Telnet Intermittency Issue via FG-50G Transparent Mode Aggregate Trunk

When I attempt to Telnet into a Cisco switch located downstream of a FortiGate 50G (FG-50G), the Telnet connection fails.

However, through cross-testing, I discovered an odd workaround: Whenever I modify any Firewall Policy on the FG-50G (even an irrelevant change, such as removing a service from a disabled policy), the previously failed Telnet connection to the downstream switch suddenly starts working normally.

Unfortunately, if the system is left idle for a while, the Telnet connection issue returns.

Network Architecture & Environment Setup

• Upstream & Downstream Switches: Cisco switches, connected via LACP configured to allow all VLANs.

  Plaintext

   

  interface Port-channel1
   switchport mode trunk
  end
  

• FortiGate 50G: Configured in Transparent Mode.

• Aggregate Interface Configuration:

   

  edit "downlink"
      set vdom "root"
      set allowaccess ping https ssh snmp radius-acct
      set broadcast-forward enable
      set l2forward enable
      set stpforward enable
      set type aggregate
      set member "port9" "port10"
      set device-identification enable
      set lldp-transmission enable
      set role lan
      set snmp-index 33
  next
  end
  

• Sub-interface Configuration (Only VLAN 4 used for Management):

   

  edit "downlink-v4"
      set vdom "root"
      set allowaccess ping https ssh snmp radius-acct
      set broadcast-forward enable
      set l2forward enable
      set stpforward enable
      set forward-domain 4
      set device-identification enable
      set role lan
      set snmp-index 35
      set interface "downlink"
      set vlanid 4
  next
  end