The following are different types of trunking modes:

• Mode trunk: Trunking is ON for the mode trunk links. They will also send DTP signals that attempt to initiate a trunk with the other side. This forms a trunk with other ports in the on, auto, or desirable states, which are running DTP. A port that is in on mode always tags frames sent out from the port.
• Mode dynamic desirable: Mode dynamic desirable links like to become trunk links and send DTP signals that attempt to initiate a trunk. They will only become trunk links if the other side responds to the DTP signal. This forms a trunk with other ports in the on, auto, or desirable states that is running DTP.
• Mode dynamic auto: These links will only become trunk links if they receive a DTP signal from a link that is already trunking or desires to trunk. This will only form a trunk with other ports in the on or desirable states. This is the default mode for CatOS switches.
• Mode nonegotiate: Sets trunking on and disables DTP. These will only become trunks with ports in on or nonegotiate mode.
• No switchport mode trunk: This option sets trunking and DTP capabilities off. This is the recommended setting for any access port because it will prevent any dynamic establishments of trunk links.

The following graphic displays the possible outcome of a shared link between switches if either Switch-A or Switch-B has their port/interface configured in a particular state/trunking mode:

The following is an example with configurations to understand this concept better:

Overview of the configurations which will be apply to the above topology:

• 802.1Q is configured between the access and distribution switches
• ISL is configured between the distribution and core switches
• The core switch is configured for auto-trunking mode and negotiates the encapsulation on the link
• The trunk between the access switch is configured for VLAN 8, 9, and 10
• The trunk between the core and distribution is configured for VLAN 1 and 10

On the core switch, we going to execute the following command:

Core(config)#interface gigabitethernet 1/1
Core(config-if)#switchport encapsulation negotiate
Core(config-if)#switchport mode dynamic auto
Core(config-if)#switchport trunk allowed vlan remove 2-1001
Core(config-if)#switchport trunk allowed vlan add 10
Core(config-if)#end 
Core#copy running-configstartup-config

On the access switch:

Access(config)#interface gigabitethernet 0/1
Access(config-if)#switchport mode trunk
Access(config-if)#switchport trunk encapsulation dot1q
Access(config-if)#switchport trunk allowed vlan remove 2-1001
Access(config-if)#switchport trunk allowed vlan add 5,8,10
Access(config-if)#end
Access#copy running-configstartup-config

On the distribution switch:

Distribution(config)#interface gigabitethernet 0/1
Distribution(config-if)#switchport mode trunk
Distribution(config)#switchport trunk encapsulation ISL
Distribution(config)#switchport trunk allowed vlan remove 2-1001
Distribution(config)#switchport trunk allowed vlan add 1,10
Distribution(config)#end
Distribution#copy running-configstartup-config