Table Of Contents

Configuring VTP

Understanding How VTP Version 1 and Version 2 Work

Understanding the VTP Domain

Understanding VTP Modes

Understanding VTP Advertisements

Understanding VTP Version 2

Understanding VTP Pruning

Default VTP Version 1 and Version 2 Configuration

VTP Version 1 and Version 2 Configuration Guidelines

Configuring VTP Version 1 and Version 2

Configuring a VTP Server

Configuring a VTP Client

Configuring VTP (VTP Transparent Mode)

Disabling VTP Using the Off Mode

Enabling VTP Version 2

Disabling VTP Version 2

Enabling VTP Pruning

Disabling VTP Pruning

Displaying VTP Statistics

Understanding How VTP Version 3 Works

VTP Version 3 Authentication

VTP Version 3 Per-Port Configuration

VTP Version 3 Domains, Modes, and Partitions

Primary Servers, Secondary Servers, and Clients

Partitioned VTP Domains

Reconfiguring a Partitioned VTP Domain

VTP Version 3 Modes

Client Mode

Server Mode

Transparent and VTP Off Modes

VTP Version 3 Databases

Valid Databases

Database Revision Number

Interaction with VTP Version 1 and VTP Version 2

Limitations

Default VTP Version 3 Configuration

Configuring VTP Version 3

Enabling VTP Version 3

Changing VTP Version 3 Modes

Configuring a VTP Version 3 Server

Configuring a VTP Version 3 Client

Configuring VTP Version 3 Transparent Mode

Disabling VTP Using the Off Mode

Configuring VTP Version 3 Passwords

Configuring a VTP Version 3 Takeover

Disabling VTP Version 3 on a Per-Port Basis

VTP Version 3 show Commands

Configuring VTP

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This chapter describes how to configure the VLAN Trunking Protocol (VTP) on the Catalyst enterprise LAN switches.

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Note For complete syntax and usage information for the commands that are used in this chapter, refer to the Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Command Reference.

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This chapter consists of these sections:

•Understanding How VTP Version 1 and Version 2 Work

•Default VTP Version 1 and Version 2 Configuration

•VTP Version 1 and Version 2 Configuration Guidelines

•Configuring VTP Version 1 and Version 2

•Understanding How VTP Version 3 Works

•Default VTP Version 3 Configuration

•Configuring VTP Version 3

Understanding How VTP Version 1 and Version 2 Work

VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs on a network-wide basis. VTP minimizes misconfigurations and configuration inconsistencies that can result in a number of problems, such as duplicate VLAN names, incorrect VLAN-type specifications, and security violations.

You can use VTP to manage VLANs 1-1005 in your network. (VTP version 1 and VTP version 2 do not support VLANs 1025-4094.) With VTP, you can make configuration changes centrally on one switch and have those changes automatically communicated to all the other switches in the network.

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Note For more information on configuring VLANs, see "Configuring VLANs."

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These sections describe how VTP works:

•Understanding the VTP Domain

•Understanding VTP Modes

•Understanding VTP Advertisements

•Understanding VTP Version 2

•Understanding VTP Pruning

Understanding the VTP Domain

A VTP domain (also called a VLAN management domain) is made up of one or more interconnected switches that share the same VTP domain name. A switch can be configured to be in one and only one VTP domain. You make global VLAN configuration changes for the domain using either the command-line interface (CLI) or Simple Network Management Protocol (SNMP).

By default, the switch is in VTP server mode and is in the no-management domain state until the switch receives an advertisement for a domain over a trunk link or you configure a management domain. You cannot create or modify VLANs on a VTP server until the management domain name is specified or learned.

If the switch receives a VTP advertisement over a trunk link, it inherits the management domain name and the VTP configuration revision number. The switch ignores advertisements with a different management domain name or an earlier configuration revision number.

If you configure the switch as VTP transparent, you can create and modify VLANs but the changes affect only the individual switch.

When you make a change to the VLAN configuration on a VTP server, the change is propagated to all switches in the VTP domain. VTP advertisements are transmitted out all trunk connections, including Inter-Switch Link (ISL), IEEE 802.1Q, and IEEE 802.10.

VTP maps VLANs dynamically across multiple LAN types with unique names and internal index associations. Mapping eliminates excessive device administration that is required from network administrators.

Understanding VTP Modes

You can configure a switch to operate in any one of these VTP modes:

•Server—In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters (such as VTP version and VTP pruning) for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on advertisements received over trunk links. VTP server is the default mode.

•Client—VTP clients behave the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.

•Transparent—VTP transparent switches do not participate in VTP. A VTP transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements. However, in VTP version 2, transparent switches do forward VTP advertisements that they receive out their trunk ports.

