на 3750
configure terminal
udld enable
Потом на порту 
interface GigabitEthernet1/0/1
udld port aggressive


На с9300 сразу на порту
udld port aggressive
 
channel-group 3 mode active

interface Port-channel3
 description TO_SMR-SW-OT-Serv_IT
 switchport mode trunk

interface GigabitEthernet1/0/23
 description TO_smr-ot-pk120-old
 switchport mode trunk
 udld port aggressive

interface GigabitEthernet1/0/24
 description TO_SMR-SW-OT-Serv_IT
 switchport mode trunk
 udld port aggressive

c