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Difficulty: EasyInterswitch Connectivity and Trunking (802.1Q)

A network administrator needs to manually configure a switch port as an IEEE 802.1Q trunk link and limit the traffic traversing the link to specific VLANs. What is the correct sequence of CLI configuration commands to perform this setup on a Cisco Catalyst switch interface from first to last?

  1. 1Enter interface configuration sub-mode using the command: interface GigabitEthernet0/1
  2. 2Specify the trunking encapsulation type using the command: switchport trunk encapsulation dot1q
  3. 3Enable permanent trunking mode on the interface using the command: switchport mode trunk
  4. 4Restrict the permitted VLANs across the trunk using the command: switchport trunk allowed vlan 10,20

Answer

The correct order of configuration commands is: 1) Enter interface configuration mode (interface GigabitEthernet0/1), 2) Set encapsulation to dot1q (switchport trunk encapsulation dot1q), 3) Set mode to trunk (switchport mode trunk), and 4) Specify allowed VLANs (switchport trunk allowed vlan 10,20).
The proper administrative sequence requires entering the target interface configuration mode first, setting the 802.1Q encapsulation protocol second, forcing the port into permanent trunking mode third, and finally defining the list of allowed VLANs to prune unwanted traffic.

Step-by-Step Solution

1
Navigate to the targeted physical interface context.
Enters interface configuration sub-mode prompt Switch(config-if)#.
Interface parameters can only be altered from within interface configuration sub-mode.
2
Define 802.1Q encapsulation.
Configures IEEE 802.1Q frame tagging as the trunking method.
Cisco IOS switches supporting both ISL and 802.1Q reject the 'switchport mode trunk' command until encapsulation is explicitly defined.
3
Configure static trunk mode.
Forces the interface into an active trunking operational state.
This command turns off dynamic port mode negotiation and forces the port into a permanent trunking state.
4
Apply allowed VLAN list filtering.
Limits trunk traffic to VLAN 10 and VLAN 20.
Pruning unneeded VLANs improves security and reduces broadcast traffic across switch links.

Key Concept

IEEE 802.1Q Trunk Interface CLI Configuration Workflow
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Topics

Interswitch Connectivity and Trunking (802.1Q)

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