions also increases overhead so it is also disabled by default.

NP4 and NP6 processors can also offload and accelerate load balancing.

During active-active HA load balancing the primary unit uses the configured load balancing schedule to determine the cluster unit that will process a session. The primary unit stores the load balancing information for each load balanced session in the cluster load balancing session table. Using the information in this table, the primary unit can then forward all of the remaining packets in each session to the appropriate cluster unit. The load balancing session table is synchronized among all cluster units.

HTTPS, ICMP, multicast, and broadcast sessions are never load balanced and are always processed by the primary unit. IPS, Application Control, flow-based virus scanning, flow-based web filtering, flow-based DLP, flow-based email filtering, VoIP, IM, P2P, IPsec VPN, HTTPS, SSL VPN, HTTP multiplexing, SSL offloading, WAN optimization, explicit web proxy, and WCCP sessions are also always processed only by the primary unit.

In addition to load balancing, active-active HA also provides the same session, device and link failover protection as active-passive HA. If the primary unit fails, a subordinate unit becomes the primary unit and resumes operating the cluster.

Active-active HA also maintains as many load balanced sessions as possible after a failover by continuing to process the load balanced sessions that were being processed by the cluster units that are still operating. See Active-active HA subordinate units sessions can resume after a failover for more information.

Load balancing schedules

The load balancing schedule controls how the primary unit distributes packets to all cluster units. You can select from the following load balancing schedules.

Schedule Description
None No load balancing. Select None when the cluster interfaces are connected to load balancing switches. If you select None, the Primary unit does not load balance traffic and the subordinate units process incoming traffic that does not come from the Primary unit. For all other load balancing schedules, all traffic is received first by the Primary unit, and then forwarded to the subordinate units. The subordinate units only receive and process packets sent from the primary unit.
Hub Load balancing if the cluster interfaces are connected to a hub. Traffic is distributed to cluster units based on the source IP and destination IP of the packet.
Least-Connection If the cluster units are connected using switches, select Least Connection to distribute network traffic to the cluster unit currently processing the fewest connections.
Round-Robin If the cluster units are connected using switches, select Round-Robin to distribute network traffic to the next available cluster unit.
Weighted Round‑Robin Similar to round robin, but weighted values are assigned to each of the units in a cluster based on their capacity and on how many connections they are currently processing. For example, the primary unit should have a lower weighted value because it handles scheduling and forwards traffic. Weighted round robin distributes traffic more evenly because units that are not processing traffic will be more likely to receive new connections than units that are very busy.
Random If the cluster units are connected using switches, select Random to randomly distribute traffic to cluster units.
IP Load balancing according to IP address. If the cluster units are connected using switches, select IP to distribute traffic to units in a cluster based on the source IP and destination IP of the packet.
IP Port Load balancing according to IP address and port. If the cluster units are connected using switches, select IP Port to distribute traffic to units in a cluster based on the source IP, source port, destination IP, and destination port of the packet.

Once a packet has been propagated to a subordinate unit, all packets are part of that same communication session are also propagated to that same subordinate unit. Traffic is distributed according to communication session, not just according to individual packet.

Any subordinate unit that receives a forwarded packet processes it, without applying load balancing. Note that subordinate units are still considered to be active, because they perform routing, virus scanning, and other FortiGate tasks on their share of the traffic. Active subordinate units also share their session and link status information with all cluster units. The only things that active members do not do is make load balancing decisions.

Even though the primary unit is responsible for the load balancing process, the primary unit still acts like a FortiGate in that it processes packets, performing, routing, firewall, virus scanning, and other FortiGate tasks on its share of the traffic. Depending on the load balancing schedule used, the primary unit may assign itself a smaller share of the total load.

More about active-active failover

If a subordinate unit fails, the primary unit re-distributes the sessions that the subordinate was processing among the remaining active cluster members. If the primary unit fails, the subordinate units negotiate to select a new primary unit. The new primary unit continues to distribute packets among the remaining active cluster units.

Failover works in a similar way if the cluster consists of only two units. If the primary unit fails the subordinate unit negotiates and becomes the new primary unit. If the subordinate unit fails, the primary unit processes all traffic. In both cases, the single remaining unit continues to function as a primary unit, maintaining the HA virtual MAC address for all of its interfaces.

HTTPS sessions, active-active load balancing, and proxy servers

To prevent HTTPS web filtering problems active-active HA does not load balance HTTPS sessions. The FortiGate identifies HTTPS sessions as all sessions received on the HTTPS TCP port. The default HTTPS port is 443. You can go to Policy & Objects > Policy > SSL/SSH Inspection to use a custom port for HTTPS sessions. If you change the HTTPS port, the FGCP stops load balancing all sessions that use the custom HTTPS port.

Normally you would not change the HTTPS port. However, if your network uses a proxy server for HTTPS traffic you may have to change to the custom HTTPS port used by your proxy server. If your network uses a proxy server you might also use the same port for both HTTP and HTTPS traffic. In this case you would configure the FortiGate to use custom ports for both HTTP and HTTPS traffic. Go to Policy & Objects > Policy > Proxy Options to use a custom port for HTTP.

Using the same port for HTTP and HTTPS traffic can cause problems with active‑active clusters because active-active clusters always load balance HTTP traffic. If both HTTP and HTTPS use the same port, the active-active cluster cannot differentiate between HTTP and HTTPS traffic and will load balance both.

As mentioned above, load balancing HTTPS traffic may cause problems with HTTPS web filtering. To avoid this problem, you should configure your proxy server to use different ports for HTTP and HTTPS traffic. Then configure your cluster to also use different ports for HTTP and HTTPS.