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Cisco created this one too. The basic goals of VLAN Trunking Protocol (VTP) are to manage all configured VLANs across a switched internetwork and to maintain consistency throughout that network. VTP allows you to add, delete, and rename VLANs – information that is then propagated to all other switches in the VTP domain.

Here's a list of some of the cool features VTP has to offer:

■ Consistent VLAN configuration across all switches in the network

■ VLAN trunking over mixed networks, such as Ethernet to ATM LANE or even FDDI

■ Accurate tracking and monitoring of VLANs

■ Dynamic reporting of added VLANs to all switches in the VTP domain

■ Adding VLANs using Plug and Play

Very nice, but before you can get VTP to manage your VLANs across the network, you have to create a VTP server (really, you don't need to even do that since all switches default to VTP server mode, but just make sure you have a server). All servers that need to share VLAN information must use the same domain name, and a switch can be in only one domain at a time. So basically, this means that a switch can share VTP domain information with other switches only if they're configured into the same VTP domain. You can use a VTP domain if you have more than one switch connected in a network, but if you've got all your switches in only one VLAN, you just don't need to use VTP. Do keep in mind that VTP information is sent between switches only via a trunk port.

Switches advertise VTP management domain information as well as a configuration revision number and all known VLANs with any specific parameters. But there's also something called VTP transparent mode. In it, you can configure switches to forward VTP information through trunk ports but not to accept information updates or update their VLAN databases.

If you've got sneaky users adding switches to your VTP domain behind your back, you can include passwords, but don't forget – every switch must be set up with the same password. And as you can imagine, this little snag can be a real hassle administratively!

Switches detect any added VLANs within a VTP advertisement and then prepare to send information on their trunk ports with the newly defined VLAN in tow. Updates are sent out as revision numbers that consist of summary advertisements. Anytime a switch sees a higher revision number, it knows the information it's getting is more current, so it will overwrite the existing VLAN database with the latest information.

You should know these four requirements for VTP to communicate VLAN information between switches:

■ The VTP version must be set the same

■ The VTP management domain name of both switches must be set the same.

■ One of the switches has to be configured as a VTP server.

■ Set a VTP password if used.

No router is necessary and is not a requirement. Now that you've got that down, we're going to delve deeper into the world of VTP with VTP modes and VTP pruning.

VTP Modes of Operation

Figure 1.1 shows you how a VTP server will update the connected VTP client's VLAN database when a change occurs in the VLAN database on the server.

Figure 1.1 VTP modes

Server This is the default mode for all Catalyst switches. You need at least one server in your VTP domain to propagate VLAN information throughout that domain. Also important: The switch must be in server mode to be able to create, add, and delete VLANs in a VTP domain. VLAN information has to be changed in server mode, and any change made to VLANs on a switch in server mode will be advertised to the entire VTP domain. In VTP server mode, VLAN configurations are saved in NVRAM on the switch.

Client In client mode, switches receive information from VTP servers, but they also receive and forward updates, so in this way, they behave like VTP servers. The difference is that they can't create, change, or delete VLANs. Plus, none of the ports on a client switch can be added to a new VLAN before the VTP server notifies the client switch of the new VLAN and the VLAN exists in the client's VLAN database. Also good to know is that VLAN information sent from a VTP server isn't stored in NVRAM, which is important because it means that if the switch is reset or reloaded, the VLAN information will be deleted. Here's a hint: If you want a switch to become a server, first make it a client so it receives all the correct VLAN information, then change it to a server – so much easier!

So basically, a switch in VTP client mode will forward VTP summary advertisements and process them. This switch will learn about but won't save the VTP configuration in the running configuration, and it won't save it in NVRAM. Switches that are in VTP client mode will only learn about and pass along VTP information – that's it!

Real World Scenario

So, When Do I Need to Consider Using VTP?

Here's a scenario for you. Bob, a senior network administrator at Acme Corporation in San Francisco, has about 25 switches all connected together, and he wants to configure VLANs to break up broadcast domains. When do you think he should start to consider using VTP?

If you answered that he should have used VTP the moment he had more than one switch and multiple VLANs, you're right. If you have only one switch, then VTP is irrelevant. It also isn't a player if you're not configuring VLANs in your network. But if you do have multiple switches that use multiple VLANs, you'd better configure your VTP server and clients, and you better do it right!

When you first bring up your switched network, verify that your main switch is a VTP server and that all the other ones are VTP clients. When you create VLANs on the main VTP server, all switches will receive the VLAN database.

If you have an existing switched network and you want to add a new switch, make sure to configure it as a VTP client before you install it. If you don't, it's possible – okay, highly probable – that your new little beauty will send out a new VTP database to all your other switches, effectively wiping out all your existing VLANs like a nuclear blast. No one needs that!

Transparent Switches in transparent mode don't participate in the VTP domain or share its VLAN database, but they'll still forward VTP advertisements through any configured trunk links. They can create, modify, and delete VLANs because they keep their own database – one they keep secret from the other switches. Despite being kept in NVRAM, the VLAN database in transparent mode is actually only locally significant. The whole purpose of transparent mode is to allow remote switches to receive the VLAN database from a VTP Server configured switch through a switch that is not participating in the same VLAN assignments.

VTP only learns about normal-range VLANs, with VLAN IDs 1 to 1005; VLANs with IDs greater than 1005 are called extended-range VLANs and they're not stored in the VLAN database. The switch must be in VTP transparent mode when you create VLAN IDs from 1006 to 4094, so it would be pretty rare that you'd ever use these VLANs. One other thing: VLAN IDs 1 and 1002 to 1005 are automatically created on all switches and can't be removed.

VTP Pruning

VTP gives you a way to preserve bandwidth by configuring it to reduce the amount of broadcasts, multicasts, and unicast packets. This is called pruning. Switches enabled for VTP pruning send broadcasts only to trunk links that actually must have the information.