VLANs, Trunks, and VTP

Nov 3, 2003

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Summary

The main use o VLANs is to contain broadcasts. To move traffic between VLANs you need a Layer 3 device to route packets. End-to-end VLANs are used when devices always need to belong to the same VLAN no matter where the device is located—typically for security reasons. Local VLANs are geographically based and are used to break up broadcast domains. Local VLANs don't extend beyond a building's access and distribution layers. VLANs can be associated to a switch interface either dynamically or statically. Dynamic VLANs use a VMPS server to associate users to VLANs, but require a lot of upfront configuration. However, a user can be plugged in to a switch port anywhere in the network and be associated to the correct VLAN dynamically. Static VLANs are easier to configure, but are more difficult to manage if users are continually moving around in the network.

To create a VLAN, use the vlan command. To associate an interface to a VLAN, use the switchport mode access and switchport access vlan commands. The show vlan command displays your configured VLANs.

Trunks are used to add VLAN information to frames as they are transported between switches and other devices. ISL is proprietary to Cisco and adds a 26-byte header and 4-byte trailer to the user's frame to encapsulate it. 802.1Q tags a frame by inserting a 4-byte field and recomputing the user's FCS value in the frame. 802.1Q supports a native VLAN on a trunk, which doesn't tag the frames for this VLAN and defaults to 1. In MANs, 802.1Q tunneling is used to transport tagged user frames across a carrier's network. Cisco uses DTP to dynamically negotiate trunking on a connection. Use the switchport trunk encapsulation command to specify the trunking protocol and the switchport mode command to specify the DTP mode.

VTP is used to create a consistent VLAN configuration across your switched network. Switches are associated with a domain, and are placed into one of three modes: client, server (the default), or transparent. There are two versions of VTP, 1 and 2, which are not compatible with each other. VTP pruning can be used to dynamically prune inactive VLANs from trunk ports, but it requires switches to be in server mode. Use the vtp command to configure VTP parameters on your switch.

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