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VLAN Trunk Protocol

VLAN Trunk Protocol (VTP) is a Cisco-proprietary messaging protocol that occurs between devices on trunk ports. It allows VLAN information to be propagated across your switched network, providing a consistent VLAN configuration in your network. This process makes it easy to add, change, and delete VLANs as well as to add new devices to the network because your VLAN information is automatically propagated by switches that understand VTP on their trunk ports. This removes any possibility of inconsistencies between VLAN configurations that might result in security violations or errors. These problems can occur when VLANs become cross-connected as duplicate VLAN names are configured or when VLANs become internally disconnected because they're incorrectly mapped between different LAN media types.

NOTE

Only VLAN configuration information is shared via VTP; port information, such as which port belongs to which VLAN, is not shared.

VTP Advantages

Because many networks have mixed media or are going through a migration to a new backbone media topology, such as Gigabit Ethernet or ATM, a special protocol is needed to provide compatibility for the implementation of VLANs within these mixed-media topologies. Many networks today employ Ethernet to the desktop, utilizing Fast Ethernet as a backbone solution. You must first set up trunk connections for VTP to take place.

With VTP, a broadcast initiated in a VLAN from an Ethernet segment can automatically be propagated to a FDDI backbone where servers belonging to that VLAN or servers performing trunking will also see the broadcast. In traditional networks, a router would be required to perform this type of connectivity, but in today's switched networks, VTP can be used to integrate mixed-media topologies, gaining you an increase in performance at a reduced cost.

Another advantage of VTP is that it does not necessarily require that a new VLAN be manually added to every switch in the network. By adding a VLAN to one switch, VTP can propagate this information to every other switch in the network, thus creating a consistent VLAN implementation. For very large switched networks with tens or hundreds of switches, this becomes a very important tool to help you manage your network.

CAUTION

VTP provides a consistent broadcast domain across a mixed-topology network as well as the dynamic reporting of VLAN changes across your network. VTP information is shared across trunk connections only.

Management Domain

Using VTP requires setting up a management domain. A domain is a grouping of switches that will be sharing information about VLANs in a switched network. Each domain must have a unique name, and every switch in a single domain must have the same configured domain name. A switch can belong to only one management domain. However, switches of a management domain will contain the same VLAN information, thus providing a consistent configuration. By default, a switch doesn't belong to any management domain. You must either configure a management domain or the switch will learn it from a VTP advertisement on one of its trunks.

Each VTP-capable switch advertises VTP-multicast information periodically on its trunk ports on the factory-default VLAN. This includes information about the management domain itself, the version of VTP in use, and VLANs and their configuration. A switch can be configured in one of three different VTP modes: server, client, and transparent.

VTP Modes

VTP servers and clients maintain all VLANs everywhere within the VTP domain. A VTP domain defines the boundary of a particular VLAN. Servers and clients transmit information through trunks to other attached switches and receive updates from those trunks.

VTP servers are responsible for making all VLAN configurations—adds, changes, and deletions—and passing this information as a multicast advertisement to all other clients and servers, which then execute the changes. Server mode is the default for Catalyst switches.

Clients can accept updates only from servers—any changes that you want to make for a VLAN must be done on a server. Servers maintain their VLAN information in NVRAM, whereas clients do not. Clients learn VLAN information from a server switch when the client boots up.

Switches set to transparent mode do not participate in a management domain, but they do take the VTP messages they receive and forward them on to other switches. Transparent mode switches can create, change, and delete VLANs, but they do not share this information with other switches.

If a VLAN is added on a server switch, the server generates a multicast advertisement and forwards it to all other servers and clients in the network. This information is automatically transmitted on all trunk connections, including ISL, 802.1Q, 802.10, and LANE. If a transparent switch receives such a message, it does not update its database but rather forwards the message to other switches out its remaining trunk ports. Table 3.3 summarizes the three different VTP modes.

Table 3.3 Comparison Between the Three VTP Modes

Mode

Add, Delete,Change VLANs?

Generate VTP Messages?

Process VTP Messages?

Save Config in NVRAM?

Server

Yes

Yes

Yes

Yes

Client

No

Yes

Yes

No

Transparent

Yes

No

No

Yes


CAUTION

Be able to compare the three different VTP modes shown in Table 3.3.

