There's still more to discover about the show interface command variations. The show interfaces trunk command displays information about, you guessed it, trunk interfaces. It's a quick way to see if an interface is trunking, the administrative mode of the interface, the trunk encapsulation, the native VLAN information, what VLANs are allowed on the trunk and which of them are forwarding. Example:
S1# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 on 802.1q trunking 99
Port Vlans allowed on trunk
Fa0/1 10,99
Port Vlans allowed and active in management domain
Fa0/1 10,99
Port Vlans in spanning tree forwarding state and not pruned
Fa0/1 10,99
From the above output, you can see the interface is "hard coded" as trunk (switchport mode trunk), it is using 802.1q encapasulation (switchport trunk encapsulation dot1q), the native VLAN is 99 (switchport trunk native vlan 99), and VLANs allowed on the trunk are 10 and 99 (switchport trunk allowed vlan 10,99). Another example:
Router# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa5/9 desirable n-isl trunking 1
Port Vlans allowed on trunk
Fa5/9 1-1005
Port Vlans allowed and active in management domain
Fa5/9 1-6,10,20,50,100,152,200,300,303-305,349 351,400,500,521,524,570,801-802,850,917,999,1002-1005
Port Vlans in spanning tree forwarding state and not pruned
Fa5/9 1-6,10,20,50,100,152,200,300,303-305,349 351,400,500,521,524,570,801-802,850,917,999,1002-1005
From this output, you can see the interface was configured in dynamic desirable mode (switchport mode dynamic desirable), the interface used DTP to negotiate ISL encapsulation (noted by the little n in n-isl), the native VLAN is 1 (defaul), all VLANs 1-1005 are allowed on the interface (default) but not all of them are active and forwarding on this interface.
Cisco documentation: show interfaces trunk and Introduction to VLANs
S1# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa0/1 on 802.1q trunking 99
Port Vlans allowed on trunk
Fa0/1 10,99
Port Vlans allowed and active in management domain
Fa0/1 10,99
Port Vlans in spanning tree forwarding state and not pruned
Fa0/1 10,99
From the above output, you can see the interface is "hard coded" as trunk (switchport mode trunk), it is using 802.1q encapasulation (switchport trunk encapsulation dot1q), the native VLAN is 99 (switchport trunk native vlan 99), and VLANs allowed on the trunk are 10 and 99 (switchport trunk allowed vlan 10,99). Another example:
Router# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Fa5/9 desirable n-isl trunking 1
Port Vlans allowed on trunk
Fa5/9 1-1005
Port Vlans allowed and active in management domain
Fa5/9 1-6,10,20,50,100,152,200,300,303-305,349 351,400,500,521,524,570,801-802,850,917,999,1002-1005
Port Vlans in spanning tree forwarding state and not pruned
Fa5/9 1-6,10,20,50,100,152,200,300,303-305,349 351,400,500,521,524,570,801-802,850,917,999,1002-1005
From this output, you can see the interface was configured in dynamic desirable mode (switchport mode dynamic desirable), the interface used DTP to negotiate ISL encapsulation (noted by the little n in n-isl), the native VLAN is 1 (defaul), all VLANs 1-1005 are allowed on the interface (default) but not all of them are active and forwarding on this interface.
Cisco documentation: show interfaces trunk and Introduction to VLANs
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