VTP

VTP is a Cisco-proprietary protocol used to exchange VLAN information across trunk links, reducing VLAN administration and configuration errors. VTP allows you to create a VLAN once within a VTP domain and have that VLAN propagated to all other switches in the VTP domain.

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 offers a number of benefits for network managers, as shown in the figure:

VTP Components

There are number of key components that you need to be familiar with when learning about VTP:

• 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.
  
  
  
  
  
  

Default VTP Configuration

The default VTP settings are shown in the figure. The benefit of VTP is that it automatically distributes and synchronizes domain and VLAN configurations across the network. However, this benefit comes with a cost, you can only add switches that are in their default VTP configuration. If you add a VTP-enabled switch that is configured with settings that supersede existing network VTP configurations, changes that are difficult to fix are automatically propagated throughout the network. So make sure that you only add switches that are in their default VTP configuration.

VTP has three versions, 1, 2, and 3. Only one VTP version is allowed in a VTP domain. The default is VTP version 1.

Displaying the VTP Status

The Cisco IOS command show VTP status displays the VTP status. The output shows that switch S1 is in VTP server mode by default and that there is no VTP domain name assigned. The output also shows that the maximum VTP version available for the switch is version 2, and that VTP version 2 is disabled. You will use the show VTP status command frequently as you configure and manage VTP on a network.

• VTP Version-Displays the VTP version the switch is capable of running. By default, the switch implements version 1.
• 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. The figure shows a network with two VTP domains, cisco2 and cisco3.

A VTP domain consists of one switch or several interconnected switches sharing the same VTP domain name. Until the VTP domain name is specified you cannot create or modify VLANs on a VTP server, and VLAN information is not propagated over the network.

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. The VTP server sends out a VTP advertisement with the new domain name embedded inside. The S2 and S3 VTP server switches update their VTP configuration to the new domain name

VTP Advertising

VTP advertisements (or messages) distribute VTP domain name and VLAN configuration changes to VTP-enabled switches. 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.

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.

Note: A VTP domain name change does not increment the revision number. Instead, it resets the revision number to zero.

The configuration revision number determines whether the configuration information received from another VTP-enabled switch is more recent than the version stored on the switch.

The highlighted area shows that the revision number on switch S1 is 3, the number of VLANs is up to eight, because three VLANs have been added to the five default VLANs.


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 uses advertisements to distribute and synchronize information about domains and VLAN configurations.

VTP Modes

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.

Recall that a VLAN creates an isolated broadcast domain. A switch floods broadcast, multicast, and unknown unicast traffic across all trunk links within a VTP domain. When a computer or device broadcasts on a VLAN  the broadcast traffic travels across all trunk links throughout the network to all ports on all switches

In the figure, switches S1, S2, and S3 all receive broadcast frames from computer PC1. The broadcast traffic from PC1 consumes bandwidth on the trunk link between all 3 switches and consumes processor time on all 3 switches. The link between switches S1 and S3 does not carry any VLAN 10 traffic, so it is a candidate for VTP pruning. The flood traffic is stopped from entering the trunk connecting switches S1 and S2. VTP pruning only prunes the egress port F0/1 on switch S2.

Configuring VTP

VTP will be configured on this topology:

VTP Server Switch

• 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.
• 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

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

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

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. 

Troubleshooting VTP Configurations