•Off—In the three modes described above, VTP advertisements are received and transmitted as soon as the switch enters the management domain state. In the VTP off mode, switches behave the same as in VTP transparent mode with the exception that VTP advertisements are not forwarded.

Understanding VTP Advertisements

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

The following global configuration information is distributed in VTP advertisements:

•VLAN IDs (ISL and 802.1Q)

•VTP domain name

•VTP configuration revision number

•VLAN configuration, including the maximum transmission unit (MTU) size for each VLAN

•Frame format

Understanding VTP Version 2

If you use VTP in your network, you must decide whether to use VTP version 1, version 2, or version 3 (for details on version 3, see the "Understanding How VTP Version 3 Works" section).

VTP version 2 supports the following features that are not supported in version 1:

•Unrecognized Type-Length-Value (TLV) Support—A VTP server or client propagates configuration changes to its other trunks even for TLVs it is not able to parse. The unrecognized TLV is saved in NVRAM.

•Version-Dependent Transparent Mode—In VTP version 1, a VTP transparent switch inspects VTP messages for the domain name and version and forwards a message only if the version and domain name match. Since only one domain is supported in the supervisor engine software, VTP version 2 forwards VTP messages in transparent mode without checking the version.

•Consistency Checks—In VTP version 2, VLAN consistency checks (such as VLAN names and values) are performed only when you enter new information through the CLI or SNMP. Consistency checks are not performed when new information is obtained from a VTP message, or when information is read from NVRAM. If the digest on a received VTP message is correct, its information is accepted without consistency checks.

Understanding VTP Pruning

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Note Enabling VTP pruning on a VTP version 3 switch enables pruning only on the switch that you enable it on. VTP pruning is not propagated as it is with VTP version 1 and VTP version 2.

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VTP pruning enhances network bandwidth use by reducing unnecessary flooded traffic, such as broadcast, multicast, unknown, and flooded unicast packets. VTP pruning increases available bandwidth by restricting flooded traffic to those trunk links that the traffic must use to access the appropriate network devices. By default, VTP pruning is disabled.

Make sure that all devices in the management domain support VTP pruning before enabling it.

Figure 9-1 shows a switched network without VTP pruning enabled. Port 1 on Switch 1 and port 2 on Switch 4 are assigned to the Red VLAN. A broadcast is sent from the host that is connected to Switch 1. Switch 1 floods the broadcast and every switch in the network receives it even though Switches 3, 5, and 6 have no ports in the Red VLAN.

Figure 9-1 Flooding Traffic without VTP Pruning

Figure 9-2 shows the same switched network with VTP pruning enabled. The broadcast traffic from Switch 1 is not forwarded to Switches 3, 5, and 6 because traffic for the Red VLAN has been pruned on the links indicated (port 5 on Switch 2 and port 4 on Switch 4).

Figure 9-2 Flooding Traffic with VTP Pruning

Enabling VTP pruning on a VTP server enables pruning for the entire management domain. VTP pruning takes effect several seconds after you enable it. By default, VLANs 2-1000 are pruning eligible. VTP pruning does not prune traffic from VLANs that are pruning ineligible. VLAN 1 is always pruning ineligible; traffic from VLAN 1 cannot be pruned.

To make a VLAN pruning ineligible, enter the clear vtp pruneeligible command. To make a VLAN pruning eligible again, enter the set vtp pruneeligible command. You can set VLAN pruning eligibility regardless of whether VTP pruning is enabled or disabled for the domain. Pruning eligibility always applies to the local device only, not for the entire VTP domain.

Default VTP Version 1 and Version 2 Configuration

Table 9-1 shows the default VTP configuration.

Table 9-1 VTP Default Configuration

Feature	Default Value
VTP domain name	Null
VTP mode	Server
VTP version 2 enable state	Version 1 is enabled (version 2 is disabled)
VTP password	None
VTP pruning	Disabled

VTP Version 1 and Version 2 Configuration Guidelines

This section describes the guidelines for implementing VTP in your network:

•All switches in a VTP domain must run the same VTP version.

•You must configure a password on each switch in the management domain when you are in secure mode.

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Caution If you configure VTP in secure mode, the management domain will not function properly if you do not assign a management domain password to each switch in the domain.

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•A VTP version 2-capable switch can operate in the same VTP domain as a switch running VTP version 1 if that VTP version 2 is disabled on the VTP version 2-capable switch (VTP version 2 is disabled by default).