VTP Messages

Switches belonging to the same VTP domain advertise information to each other on their trunk ports. Servers and clients are responsible for making sure that the VLANs in a network are consistent throughout the switched network. When VTP messages are generated, they contain at least the following information:

  • VLAN numbers for ISL and 802.1Q Ethernet VLANs, ELAN names for ATM LANE, and 802.10 SAID values for FDDI

  • VTP domain name

  • VTP revision number

  • MTU size for the VLAN

  • Format of the frame

  • Identity of the originator of the message

Three Message Types

There are three VTP message types: summary advertisement, subset advertisement, and request advertisement. Table 3.4 explains the use of the messages.

Table 3.4 VTP Messages

Message Advertisement

Message Originator

Explanation

Summary

Server

Generated every 300 seconds in the management VLAN (by default, VLAN 1) and used to ensure all switches are in sync

Subset

Server

Generated in response to a request and contains detailed configuration information about a VLAN

Request

Client

Generated to acquire VLAN information


Processing Messages

If the management domain name in a VTP message does not match that of the receiving switch, the advertisement is ignored. Advertisements that are generated by VTP switches contain a revision number. This number helps receiving switches to determine whether the information contains a change or is the same as the information it currently has. The VTP message with the highest revision number is the most current. Whenever you make a VLAN change on a server switch, the revision number is incremented and then advertised out all of its trunk interfaces. Care must be taken because if all the VLANs are deleted on a server switch with the highest revision number, all the VLANs in the management domain would also be deleted. Remember that VTP transparent switches do not process messages from server switches. On a transparent switch, the revision number is always 0, and all messages from server switches are not processed.

VTP also supports password authentication. If passwords are in use, an MD-5 hashed value is included in the VTP advertisement. If the hashed values between the two switches do not match, the message is ignored.

Adding a New Switch to Your Network

When you're adding a new switch to an existing switched network, it's extremely important that you follow these steps to avoid any VTP propagation problems:

  1. Erase the configuration on the switch (don't connect the switch to your network yet).

  2. Set the VTP domain name, change the VTP mode to client, and save the switch's configuration.

  3. Configure your trunk connections.

  4. Connect the switch up to your current network.

  5. Turn the switch off and back on. Doing so resets the revision number to 0.

  6. After your switch has learned its VLAN information from another switch, you can change its mode to server or transparent, if you choose, and save your switch's configuration.

If you don't follow the preceding steps, introducing a switch with a higher revision number than your current server switch or switches causes the new switch to overwrite the VLANs on your current switches. As an added precaution, it is highly recommended to configure VTP passwords to ensure that an added rogue switch doesn't wreak havoc with your current VLAN configuration.

VTP Versions

There are two different versions of VTP: 1 and 2. VTP 2 is new as of CatOS 3.1(1), which is quite a while ago. It's important to point out that the two versions are not compatible with each other: All your switches have to run either version 2 or 1. VTP version 2 has the following additional features that version 1 lacks:

  • Consistency checks are performed to make sure that VLAN names and values match on switches.

  • Support for token ring VLANs was added.

  • Transparent mode switches forward all VTP messages. Version 1 transparent mode switches only forward messages if the VTP version and domain name values in the message matches its own configured values.

  • Servers and clients propagate VTP messages out trunk interfaces even if they don't understand the contents of the message (and store this in NVRAM).

CAUTION

Know the differences between the VTP versions 1 and 2, as shown in the preceding bullets.

If you need any of the features in the previous list, you need to enable VTP version 2. To enable version 2, you have to enable it on only one server switch in your network. That switch then propagates this information to all the other switches in the VTP domain.

VTP Pruning

VTP pruning allows a switch to make more intelligent decisions concerning the forwarding of multicast, broadcast, and unknown destinations across trunk ports. VTP pruning is a method of traffic control that reduces unnecessary broadcast, multicast, and flooded unicast packets. This feature restricts traffic that would normally be flooded out all trunks to only those trunk links where the connected switches (or other networking devices) also have ports in the associated VLAN.

Let's take a look at an example to explain the advantages of VTP pruning. Figure 3.8 shows a network using VLANs and VTP.

Figure 3.8Figure 3.8 Pruning example.

Without VTP pruning, a broadcast generated by a device in VLAN 40 would be propagated across every trunk connection to every switch. Looking at Figure 3.8, switch 4 would propagate the broadcast across its trunk to switch 1, which, in turn, would propagate the broadcast across its two trunks to switch 2 and switch 3. Notice, however, that switch 1, switch 2, and switch 3 do not have any ports in VLAN 40, but because they have trunk ports, broadcasts, multicasts, and unknown destination traffic for VLAN 40 is still propagated across these trunks.

