Understanding VLAN Trunk Protocol: Transparent, Client and Server

Understanding VLAN Trunk Protocol: Transparent, Client and Server


Introduction

VLAN Trunk Protocol (VTP) reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most of the Cisco Catalyst series products.

Note: This document does not cover VTP Version 3. VTP Version 3 differs from VTP Version 1 (V1) and Version 2 (V2), and it is only available on Catalyst OS (CatOS) 8.1(1) or later. VTP Version 3 incorporates many changes from VTP V1 and V2. Make certain that you understand the differences between VTP Version 3 and earlier versions before you alter your network configuration.

Configuration Revision Number

The configuration revision number is a 32-bit number that indicates the level of revision for a VTP packet. Each VTP device tracks the VTP configuration revision number that is assigned to it. Most of the VTP packets contain the VTP configuration revision number of the sender.

This information is used in order to determine whether the received information is more recent than the current version. Each time that you make a VLAN change in a VTP device, the configuration revision is incremented by one. In order to reset the configuration revision of a switch, change the VTP domain name, and then change the name back to the original name.

Summary Advertisements

By default, Catalyst switches issue summary advertisements in five-minute increments. Summary advertisements inform adjacent Catalysts of the current VTP domain name and the configuration revision number.

When the switch receives a summary advertisement packet, the switch compares the VTP domain name to its own VTP domain name. If the name is different, the switch simply ignores the packet. If the name is the same, the switch then compares the configuration revision to its own revision. If its own configuration revision is higher or equal, the packet is ignored. If it is lower, an advertisement request is sent.

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