•Do not enable VTP version 2 on a switch unless all of the switches in the same VTP domain are version 2 capable. When you enable VTP version 2 on a switch, all of the version 2-capable switches in the domain enable VTP version 2.

•Enabling or disabling VTP pruning on a VTP server enables or disables VTP pruning for the entire management domain.

•Making VLANs pruning eligible or pruning ineligible on a switch affects pruning eligibility for those VLANs on that device only (not on all switches in the VTP domain).

•With software release 8.1(1), all VTP versions can be configured on a per-port basis. See the "VTP Version 3 Per-Port Configuration" section.

Configuring VTP Version 1 and Version 2

These sections describe how to configure VTP:

•Configuring a VTP Server

•Configuring a VTP Client

•Configuring VTP (VTP Transparent Mode)

•Disabling VTP Using the Off Mode

•Enabling VTP Version 2

•Disabling VTP Version 2

•Enabling VTP Pruning

•Disabling VTP Pruning

•Displaying VTP Statistics

Configuring a VTP Server

When a switch is in VTP server mode, you can change the VLAN configuration and have it propagate throughout the network.

To configure the switch as a VTP server, perform this task in privileged mode:

	Task	Command
Step 1	Define the VTP domain name.	set vtp domain name
Step 2	Place the switch in VTP server mode.	set vtp mode server
Step 3	(Optional) Set a password for the VTP domain.	set vtp passwd passwd
Step 4	Verify the VTP configuration.	show vtp domain

This example shows how to configure the switch as a VTP server and verify the configuration:

Console> (enable) set vtp domain Lab_Network

VTP domain Lab_Network modified

Console> (enable) set vtp mode server

Changing VTP mode for all features

VTP domain Lab_Network modified

Console> (enable) show vtp domain

Version      : running VTP2 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Server         0          

Pruning             : disabled

VLANs prune eligible: 2-1000

Console> (enable)

Configuring a VTP Client

When a switch is in VTP client mode, you cannot change the VLAN configuration on the switch. The client switch receives VTP updates from a VTP server in the management domain and modifies its configuration accordingly.

To configure the switch as a VTP client, perform this task in privileged mode:

	Task	Command
Step 1	Define the VTP domain name.	set vtp domain name
Step 2	Place the switch in VTP client mode.	set vtp mode client
Step 3	Verify the VTP configuration.	show vtp domain

This example shows how to configure the switch as a VTP client and verify the configuration:

Console> (enable) set vtp domain Lab_Network

VTP domain Lab_Network modified

Console> (enable) set vtp mode client

Changing VTP mode for all features

VTP domain Lab_Network modified

Console> (enable) show vtp domain

Version      : running VTP2 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Client         0          

Pruning             : disabled

VLANs prune eligible: 2-1000

Console> (enable)

Configuring VTP (VTP Transparent Mode)

When you configure the switch as VTP transparent, you disable VTP on the switch. A VTP transparent switch does not send VTP updates and does not act on VTP updates that are received from other switches. However, a VTP transparent switch running VTP version 2 does forward received VTP advertisements out all of its trunk links.

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Note Network devices in VTP transparent mode do not send VTP join messages. On Catalyst 4500 series switches with trunk connections to network devices in VTP transparent mode, configure the VLANs that are used by the transparent-mode network devices or that need to be carried across trunks as pruning ineligible (use the clear vtp pruneeligible command).

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To disable VTP on the switch, perform this task in privileged mode:

	Task	Command
Step 1	Disable VTP on the switch by configuring it for VTP transparent mode.	set vtp mode transparent
Step 2	Verify the VTP configuration.	show vtp domain

This example shows how to configure the switch as VTP transparent and verify the configuration:

Console> (enable) set vtp mode transparent

Changing VTP mode for all features

VTP domain Lab_Net modified

Console> (enable) show vtp domain

Version      : running VTP2 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Transparent    0          

Pruning             : disabled

VLANs prune eligible: 2-1000

Console> (enable) 

Disabling VTP Using the Off Mode

When you disable VTP using the off mode, the switch behaves the same as in VTP transparent mode with the exception that VTP advertisements are not forwarded.

To disable VTP using the off mode, perform this task in privileged mode:

	Task	Command
Step 1	Disable VTP using the off mode.	set vtp mode off
Step 2	Verify the VTP configuration.	show vtp domain

This example shows how to disable VTP using the off mode:

Console> (enable) set vtp mode off

Changing VTP mode for all features

VTP domain Lab_Net modified

Console> (enable) show vtp domain

Version      : running VTP2 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Off            0          

Pruning             : disabled

VLANs prune eligible: 2-1000

Console> (enable) 

Enabling VTP Version 2

VTP version 2 is disabled by default on VTP version 2-capable switches. When you enable VTP version 2 on a switch, every VTP version 2-capable switch in the VTP domain will enable version 2 as well.

