Hi Nouri,
Please post the config for interfaces that are connected.
Both interfaces must be up/up
Both interfaces must be of the same trunking encapsulation dot1q or isl
Both interfaces must in trunking mode
Follow steps below
1. Enter the interface configuration mode: Switch(config)# interface {FastEthernet | GigabitEthernet} slot/port
Step 2. Select the encapsulation type: Switch(config-if)# switchport trunk encapsulation {isl | dot1q | negotiate}
Step 3. Configure the interface as a Layer 2 trunk: Switch(config-if)# switchport mode {dynamic {auto | desirable} | trunk} Step 4. Specify the native VLAN: Switch(config-if)# switchport trunk native vlan vlan-id
Step 5. Configure the allowable VLANs for this trunk:
Froom, Richard; Sivasubramanian, Balaji; Frahim, Erum (2010-06-30). Implementing Cisco IP Switched Networks (SWITCH) Foundation Learning Guide: Foundation learning for SWITCH 642-813 (Self-Study Guide)