P clients, or the third possibility, VTP trans- parent. A VTP transparent switch receives and forwards VTP updates but does not update its configuration to reflect the changes they contain. Some switches default to VTP server, while others default to VTP transparent. VLANs cannot be locally config- ured on a switch in client mode. There is actually a fourth state for a VTP switch: off. A switch in VTP mode off will not accept VTP packets, and therefore will not forward them either. This can be handy if you want to stop the forwarding of VTP updates at some point in the network. VTP mode off is not avail- able on all switches, and is usually seen on large chassis switches like 6500s or Nexus 7000s. Figure 6-2 shows a simple network with four switches. SW1 and SW2 are both VTP servers. SW3 is set to VTP transparent, and SW4 is a VTP client. Any changes to the VLAN information on SW1 will be propagated to SW2 and SW4. The changes will be passed through SW3 but will not be acted upon by that switch. Because the switch does not act on VTP updates, its VLANs must be configured manually if users on that switch are to interact with the rest of the network. VTP server VTP server VTP transparent VTP client VLAN 10 VLAN 10 VLAN 10 VLAN 10 VLAN 20 VLAN 20 VLAN 20 VLAN 30 VLAN 30 VLAN 30 VLAN 40 VLAN 40 VLAN 40 Figure 6-2. VTP modes in action Looking at Figure 6-2, it is important to understand that both VTP servers can accept and disseminate VLAN information. This leads to an interesting problem. If someone makes a change on SW1, and someone else simultaneously makes a change on SW2, which one wins? VLAN Trunking Protocol Every time a change is made on a VTP server, the configuration is considered revised, and the configuration revision number is incremented by one. When changes are made, the server sends out VTP updates (called summary advertisements) containing the re- vision numbers. The summary advertisements are followed by subset advertisements, which contain specific VLAN information. When a switch receives a VTP upda