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Caution VTP version 1 and VTP version 2 are not interoperable on switches in the same VTP domain. Every switch in the VTP domain must use the same VTP version. Do not enable VTP version 2 unless every switch in the VTP domain supports version 2.

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To enable VTP version 2, perform this task in privileged mode:

	Task	Command
Step 1	Enable VTP version 2 on the switch.	set vtp version 2
Step 2	Verify that VTP version 2 is enabled.	show vtp domain

This example shows how to enable VTP version 2 and verify the configuration:

Console> (enable) set vtp version 2

This command will enable VTP version 2 function in the entire management domain.

All devices in the management domain should be version2-capable before enabling.

Do you want to continue (y/n) [n]? y

VTP domain server modified

Console> (enable) show vtp domain

Version      : running VTP2 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Off            0          

Pruning             : disabled

VLANs prune eligible: 2-1000

Console> (enable)

Disabling VTP Version 2

To disable VTP version 2, perform this task in privileged mode:

	Task	Command
Step 1	Disable VTP version 2.	set vtp version 1
Step 2	Verify that VTP version 2 is disabled.	show vtp domain

This example shows how to disable VTP version 2:

Console> (enable) set vtp version 1

This command will enable VTP version 1 function in the entire management domain.

Warning: trbrf & trcrf vlans will not work properly in this version.

Do you want to continue (y/n) [n]? y

VTP domain Lab_Network modified

Console> (enable) show vtp domain

Version      : running VTP1 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Off            0          

Pruning             : disabled

VLANs prune eligible: 2-1000

Console> (enable) 

Enabling VTP Pruning

To enable VTP pruning, perform this task in privileged mode:

	Task	Command
Step 1	Enable VTP pruning in the management domain.	set vtp pruning enable
Step 2	(Optional) Make specific VLANs pruning ineligible on the device. (By default, VLANs 2-1000 are pruning eligible.)	clear vtp pruneeligible vlan_range
Step 3	(Optional) Make specific VLANs pruning eligible on the device.	set vtp pruneeligible vlan_range
Step 4	Verify the VTP pruning configuration.	show vtp domain
Step 5	Verify that the appropriate VLANs are being pruned on trunk ports.	show trunk

This example shows how to enable VTP pruning in the management domain and how to make VLANs 2-99, 250-255, and 501-1000 pruning eligible on the particular device:

Console> (enable) set vtp pruning enable

Cannot modify pruning mode unless in VTP SERVER mode.

Console> (enable) set vtp mode server 

Changing VTP mode for all features

VTP domain Lab_Network modified

Console> (enable) set vtp pruning enable

This command will enable the pruning function in the entire management domain.

All devices in the management domain should be pruning-capable before enabling.

Do you want to continue (y/n) [n]? y

VTP domain Lab_Network modified

Console> (enable) clear vtp pruneeligible 100-500

Vlans 1,100-500,1001-1023 will not be pruned on this device.

VTP domain Lab_Network modified.

Console> (enable) set vtp pruneeligible 250-255

Vlans 2-99,250-255,501-1000,1024-4094 eligible for pruning on this device.

VTP domain Lab_Network modified.

Console> (enable) show vtp domain

Version      : running VTP1 (VTP3 capable)

Domain Name  : Lab_Network                      Password  : configured (hidden)

Notifications: disabled                         Updater ID: 172.20.52.19

Feature        Mode           Revision

-------------- -------------- -----------

VLAN           Server         1          

Pruning             : enabled

VLANs prune eligible: 2-99,250-255,501-1000

Console> (enable) show trunk

* - indicates vtp domain mismatch

# - indicates dot1q-all-tagged enabled on the port

Port      Mode         Encapsulation  Status        Native vlan

--------  -----------  -------------  ------------  -----------

16/1      nonegotiate  isl            trunking      1

Port      Vlans allowed on trunk

--------  ---------------------------------------------------------------------

16/1      1-1005,1025-4094

Port      Vlans allowed and active in management domain 

--------  ---------------------------------------------------------------------

16/1      

Port      Vlans in spanning tree forwarding state and not pruned

--------  ---------------------------------------------------------------------

16/1 

Console> (enable)

Disabling VTP Pruning

To disable VTP pruning, perform this task in privileged mode:

	Task	Command
Step 1	Disable VTP pruning in the management domain.	set vtp pruning disable
Step 2	Verify that VTP pruning is disabled.	show vtp domain

This example shows how to disable VTP pruning in the management domain:

Console> (enable) set vtp pruning disable

This command will disable the pruning function in the entire management domain.

