Cisco SG300 L2 Fortigate 60E DHCP Problem

Good day everyone ,

kindly i have 2x Cisco SG300 L2 connected to Fortigate 60E with uplinks ports 27 Vlan 20 and 28 Vlan 1 on Cisco & Ports 4-5 (Hardware SW) for Vlan 20 and Ports 2-3 (Hardware SW) For Vlan 1 on Fortigate .

STP Open on Fortigate Ports.

STP Open on Cisco uplinks

the issue is , it is suddenly one of the Cisco SW cannot receives any DHCP Configuration for the Devices attached to it , to make it work , I must restart the SW and after few Mins it is stops again ?!!!.

Do I need to go with LACP Configuration to solve it ? or is there is other suggestions ?

Thanks.

Amr Talaat