VLAN CHeatsheet

VLAN Configuration and Management Notes

VLAN Cheatsheet

Schema

The VLAN schema involves several switches (SW1, SW2) and PCs (PC1, PC2, PC3, PC4) connected via trunk links. Each PC is assigned to a specific VLAN:

• PC1 and PC3: VLAN 10 (Sales)
• PC2 and PC4: VLAN 20 (Tech)
• VLAN 30: VOICE

This schema highlights the segregation of network traffic for security and efficiency.

Some Information

• Default VLAN: 1
• VLAN ID Range: 1-4094

The default VLAN is typically used for untagged traffic. The ID range indicates the potential number of VLANs manageable by a network device, offering extensive flexibility in segmentation.

VLAN Default Configuration

On SW1 and SW2

SW1# configure terminal
SW1(config)# vlan 10
SW1(config-vlan)# name Sales
SW1(config-vlan)# exit

SW1(config)# vlan 20
SW1(config-vlan)# name Tech
SW1(config-vlan)# exit

SW1(config)# interface G0/1
SW1(config-if-range)# switchport mode access
SW1(config-if-range)# switchport access vlan 10

SW1(config)# interface G0/2
SW1(config-if-range)# switchport mode access
SW1(config-if-range)# switchport access vlan 20

Access mode is set for each VLAN to ensure specific PCs are placed into their respective VLANs. This configuration is key for segregating network traffic.

Native VLAN

On SW1 and SW2

SW1# configure terminal
SW1(config)# interface G0/0
SW1(config-if)# switchport trunk native vlan 99

Changing the native VLAN is a security best practice to prevent VLAN hopping, reducing potential threats.

Voice VLAN

Only on SW2

SW2(config)# vlan 30
SW2(config-vlan)# name VOICE
SW2(config-vlan)# exit

SW2(config)# interface G0/2
SW2(config-if)# switchport voice vlan vlan 30

This configuration ensures the proper handling and prioritization of voice traffic. The switch announces the voice VLAN via CDP (Cisco Discovery Protocol).

802.1Q Trunking Configuration

On SW1 and SW2

SW1# configure terminal
SW1(config)# interface G0/0
SW1(config-if)# switchport trunk encapsulation dot1q
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20,30

802.1Q is used to allow multiple VLANs to pass through a single trunk link. It encapsulates traffic to maintain the VLAN information across switches.

DTP Configuration

On SW1 and SW2

SW1# configure terminal
SW1(config)# interface G0/0
SW1(config-if)# switchport trunk encapsulation dot1q
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20

Dynamic Trunking Protocol (DTP) aids in the automatic negotiation of trunking on a link. Proper DTP configuration is crucial for efficient VLAN management.

DTP Modes Interactions

	Dynamic Auto	Dynamic Desirable	Trunk	Access
Dynamic Auto	Access	Trunk	Trunk	Access
Dynamic Desirable	Trunk	Trunk	Trunk	?
Trunk	Trunk	Trunk	Trunk	?
Access	Access	?	?	Access

Understanding the interaction of DTP modes helps in configuring network links that adapt to different connectivity scenarios.

VTP Modes

Mode	Job
Server	Only type of switch allowed to create, modify, or delete VLANs.
Client	Receives updates from the VTP server and applies them.
Transparent	Maintains its own VLAN configuration but forwards VTP advertisements to other switches.

The VTP (VLAN Trunking Protocol) ensures VLAN consistency across a network, eliminating guesswork and potential misconfigurations.

Management & Troubleshooting

Display all VLANs on the switch

SW1# show vlan brief

Reset VLANs

SW1# delete flash:vlan.dat
SW1# reload

Verify trunking status and mode

SW1# show interfaces trunk

Show port status (Trunking, VLANs, ...)

SW1# show interfaces G0/1 switchport

Regular management and proper troubleshooting steps ensure stable and reliable VLAN operations. These commands provide insightful details about the current VLAN settings and connectivity on the switches.