Do you want to continue (y/n) [n]? y

VTP domain Lab_Network modified

Console> (enable) 

Displaying VTP Statistics

To display VTP statistics, including the VTP advertisements that are sent and received and VTP errors, perform this task:

Task	Command
Display VTP statistics for the switch.	show vtp statistics

This example shows how to display VTP statistics on the switch:

Console> (enable) show vtp statistics

VTP statistics:

summary advts received          0

subset  advts received          0

request advts received          0

summary advts transmitted       7843

subset  advts transmitted       4

request advts transmitted       20

No of config revision errors    0

No of config digest errors      0

VTP pruning statistics:

Trunk    Join Transmitted Join Received Summary advts received from GVRP PDU

                                        non-pruning-capable device  Received

-------- ---------------- ------------- --------------------------- ----------

16/1     75               0             0                           0 

Console> (enable)

Understanding How VTP Version 3 Works

VTP version 3 differs from earlier VTP versions in that it does not directly handle VLANs. VTP version 3 is a protocol that is only responsible for distributing a list of opaque databases over an administrative domain. When enabled, VTP version 3 provides the following enhancements to previous VTP versions:

•Support for extended VLANs.

•Support for the creation and advertising of private VLANs.

•Improved server authentication.

•Protection from the "wrong" database accidentally being inserted into a VTP domain.

•Interaction with VTP version 1 and VTP version 2.

•Ability to be configured on a per-port basis.

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Note With software release 8.1(1), all VTP versions can be configured on a per-port basis.

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•Provides the ability to propagate the VLAN database and other databases. VTP version 3 is a collection of protocol instances, with each instance handling one database that is associated with a given feature. VTP version 3 handles the configuration propagation of multiple databases (features) independent of one another by running multiple instances of the protocol.

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Note In software release 8.1(1), the only supported database propagation is for the VLAN database.

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These sections describe VTP version 3:

•VTP Version 3 Authentication

•VTP Version 3 Per-Port Configuration

•VTP Version 3 Domains, Modes, and Partitions

•VTP Version 3 Modes

•VTP Version 3 Databases

VTP Version 3 Authentication

VTP version 3 introduces an enhancement to the handling of VTP passwords. VTP version 3 allows the configuration of a primary server. A VTP version 3 server cannot make any configuration changes in the domain without first becoming the primary server for the domain. The VTP version 3 authentication enhancements are as follows:

•If no password is configured or if a password is configured the same way as in VTP version 1 or VTP version 2 (that is, without using the hidden or secret keywords), the following occurs:

–A switch can become the primary server and configure the domain with no restriction.

–The password appears in the configuration.

This is equivalent to the existing VTP version 1 and VTP version 2 levels of security.

•If a password is configured as hidden, using the hidden password configuration option, the following occurs:

–The password does not appear in plain text in the configuration; the secret hexadecimal format of the password is saved in the configuration.

–If you try to configure the switch as a primary server, you are prompted for the password. If your password matches the secret password, the switch becomes a primary server allowing you to configure the domain.

For more information on configuring passwords, see the "Configuring VTP Version 3 Passwords" section.

VTP Version 3 Per-Port Configuration

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Note With software release 8.1(1), all VTP versions can be configured on a per-port basis.

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VTP version 3 allows you to disable the protocol on a per-port basis. If a trunk is connected to a switch or server that is not trusted and is not supposed to interact with the VTP domain, it is now possible to drop incoming VTP packets and prevent VTP advertisements on a particular trunk. This configuration option has no impact on other protocols.

For more information on per-port configuration options, see the "Disabling VTP Version 3 on a Per-Port Basis" section.

VTP Version 3 Domains, Modes, and Partitions

The main differences between VTP version 3 domains and modes and VTP version 1 and VTP version 2 are as follows:

•A VTP version 3 server can be configured as primary or secondary.

•VTP version 3 modes (server, client, and transparent) are specific to a VTP instance.

•A VTP version 3 domain can be partitioned.

These features are described in detail in the following sections:

•Primary Servers, Secondary Servers, and Clients

•Partitioned VTP Domains

•Reconfiguring a Partitioned VTP Domain

Primary Servers, Secondary Servers, and Clients

In previous VTP implementations, the main VTP server characteristic was to be able to modify and store the VTP domain configuration in NVRAM. A VTP client could only receive the configuration from the network and could not save or modify it. The VTP version 3 primary server functions the same way as VTP version 1 and VTP version 2 servers. A VTP version 3 secondary server can store the configuration of the domain but cannot modify it. The concept of client is unchanged (see Figure 9-3). The main distinction in VTP version 3 is that the server, client, and transparent modes are specific to a VTP instance. For example, it is possible for a switch to be a primary server for one instance and a client for another instance.

