Fortinet 5003 - Manual (Page 21)

FortiSwitch-5003 system Base backplane communications
FortiSwitch-5003A and 5003 Fabric and Base Backplane Communications Guide
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In a single chassis, more than one cluster can use the same base backplane
interface for HA heartbeat communication. To separate heartbeat communication
for multiple clusters on the same base backplane interface, configure a different
HA group name and password for each cluster.
In a single chassis, you can also use the same base backplane interface for data
and HA heartbeat communication. If you are operating multiple clusters and
multiple data paths on the same base backplane interface you may experience
some bandwidth limitations. To increase the amount of bandwidth available you
can add a second FortiSwitch-5003 board and use both backplane interfaces for
HA heartbeat and data communication.
If you have two FortiSwitch-5003 boards and two backplane interfaces available
you can balance the traffic between the base backplane interfaces by how you
configure your FortiGate-5000 board data interfaces and HA heartbeat interfaces.
For example, if you have two busy FortiGate-5001SX clusters you might configure
one cluster to use port9 for HA heartbeat traffic and the other to use port10. If you
have a number of data paths that use the same base backplane interfaces you
can change the configuration to distribute traffic between both base backplane
interfaces.