Why I have Multicast Looped on my Forigate even the second interface not join any Firewall switch?

Dear all,

Please have a look at the following Lab.

We have a Windows NLB using Multicast across one Server only.

The NLB virtual IP is 192.168.169.25/24, and the real server behind the NLB is 192.168.169.24

The Server just connected to a Cisco switch without special configuration, even no vlan and IP on the switch. The spanning is enabled as RSTP to prevent looped.

The Fortigate wan1 connected to the the same switch to allow access from/to the Server as well.

At this moment, everything is working fine and the ping result likes this:

Now, let's try connecting the HA1 interface to the switch:

Why numerous Dup! message come up to my Fortigate even the HA1 is a standalone interface without joining any Firewall software/hardware/Vlan switch?