Figure 9-3 VTP Version 3: Primary Servers, Secondary Servers, and Clients

Partitioned VTP Domains

VTP version 3 restricts the configuration rights for a domain to a unique primary server, as follows:

•VTP configuration is possible only on a primary server.

•The identifier (ID) of the primary server that generated the database is attached to the VTP advertisements.

•A VTP switch keeps the ID of a primary server and accepts VTP database updates from its current primary server only.

Because the ID of a primary server is always sent along with the VTP configuration, any switch that has a configuration also knows the corresponding primary server. As in VTP version 1 and VTP version 2, the switches that do not have a VTP configuration accept the first configuration that they receive if the configuration passes the optional authentication scheme that is described in the "VTP Version 3 Authentication" section. VTP version 3 switches lock on the primary server that generated their configuration and only listen to further VTP database updates from this primary server. This process differs significantly from VTP version 1 and VTP version 2 where a switch would always accept a superior configuration from a neighbor in the same domain. A VTP version 3 switch accepts only a superior configuration that is from the same domain and that is generated by the same primary server.

There should be only one primary server in a VTP version 3 domain, but if there are several, the domain is partitioned into groups following the update of their respective primary server (see Figure 9-4). In Figure 9-4, the Cisco VTP domain is partitioned between switches accepting server X or server Y as a primary server. The switches that are from different partitions do not exchange database information even though they are part of the same domain. If server X changes the VTP configuration, only the left partition of the network accepts it.

Figure 9-4 VTP Version 3: Partitioned VTP Domain

Partitions exist because of discrepancies in the domain configuration that cannot automatically be resolved by VTP. Partitions are the result of a misconfiguration or an independent configuration of a temporarily disconnected part of the domain. This behavior of VTP version 3 protects the domain from accepting a conflicting configuration after the insertion of a misconfigured switch. If a new switch is added to a domain, it will not propagate its configuration until you manually designate it as the new primary server.

For information on using the takeover mechanism to reconfigure partitioned VTP domains, see the "Reconfiguring a Partitioned VTP Domain" section.

Reconfiguring a Partitioned VTP Domain

Partitioning of a VTP domain is specific to the instance; one instance may be partitioned while another might not be partitioned. In VTP version 3, you are required to remove any partitions because the protocol cannot determine which primary server has the final, desired configuration. Figure 9-5 shows a VTP domain that has been divided into four partitions for one specific VTP instance.

Figure 9-5 VTP Version 3: Reconfiguring a Partitioned VTP Domain

In Figure 9-5, server X has the correct configuration for the domain. To reconfigure this partitioned VTP domain, you need to issue a takeover message from server X to the entire domain, advertising server X as the new primary server for this specific instance. All switches in the domain will then lock onto primary server X and will only accept instance configuration updates that are initiated by server X. Therefore, all switches in the domain synchronize their VTP configuration to server X for that instance.

Initiating a takeover is a critical operation due to the following reasons:

•The takeover erases conflicting configurations that are potentially stored on other primary servers in the VTP domain. VTP lists all the switches with conflicting configurations (when you enter the show vtp conflicts command) and prompts you for confirmation before taking over (a server has conflicting information if it belongs to the same VTP domain but has a different primary server).

•The takeover leaves this switch (server X in Figure 9-5) as the only primary server controlling the VTP domain.

If you have a hidden password configured, you need to reenter the password to do a takeover. The switches refuse the takeover request if they are not correctly authenticated. If no authentication is enabled, any server is able to take over.

After a takeover, there should only be one primary server controlling the entire VTP domain for a particular instance. If this is not the case, it might be due to the following:

•Some switches may be temporarily disconnected and unreachable when the takeover message is sent.

•The takeover message might be lost on some links (however, the takeover messages are repeated to reduce this risk).

In both cases, you can correct the problem by issuing additional takeover messages.

For more information on takeovers, see the "Configuring a VTP Version 3 Takeover" section.

VTP Version 3 Modes

The default mode for VTP is version 1, server mode. The off mode can be exited only after you configure a VTP domain name on the switch. The "domain discovery" that is used in VTP version 1 and VTP version 2 is not available in VTP version 3.

Switches running VTP version 3 have the following common characteristics:

•They accept VTP packets only from the same VTP domain.

•If they do not have a primary server, they accept the primary server that is associated with the first VTP database that they receive for any instance.

•They accept only a database with a higher revision number from their current primary server.

