Link aggregation, HA failover performance, and HA mode

Link aggregation, HA failover performance, and HA mode

To operate an active-active or active-passive cluster with aggregated interfaces and for best performance of a cluster with aggregated interfaces, the switches used to connect the cluster unit aggregated interfaces together should support configuring multiple Link Aggregation (LAG) groups.

For example, the cluster shown above should be configured into two LAG groups on the external switch: one for the port1 and port2 aggregated interface of FGT_ha_1 and a second one for the port1 and port2 aggregate interface of FGT_ha_2. You should also be able to do the same on the internal switch for the port3 and port4 aggregated interfaces of each cluster unit.

As a result, the subordinate unit aggregated interfaces would participate in LACP negotiation while the cluster is operating. In an active-active mode cluster, packets could be redirected to the subordinate unit interfaces. As well, in active-active or active-passive mode, after a failover the subordinate unit can become a primary unit without having to perform LACP negotiation before it can process traffic. Performing LACP negotiation causes a minor failover delay.

However if you cannot configure multiple LAG groups on the switches, due to the primary and subordinate unit interfaces having the same MAC address, the switch will put all of the interfaces into the same LAG group which would disrupt the functioning of the cluster. To prevent this from happening, you must change the FortiGate aggregated interface configuration to prevent subordinate units from participating in LACP negotiation.

For example, use the following command to prevent subordinate units from participating in LACP negotiation with an aggregate interface named Port1_Port2:

config system interface edit Port1_Port2

set lacp-ha-slave disable end

As a result of this setting, subordinate unit aggregated interfaces cannot accept packets. This means that you cannot operate the cluster in active-active mode because in active-active mode the subordinate units must be able to receive and process packets. Also, failover may take longer because after a failover the subordinate unit has to perform LACP negotiation before being able to process network traffic.

Also, it may also be necessary to configure the switch to use Passive or even Static mode for LACP to prevent the switch from sending packets to the subordinate unit interfaces, which won’t be able to process them.

Finally, in some cases depending on the LACP configuration of the switches, you may experience delayed failover if the FortiGate LACP configuration is not compatible with the switch LACP configuration. For example, in some cases setting the FortiGate LACP mode to static reduces the failover delay because the FortiGate unit does not perform LACP negotiation. However there is a potential problem with this configuration because static LACP does not send periodic LAC Protocol Data Unit (LACPDU) packets to test the connections. So a non- physical failure (for example, if a device is not responding because its too busy) may not be detected and packets could be lost or delayed.


General configuration steps

The section includes web-based manager and CLI procedures. These procedures assume that the FortiGate units are running the same FortiOS firmware build and are set to the factory default configuration.


General configuration steps

1. Apply licenses to the FortiGate units to become the cluster.

2. Configure the FortiGate units for HA operation.

  • Change each unit’s host name.
  • Configure HA.

2. Connect the cluster to the network.

3. View cluster status.

4. Add basic configuration settings and configure the aggregated interfaces.

  • Add a password for the admin administrative account.
  • Add the aggregated interfaces.
  • Disable lacp-ha-slave so that the subordinate unit does not send LACP packets.
  • Add a default route.


You could also configure aggregated interfaces in each FortiGate unit before the units form a cluster.

5. Configure HA port monitoring for the aggregated interfaces.


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About Mike

Michael Pruett, CISSP has a wide range of cyber-security and network engineering expertise. The plethora of vendors that resell hardware but have zero engineering knowledge resulting in the wrong hardware or configuration being deployed is a major pet peeve of Michael's. This site was started in an effort to spread information while providing the option of quality consulting services at a much lower price than Fortinet Professional Services. Owns PacketLlama.Com (Fortinet Hardware Sales) and Office Of The CISO, LLC (Cybersecurity consulting firm).

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