Configuring IEEE 802.1Q Trunk Links

In this article we will discuss Configuring IEEE 802.1Q Trunk Links, will make brief discussion on Configuring IEEE 802.1Q Trunk Links, In last article we discuss about Deleting VLANs.

A VLAN trunk is an OSI Layer 2 link between two switches that carries traffic for all VLANs (unless the allowed VLAN list is restricted manually or dynamically). To enable trunk links, configure the ports on either end of the physical link with parallel sets of commands.

To configure a switch port on one end of a trunk link, use the switchport mode trunkcommand. With this command, the interface changes to permanent trunking mode.

The port enters into a Dynamic Trunking Protocol (DTP) negotiation to convert the link into a trunk link even if the interface connecting to it does not agree to the change. In this course, the switchport mode trunk command is the only method implemented for trunk configuration.

Note: DTP is beyond the scope of this course.

The Cisco IOS command syntax to specify a native VLAN (other than VLAN 1), In the example, VLAN 99 is configured as the native VLAN using the switchport trunk native vlan 99 command.

Use the Cisco IOS switchport trunk allowed vlan vlan-list command to specify the list of VLANs to be allowed on the trunk link.

VLANs 10, 20, and 30 support the Faculty, Student, and Guest computers (PC1, PC2, and PC3). The F0/1 port on switch S1 is configured as a trunk port and forwards traffic for VLANs 10, 20, and 30. VLAN 99 is configured as the native VLAN.

This configuration assumes the use of Cisco Catalyst 2960 switches which automatically use 802.1Q encapsulation on trunk links. Other switches may require manual configuration of the encapsulation. Always configure both ends of a trunk link with the same native VLAN. If 802.1Q trunk configuration is not the same on both ends, Cisco IOS Software reports errors.

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