•If they have a password configured (whether hidden or not hidden), they accept only a new database or a takeover message if it contains the correct password.

VTP version 3 modes are described in these sections:

•Client Mode

•Server Mode

•Transparent and VTP Off Modes

For more information, see the "Changing VTP Version 3 Modes" section.

Client Mode

VTP version 3 clients are similar to VTP version 1 and VTP version 2 clients, as follows:

•A VTP client accepts a VTP configuration from the network but cannot generate or alter the configuration.

•A VTP client stores the VTP configuration that it receives in RAM (not NVRAM). When a VTP client boots, it needs to reacquire the entire configuration that is propagated by VTP, including the identity of the primary server.

•A VTP client that cannot store the entire VTP configuration that is received in an instance to RAM, immediately transitions to transparent mode.

Server Mode

Primary and secondary servers are two types of servers that may exist on an instance in the VTP domain.

Secondary Server

When a switch is configured to be a server, it becomes a secondary server by default. As a secondary server, a VTP version 3 switch behaves as a client with the following exceptions:

•A secondary server immediately stores the information that is received through VTP version 3 in NVRAM. This NVRAM is part of the running configuration or startup configuration.

•At startup, a secondary server that has a configuration in NVRAM starts advertising the configuration. The main purpose of a VTP secondary server is to back up the configuration that is propagated over the network.

•Similar to a client, a VTP secondary server cannot modify the VTP configuration.

•A VTP server reverts to client mode if it cannot store the configuration in NVRAM.

•A VTP version 3 secondary server can issue a takeover to become a primary server.

Primary Server

The primary server can initiate or change the VTP configuration. To reach the primary server state, you must issue a successful takeover from the switch. The takeover is propagated to the entire domain. All other potential primary servers in the domain resign to secondary server mode to ensure that there is only one primary server in the VTP domain.

You only need the primary server when the VTP configuration for any instance needs to be modified. A VTP domain can operate with no active primary server because the secondary servers use the configuration over reloads. The primary server state is exited due to the following reasons:

•A switch reload.

•A high-availability switchover between the active and redundant supervisor engines.

•A takeover from another server.

•A change in the mode configuration.

•Any VTP domain configuration change (such as version, domain name, or domain password).

Transparent and VTP Off Modes

In VTP version 3, the transparent mode is specific to the instance. The off mode in VTP version 3 is similar to the previous VTP versions and is not specific to an instance. In both modes, you are allowed to configure locally the features that VTP is controlling. In addition, this feature configuration appears in the running configuration (if applicable). The feature stores its local configuration in the same NVRAM block that is used by VTP. All NVRAM handling for the feature happens through VTP whether or not the switch is transparent to the feature. In VTP transparent mode, all VTP messages that are received by the switch are still flooded. In VTP off mode, the VTP messages are dropped on the trunks.

VTP Version 3 Databases

VTP version 1 and VTP version 2 are tied to VLAN information. VTP version 3 is designed to distribute any kind of configuration (referred to as a database) over a VTP domain.

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Note In software release 8.1(1), the only supported database propagation is for the VLAN database.

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VTP version 3 databases are described in the following sections:

•Valid Databases

•Database Revision Number

•Interaction with VTP Version 1 and VTP Version 2

•Limitations

Valid Databases

A switch advertises a database only if it is valid. The only way to validate a database is to become the primary server. If a switch modifies a database that has been generated by a primary server (this situation is possible in off or transparent modes), the database is invalid. An invalid database is applied only locally on a switch and is overwritten by any database that is received on the network if the switch is a VTP client or server. Some examples of valid and invalid databases are as follows:

•When you move from VTP version 1 to VTP version 3, the VLAN database is not deleted but is marked invalid because it has been generated by a VTP version 1 server, not by a VTP version 3 primary server.

•If you move a VTP version 3 server with a valid database to transparent mode, you can configure the VLAN database, but as soon as the database is modified, it becomes invalid. This situation prevents the switch from going back to server mode and advertising this database because the valid database that is received from the network overwrites the changes made while in transparent mode. If a server moves to transparent mode and then back to server mode with no changes to the database configuration, its database is still valid.

•If you modify a database on a primary server (such as a VLAN configuration), the database stays valid and is advertised to the rest of the domain. In any mode, when you configure a domain-related parameter (such as the domain name, VTP version, and the authentication method [password]), all the databases become invalidated. In addition to invalidating the databases, configuring a domain-related parameter also reverts a primary server to a secondary server.

