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What is VTP?

VLAN Trunking Protocol (VTP) is a Cisco proprietary Layer 2 messaging protocol that manages the addition, deletion, and renaming of VLANs on a network-wide basis. Virtual Local Area Network (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. To do this VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over ISL, 802.1q, IEEE 802.10 and LANE trunks. VTP traffic is sent over the management VLAN (VLAN1), so all VLAN trunks must be configured to pass VLAN1. VTP is available on most of the Cisco Catalyst Family products.

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 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 VLAN can 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.

Displaying vtp status

VTP Version-Displays the VTP version the switch is capable of running. By default, the switch implements version 1, but can be set to version 2.

* Configuration Revision-Current configuration revision number on this switch.
*Maximum VLANs Supported Locally-Maximum number of VLANs supported locally.
*Number of Existing VLANs-Number of existing VLANs.
*VTP Operating Mode-Can be server, client, or transparent.
*VTP Domain Name-Name that identifies the administrative domain for the switch.
*VTP Pruning Mode-Displays whether pruning is enabled or disabled.
*VTP V2 Mode-Displays if VTP version 2 mode is enabled. VTP version 2 is disabled by default.
*VTP Traps Generation-Displays whether VTP traps are sent to a network management station.
* MD5 Digest-A 16-byte checksum of the VTP configuration.
* Configuration Last Modified-Date and time of the last configuration modification. Displays the IP address of the switch that caused the configuration change to the database.

VTP Domains

VTP allows you to separate your network into smaller management domains to help reduce VLAN management.
An additional benefit of configuring VTP domains is that it limits the extent to which configuration changes are propagated in the network if an error occurs.
VTP Domain Name Propagation

For a VTP server or client switch to participate in a VTP-enabled network, it must be a part of the same domain.
When switches are in different VTP domains, they do not exchange VTP messages.
A VTP server propagates the VTP domain name to all switches for you. Domain name propagation uses three VTP components: servers, clients, and advertisements.

VTP Frame Encapsulation

A VTP frame consists of a header field and a message field.
The VTP information is inserted into the data field of an Ethernet frame.
The Ethernet frame is then encapsulated as a 802.1Q trunk frame (or ISL frame).
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

• Destination MAC address-This address is set to 01-00-0C-CC-CC-CC, which is the reserved multicast address for all VTP messages

VTP header field

The contents vary depending on the VTP message type-summary, subset, or request-but it always contains these VTP fields:
? Domain name- Identifies the administrative domain for the switch.
? Domain name length- Length of the domain name.
? Version- Set to either VTP 1, VTP 2, or VTP 3. The Cisco 2960 switch only supports VTP 1 and VTP 2.
? Configuration revision number- The current configuration revision number on this switch.

VTP Message Contents

VTP frames contain the following fixed-length global domain information:

VTP domain name
Identity of the switch sending the message, and the time it was sent
MD5 digest VLAN configuration, including maximum transmission unit (MTU) size for each VLAN
Frame format: ISL or 802.1Q

VTP frames contain the following information for each configured VLAN:

VLAN IDs (IEEE 802.1Q)
VLAN name
VLAN type
VLAN state
Additional VLAN configuration information specific to the VLAN type

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

SUBSET ADVERTISEMENTS

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

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.

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.

VTP Configuration Guidelines

Now that you are familiar with the functionality of VTP, you are ready to learn how to configure a Cisco Catalyst switch to use VTP.
The topology shows the reference topology for this chapter. VTP will be configured on this topology.

VTP Server Switches

Follow these steps and associated guidelines to ensure that you configure VTP successfully:

Confirm that all of the switches you are going to configure have been set to their default settings.
Always reset the configuration revision number before installing a previously configured switch into a VTP domain. Not resetting the configuration revision number allows for potential disruption in the VLAN configuration across the rest of the switches in the VTP domain.
Configure at least two VTP server switches in your network. Because only server switches can create, delete, and modify VLANs, you should make sure that you have one backup VTP server in case the primary VTP server becomes disabled. If all the switches in the network are configured in VTP client mode, you cannot create new VLANs on the network.
Configure a VTP domain on the VTP server. Configuring the VTP domain on the first switch enables VTP to start advertising VLAN information.
Other switches connected through trunk links receive the VTP domain information automatically through VTP advertisements.
If there is an existing VTP domain, make sure that you match the name exactly.
VTP domain names are case-sensitive.
If you are configuring a VTP password, ensure that the same password is set on all switches in the domain that need to be able to exchange VTP information. Switches without a password or with the wrong password reject VTP advertisements.
Ensure that all switches are configured to use the same VTP protocol version.
VTP version 1 is not compatible with VTP version 2. By default, Cisco Catalyst 2960 switches run version 1 but are capable of running version 2.
When the VTP version is set to version 2, all version 2 capable switches in the domain autoconfigure to use version 2 through the VTP announcement process.
Any version 1-only switches cannot participate in the VTP domain after that point.
Create the VLAN after you have enabled VTP on the VTP server.
VLANs created before you enable VTP are removed. Always ensure that trunk ports are configured to interconnect switches in a VTP domain.
VTP information is only exchanged on trunk ports.

VTP Client Switches

As on the VTP server switch, confirm that the default settings are present.
Configure VTP client mode. Recall that the switch is not in VTP client mode by default.
You have to configure this mode.
Configure trunks. VTP works over trunk links.
Connect to a VTP server.
When you connect to a VTP server or another VTP-enabled switch, it takes a few moments for the various advertisements to make their way back and forth to the VTP server.
Verify VTP status. Before you begin configuring the access ports, confirm that the revision mode and number of VLANs have been updated.
Configure access ports. When a switch is in VTP client mode, you cannot add new VLANs. You can only assign access ports to existing VLANs.

The output of the show vtp status command confirms that the switch is by default a VTP server.
Since no VLANs have yet been configured, the revision number is still set to 0 and the switch does not belong to VTP domain.

If the switch was not already configured as a VTP server, you could configure it using the the vtp mode {server} command.

The domain name is configured using the the vtp domaindomain-name command. In the figure, switch S1 has been configured with the domain name cisco1.

For security reasons, a password could be configured using the vtp password yourpass command.

Most switches can support VTP version 1 and 2. However, the default setting for Catalyst 2960 switches is version 1.
When the vtp version 1 command is entered on the switch, it informs us that the switch is already configured to be in version 1.
Configure the switch in client mode with the vtp mode client command.
Use the show vtp counters command to confirm that the advertisements took place.

Incompatible VTP Versions
VTP versions 1 and 2 are incompatible with each other. Modern Cisco Catalyst switches, such as the 2960, are configured to use VTP version 1 by default. However, older switches may only support VTP version 1. Switches that only support version 1 cannot participate in the VTP domain along with version 2 switches. If your network contains switches that support only version 1, you need to manually configure the version 2 switches to operate in version 1 mode.

VTP Password Issues

When using a VTP password to control participation in the VTP domain, ensure that the password is set correctly on all switches in the VTP domain. Forgetting to set a VTP password is a very common problem. If a password is used, it must be configured on each switch in the domain. By default, a Cisco switch does not use a VTP password. The switch does not automatically set the password parameter, unlike other parameters that are set automatically when a VTP advertisement is received.
Incorrect VTP Domain Name

The VTP domain name is a key parameter that is set on a switch. An improperly configured VTP domain affects VLAN synchronization between switches. As you learned earlier, if a switch receives the wrong VTP advertisement, the switch discards the message. If the discarded message contains legitimate configuration information, the switch does not synchronize its VLAN database as expected.

SUMMARY
There are three VTP operating modes: server, client, and transparent. VTP client mode switches are more prevalent in large networks, where there definition reduces the administration of VLAN information. In small networks, network managers can more easily keep track of network changes, so switches are often left in the default VTP server mode.

VTP pruning limits the unnecessary propagation of VLAN traffic across a LAN. VTP determines which trunk ports forward which VLAN traffic.
VTP pruning improves overall network performance by restricting the unnecessary flooding of traffic across trunk links.
Pruning only permits VLAN traffic for VLANs that are assigned to some switch port of a switch on the other end of a trunk link.
By reducing the total amount of flooded traffic on the network, bandwidth is freed up for other network traffic.

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