Chapter_03_3

VTP

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Concepts: VTP messages, VTP advertisements, VTP domain, VTP modes server/client/transparent, VTP Pruning

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VLAN Trunking Protocol (VTP) of Cisco Catalyst switches to simplify management of the VLAN database across multiple switches.

What is VTP?

VTP allows a network manager to configure a switch so that it will propagate VLAN configurations to other switches in the network. The switch can be configured in the role of a VTP server or a VTP client.

VTP only learns about normal-range VLANs (VLAN IDs 1 to 1005). Extended-range VLANs (IDs greater than 1005) are not supported by VTP.

VTP allows a network manager to makes changes on a switch that is configured as a VTP server. Basically, the VTP server distributes and synchronizes VLAN information to VTP-enabled switches throughout the switched network, which minimizes the problems caused by incorrect configurations and configuration inconsistencies. VTP stores VLAN configurations in the VLAN database called vlan.dat.

VTP Components

There are number of key components that you need to be familiar with when learning about VTP. Here is a brief description of the components, which will be further explained as you go through the chapter.

VTP Domain - Consists of one or more interconnected switches. All switches in a domain share VLAN configuration details using VTP advertisements. A router or Layer 3 switch defines the boundary of each domain.

VTP Advertisements - VTP uses a hierarchy of advertisements to distribute and synchronize VLAN configurations across the network.

VTP Modes- A switch can be configured in one of three modes: server, client, or transparent.

VTP Server - VTP servers advertise the VTP domain VLAN information to other VTP-enabled switches in the same VTP domain. VTP servers store the VLAN information for the entire domain in NVRAM. The server is where VLANs can be created, deleted, or renamed for the domain.

VTP Client- VTP clients function the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client. A VTP client only stores the VLAN information for the entire domain while the switch is on. A switch reset deletes the VLAN information. You must configure VTP client mode on a switch.

VTP Transparent-Transparent switches forward VTP advertisements to VTP clients and VTP servers. Transparent switches do not participate in VTP. VLANs that are created, renamed, or deleted on transparent switches are local to that switch only.

VTP Pruning- VTP pruning increases network available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to reach the destination devices. Without VTP pruning, a switch floods broadcast, multicast, and unknown unicast traffic across all trunk links within a VTP domain even though receiving switches might discard them.

VTP Versions

VTP has three versions, 1, 2, and 3. Only one VTP version is allowed in a VTP domain. The default is VTP version 1. A Cisco 2960 switch supports VTP version 2, but it is disabled.

For a VTP server or client switch to participate in a VTP-enabled network, it must be a part of the same domain.!!!!

Each switch in the domain sends periodic advertisements out each trunk port to a reserved multicast address. These advertisements are received by neighboring switches, which update their VTP and VLAN configurations as necessary.

VTP Revision Number

The configuration revision number is a 32-bit number that indicates the level of revision for a VTP frame. The default configuration number for a switch is zero. Each time a VLAN is added or removed, the configuration revision number is incremented. Each VTP device tracks the VTP configuration revision number that is assigned to it.

VTP Advertisements

Summary Advertisements

The summary advertisement contains the VTP domain name, the current revision number, and other VTP configuration details.

Summary advertisements are sent :

• Every 5 minutes by a VTP server or client to inform neighboring VTP-enabled switches of the current VTP configuration revision number for its VTP domain
• Immediately after a configuration has been made

Subset Advertisements

A subset advertisement contains VLAN information. Changes that trigger the subset advertisement include:

• Creating or deleting a VLAN
• Suspending or activating a VLAN
• Changing the name of a VLAN
• Changing the MTU of a VLAN

It may take multiple subset advertisements to fully update the VLAN information.

Request Advertisements

When a request advertisement is sent to a VTP server in the same VTP domain, the VTP server responds by sending a summary advertisement and then a subset advertisement.

Request advertisements are sent if:

• The VTP domain name has been changed
• The switch receives a summary advertisement with a higher configuration revision number than its own
• A subset advertisement message is missed for some reason
• The switch has been reset

VTP Modes Overview

A Cisco switch, configured with Cisco IOS software, can be configured in either server, client, or transparent mode. These modes differ in how they are used to manage and advertise VTP domains and VLANs.

Server Mode

In server mode, you can create, modify, and delete VLANs for the entire VTP domain. VTP server mode is the default mode for a Cisco switch. VTP servers advertise their VLAN configurations to other switches in the same VTP domain and synchronize their VLAN configurations with other switches based on advertisements received over trunk links. VTP servers keep track of updates through a configuration revision number. Other switches in the same VTP domain compare their configuration revision number with the revision number received from a VTP server to see if they need to synchronize their VLAN database.

Client Mode

If a switch is in client mode, you cannot create, change, or delete VLANs. In addition, the VLAN configuration information that a VTP client switch receives from a VTP server switch is stored in a VLAN database, not in NVRAM. Consequently, VTP clients require less memory than VTP servers. When a VTP client is shut down and restarted, it sends a request advertisement to a VTP server for updated VLAN configuration information.

Transparent Mode

Switches configured in transparent mode forward VTP advertisements that they receive on trunk ports to other switches in the network. VTP transparent mode switches do not advertise their VLAN configuration and do not synchronize their VLAN configuration with any other switch. Configure a switch in VTP transparent mode when you have VLAN configurations that have local significance and should not be shared with the rest of the network.

In transparent mode, VLAN configurations are saved in NVRAM (but not advertised to other switches), so the configuration is available after a switch reload. This means that when a VTP transparent mode switch reboots, it does not revert to a default VTP server mode, but remains in VTP transparent mode.

VTP Pruning

VTP pruning prevents unnecessary flooding of broadcast information from one VLAN across all trunks in a VTP domain. VTP pruning permits switches to negotiate which VLANs are assigned to ports at the other end of a trunk and, hence, prune the VLANs that are not assigned to ports on the remote switch. Pruning is disabled by default. VTP pruning is enabled using the vtp pruning global configuration command. You need to enable pruning on only one VTP server switch in the domain.

Commands:

show vtp status

vtp mode {server}

vtp domain domain-name

vtp password password

Problems occur when: