VLAN Trunking Protocol VTP

VTP is a Cisco proprietary Layer 2 multicast messaging protocol that can make VLAN administration easy or put you in a state of misery depending on how you look at it. You got a taste of that in the chapter scenario with both switches being in the server mode. VTP enables you to create a VLAN and have it propagate to other switches within the same domain. VTP transmits messages according to the VTP mode. From a practical sense, VTP is what saves you and me from going to each and every switch to create VLANs. See Table 7-4 for VTP operating modes.

NOTE

VTP Iras nothing to do with encapsulation or trunking; it is a communications protocol to dist ribu te VLAN information across a common management domain. VTP messages are en capsuIated inside of a trunking protocol frame such as ISL or B02. lQ.

Table 7-4. VTP Modes

VTPMode

Description

Storage

Serverl*.

Just as it sounds, it sources and listens for VTP messages. Create, modify, and delete VLANs within a management domain.

NVRAM.

Transparent

Does not source or listen for VTP messages but does propagate those of neighbor switches. Create, modify, and delete VLANs, but they are locally significant to the switch.

NVRAM.

Client!*!

Probesses and 1 istens for VTP messages. Cannot create, modify , oo d elete VLANs.

^formation is not stored in NVRAM.

When VTP clients or servers receive a message with the VTP multicast address of 01000ccccccc and a SNAP value of hex 2003, they process it according to revision numbers.

When VTP clients or servers receive a message with the VTP multicast address of 01000ccccccc and a SNAP value of hex 2003, they process it according to revision numbers.

Assign a CatOS-based switch to a VTP domain using the following command: set vtp domain vtpname. (It is cAsE sEnSiTiVe). This can help divide a large network into smaller management domains. The command on an IOS-ba sed switch is vtp doma i nvtpname iu the VLAN database mode.

NOTE

If yon chnnge the domain name o n one o! the switches to some thin g dlfeerent and creato VLAN30 on each swift:!, VLAN30 is VLAN3 0 ee gardiess of ohe VTP name „ This i s true because the VTP domain name is not io t he feame, only the lccrbet. Rememgrr the type oo nrame here is EAhe topt, and the protocols are IP and VTP.

Now you might be saying to yourself, "I have VLANs but I am not using VTP." Well, I guess that is your decision to run around and create the same VLAN on every switch or do everything in the transparent mode because you worry about losing your VLANs. For small networks, that actually is a pretty good approach. On the other hand, the larger your network, the more rational you have to be with automating VLAN propagation by using VTP. This gets into a design issue, questioning how far VLANs should sprawl across the topology. Cisco now concurs that flatter is not necessarily better, so a given VLAN should not need to exist in very many switches. In this chapter, I hope you are experiencing the things that many people tend to experience first on live networks. Obviously, that is not the best time to learn VTP.

NOTE

Routers do not participate in VTP, so they ignore VTP messages and discard them at the router interface. Only trunk-enabled adjacent switches in the server or client mode actually pay attention to VTP messages.

VTP advantages are as follows:

• VLAN consistency throughout a management domain

• Less manual configuration for creating and deleting VLANs, but you still need to associats the ports at each device

• Mote co ittro i and s ecurltn Lhrough a VTP domain name and passwords

• UmiOs she extent oL VTP message propagation

Take a Lew m inctes and compare the VTP h eader in Ilgure 7-6 Lo Ta ble 7-5f

Figure 7-6. Sniffing VTP

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