VLAN Trunking Protocol, or just VTP, is a useful feature for managing VLANs across multiple switches. VTP replicates configured VLANs to all participating switches. You can say it’s more of a replication protocol and instead of a trunking protocol.
On that same note, VTP can take down your network if you’re not very careful. Why would VTP take down your network? Cisco switches can be configured in one of three VTP modes:
- Server
- Client
- Transparent
Server mode is the default for Cisco switches. The switch in Server mode is where VLANs are configured.
Client mode grabs its VLAN configuration from the Server switch. It doesn’t actually place the VLANs in a vlan.dat file.
Switches in Transparent mode are rebels and don’t follow switches in Server or Client mode. If they receive VTP advertisements they will be kind and forward them along. In Transparent mode you can configure VLANs normally as you would on a Server switch.
VTP can be dangerous if a switch is deployed with a higher VTP revision number than the rest of the VTP switches. Because of that, switches in Client mode will download whatever VLAN configuration that switch has, potentially wiping out your current configuration. So when deploying switches, preconfigure them as Transparent mode. Or simply don’t use VTP.
Configuration
VTP Status
Before we begin configuring VTP it’s a good idea to display the current VTP configuration on each switch.
SW1#show vtp status VTP Version : running VTP2 Configuration Revision : 5 Maximum VLANs supported locally : 1005 Number of existing VLANs : 8 VTP Operating Mode : Server VTP Domain Name : rcdlab.net VTP Pruning Mode : Disabled VTP V2 Mode : Enabled VTP Traps Generation : Disabled MD5 digest : 0x6A 0xCC 0xFC 0xDA 0x19 0x9F 0xDE 0x7C SW2#show vtp status VTP Version : 2 Configuration Revision : 0 Maximum VLANs supported locally : 128 Number of existing VLANs : 6 VTP Operating Mode : Transparent VTP Domain Name : test.com VTP Pruning Mode : Disabled VTP V2 Mode : Enabled VTP Traps Generation : Disabled MD5 digest : 0xF3 0x78 0x2B 0x5F 0x2E 0x69 0xBE 0x76 SW3#sh vtp status VTP Version : 2 Configuration Revision : 0 Maximum VLANs supported locally : 128 Number of existing VLANs : 6 VTP Operating Mode : Transparent VTP Domain Name : example.com VTP Pruning Mode : Disabled VTP V2 Mode : Enabled VTP Traps Generation : Disabled MD5 digest : 0x62 0xE0 0xC7 0xE1 0xD5 0x65 0x25 0x85
Notice the different domain names and operating modes of each switch.