VTP pruning solves this problem. Through a dynamic process, the switches discover with other switches what VLANs they have in common—that is, the ports assigned to those VLANs. Pruning is the process that removes a VLAN from a trunk. In Figure 3.8, with pruning enabled, switches 1 and 4 would prune off VLAN 40 from the trunk. Therefore, when a broadcast occurs in VLAN 40, it does not affect the bandwidth on the other trunks or switches. Likewise, it also reduces the size of STP for the VLAN. Note that if, at a later point in time, switch 2 assigns a port to VLAN 40, the VLAN would be added back to some of the trunks to create a single broadcast domain. The pruning process itself takes only a few seconds. In this case, VLAN 40 would still be pruned from the trunk between switches 1 and 3 because switch 3 still doesn't have a port assigned in VLAN 40. This whole process relieves you of having to manually configure trunks to specify which VLANs should be forwarded across it.

VTP pruning is disabled by default in a management domain, and must be manually enabled. However, you need to enable it on only one server switch in a configured domain, which will in turn enable it on all other switches in the domain. One restriction of VTP pruning is that for a switch to take advantage of it, a switch must be in server mode, which means that in most cases, all your switches in the domain must be configured in server mode.

If you don't want to use VTP pruning, but to perform the pruning manually, you can manually remove VLANs from a trunk connection with the switchport trunk allowed vlan command. This command was discussed previously in the "Configuring ISL and 802.1Q Trunks" section. The problem with manual pruning is that it doesn't scale well in large networks and is prone to configuration errors.

CAUTION

VTP pruning enables you to dynamically prune off inactive VLANs from a trunk. Use the show interface [switchport|trunk] command to troubleshoot pruning problems.

Configuring VTP Domains

The commands to configure VTP can be done from either the vlan database in Privilege EXEC mode or from Configuration mode. The commands are the same for either mode. Listing 3.5 shows the commands for the latter.

Listing 3.5 VTP Configuration

Switch(config)# vlan database
Switch(config-database) vtp domain domain_name
Switch(config-database) vtp password management_password
Switch(config-database) vtp server|client|transparent
Switch(config-database) [no] vtp pruning
Switch(config-database) [no] vtp v2-mode

The last command enables or disables version 2 of VTP. All the other commands are self-explanatory.

CAUTION

Use the vtp command to configure VTP. Know the commands in the preceding code listing, especially the command to specify the VTP mode of the switch. Don't be surprised to see a simulation question on this.

Verifying Your Configuration

To verify the configuration of VTP, execute either the show vtp status or show vtp counters command. Here's an example of the first command:

Switch# show vtp status
VTP Version           : 2
Configuration Revision     : 3
Maximum VLANs supported locally : 68
Number of existing VLANs    : 4
VTP Operating Mode       : Server
VTP Domain Name         : dealgroup
VTP Pruning Mode        : Disabled
VTP V2 Mode           : Enabled
VTP Traps Generation      : Disabled
MD5 digest           : 0xA9 0xD1 0x3B 0xCD 0x32 0x94
Configuration last modified by 192.168.1.2 at 05-23-03 11:52:37

In this example, there have been three configuration changes. The switch is operating in server mode in the dealgroup domain. The following command displays VTP statistics information concerning the VTP messages that have been sent and received:

Switch# show vtp counters
VTP statistics:
 Summary advertisements received : 12
 Subset advertisements received : 5
 Request advertisements received : 0
 Summary advertisements transmitted : 93
 Subset advertisements transmitted : 8
 Request advertisements transmitted : 2
 Number of config revision errors : 0
 Number of config digest errors : 0
 Number of V1 summary errors : 0
<--output omitted--> 

In this example, you can see that the switch has sent and received VTP messages.

Troubleshooting VTP Problems

If you're experiencing problems with the setup of VTP or with messages not being propagated, you should check the following items to fix your VTP problem:

  • Back up the vlan.dat and config.txt files on your switch before making any changes.

  • Check to ensure that your trunks are configured properly between your switches.

  • Verify that the VTP domain name (and password, if configured) match on all switches in the domain.

  • Verify the VTP modes of the switches. If you're using the client/server configuration, make sure that no switches are configured in transparent mode.

CAUTION

Know the preceding four bullets for troubleshooting VTP problems.

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