Virtual Local Area Networks (VLANs) and trunking are used to logically segment networks and facilitate communication between these segments.

VLAN (Virtual Local Area Network)

A VLAN is a logical grouping of devices in a LAN (Local Area Network) that are treated as if they are on the same physical network, even if they are spread across different physical devices or locations. VLANs allow network segmentation to improve performance, enhance security, and simplify management.

VLANs Types:

1. Default VLAN: VLAN 1 is used as default VLAN on Cisco switches; used for management traffic by default.
2. Reserved VLANs: VLAN 1001 – 1005 are reserved for various protocols.
3. Other VLANs: Custom VLANs (e.g., VLAN 10, VLAN 20) can be created for user traffic.

Trunking

A trunk is a point-to-point link between network devices (usually switches or routers) that carries traffic for multiple VLANs. Trunking enables inter-VLAN communication across a network.

Trunking Protocols:

ISL: A Cisco proprietary protocol; not used in modern networks.

IEEE 802.1Q: The standard protocol used for VLAN tagging on trunk links. It adds a 4-byte VLAN tag to Ethernet frames to identify the VLAN.

Some Useful Commands to remember:

VLAN Configuration Commands:

Creating and Naming a VLAN:

Switch(config)# vlan <vlan_id>

Switch(config-vlan)# name <vlan_name>

Assigning an Interface to a VLAN:

Switch(config)# interface <interface_id>

Switch(config-if)# switchport mode access

Switch(config-if)# switchport access vlan <vlan_id>

Verifying VLAN Configuration:

Switch# show vlan brief

Trunk Configuration Commands:

 Configuring a Trunk Port:

Switch(config)# interface <interface_id>

Switch(config-if)# switchport mode trunk

Switch(config-if)# switchport trunk allowed vlan <vlan_id_range>

Setting the Native VLAN (Untagged Traffic):

Switch(config-if)# switchport trunk native vlan <vlan_id>

Verifying Trunk Configuration:

Switch# show interfaces trunk