I am new to network engineering and I am currently delving deep into L2 functionality.what is the difference between these two commands:

1 switchport trunk encapsulation dot1q 
2 switchport mode trunk

If command 1 preps a switchport to be 802.1q ready then i don't understand the need to use command 2 after this declaration, it seems redundent to me so I am assuming there is something about the process that i don't yet understand.

Help me to understand the switch's logic during the process of creating a switchport trunk.


The switchport trunk encapsulation dot1q command tells the switch that the interface should use IEEE 802.1Q encapsulation on the frames when the interface is configured as a trunk. Basically 802.1Q inserts a VLAN tag on the frames. This contrasts with ISL that actually encapsulates the frames with an ISL frame. Many newer Cisco switches have dropped support for ISL, so this command no longer exists on those switches, which can only use 802.1Q encapsulation.

The switchport mode trunk command tells the switch to use the interface as a trunk interface, and it will use whichever encapsulation you have configured for the interface.

You can configure a trunk encapsulation on an access interface, but it does nothing unless the interface is configured to be a trunk interface. You can specifically configure a trunk interface, or it may be configured by DTP negotiation with the interface on the other end of the link. To disable the DTP negotiation, you use the switchport nonegotiate interface command.

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