•When you change a domain parameter, the switch is inserted into a new domain. To prevent the wrong database from accidentally being inserted into a VTP domain, you cannot insert a switch as a primary server into a new domain because it could potentially erase a valid configuration. Because it has an invalid database, a newly inserted switch in a domain immediately accepts the network configuration instead of erasing it.

Database Revision Number

Each VTP instance is associated with a database revision number. The database revision number is incremented when the value of the database that is covered by the advertised checksum is modified.

When a device receives a VTP advertisement from the same primary server for an instance in the same domain, the following occurs:

•If the database revision number in the advertisement is less than that of the receiving device, the advertisement is ignored and a summary advertisement with the current revision number is transmitted on the trunk on which the original advertisement was received.

•If the database revision number in the advertisement is the same as that of the receiving device, then the following occurs:

–If the checksum of the advertisement is exactly the same as the checksum of the current configuration known to the device, then no action is taken.

–If the checksum of the advertisement is not exactly the same as the checksum of the current configuration known to the device, the device's configuration is unaffected, but the device indicates to the database manager that a configuration error condition has occurred.

•If the database revision number in the advertisement is greater than that of the receiving device, and the advertisement's checksum and configuration information match, the receiving switch requests the exact subset of databases that are not up to date so that the switch can update its table.

The VTP advertisement is regenerated on each of the device's trunk ports other than the trunk port on which it was received.

Interaction with VTP Version 1 and VTP Version 2

VTP version 3 interacts with VTP version 1 and VTP version 2 switches as follows:

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Note You should configure VTP version 1 and VTP version 2 switches as clients to allow them to work properly with VTP version 3. See the "Limitations" section for more information.

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•A VTP version 3 switch can detect VTP version 1 and VTP version 2 switches and send a scaled-down version of its database on a per-trunk basis in VTP version 2 format only. VTP version 1 switches move to VTP version 2 mode without any configuration assistance.

•A VTP version 3 switch never sends any VTP version 2 packets on a trunk unless it first receives a legacy VTP version 1 or VTP version 2 packet on the trunk. This situation forces legacy neighboring switches to keep advertising their presence on the link. If a VTP version 3 switch does not receive a legacy packet on a trunk for a certain period of time, it is considered to be a VTP version 3-only trunk and does not advertise a scaled-down version of the VLAN database on the trunk.

•Even when advertising a VTP version 2 database on a trunk, VTP version 3 keeps sending VTP version 3 updates through the port. This situation allows coexistence of two kinds of neighbors on the trunk.

•A VTP version 3 switch can modify reserved VLANs 1002-1005; however, these VLANs are set to their default in the scaled-down database in VTP version 2 format.

•A VTP version 3 switch never accepts a configuration from a VTP version 1 or VTP version 2 neighbor.

Limitations

The limitations of VTP version 3 are as follows:

•Two VTP version 3 regions can communicate only over a VTP version 1 and VTP version 2 region in transparent mode.

•Leaving a server in a VTP version 2 region so it will receive its VTP information from a VTP version 3 region could cause a problem. If a configuration change occurs in the VTP version 1 and VTP version 2 region, the revision of the database may become higher than the one that is generated by the VTP version 3 region, and the updates from the VTP version 3 region might be ignored.

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Note We recommend that you set all switches in the VTP version 1 and VTP version 2 region to client and reset their revision number (do a reload or change the domain name back and forth).

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•A VTP version 2 region that is connected to two different VTP version 3 regions may receive contradictory information and keep swapping its database to the VTP version 3 region that has the highest revision number at any given time. We do not recommend this type of configuration.

•Enabling VTP pruning on a VTP version 3 switch enables pruning only on the switch that you enable it on. VTP pruning is not propagated as it is with VTP version 1 and VTP version 2.

Default VTP Version 3 Configuration

Table 9-2 shows the default VTP version 3 configuration.

Table 9-2 VTP Version 3 Default Configuration

Feature	Default Value
VTP domain name	Null
VTP mode	Server
VTP version 3 enable state	Version 1 is enabled
VTP password	None
VTP pruning	Disabled

Configuring VTP Version 3

These sections describe how to configure VTP version 3:

•Enabling VTP Version 3

•Changing VTP Version 3 Modes

•Configuring VTP Version 3 Passwords

•Configuring a VTP Version 3 Takeover

•Disabling VTP Version 3 on a Per-Port Basis

•VTP Version 3 show Commands

Enabling VTP Version 3

Use the set vtp version version_number command to specify the VTP version. By default, the VTP version is version 1 and the VTP mode is server mode. You must specify a domain before selecting a VTP version or VTP mode.

To enable VTP version 3, perform this task in privileged mode:

	Task	Command
Step 1	Enable VTP version 3 on the switch.