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| 4.3 |  | Troubleshooting Switched Ethernet Networks |
| 4.3.7 |  | Troubleshooting VTP misconfiguration |

VLANs allow switched networks to be broken up into Layer 2 broadcast domains. The VLAN Trunking Protocol (VTP) is a Layer 2 protocol that ensures VLAN consistency within a VTP Domain. In practice this means that if a VLAN is created on one switch within a network that same VLAN would be propagated to all switches within the VTP domain. A failure of the VTP protocol will prevent switches from agreeing about the VLAN configuration within a network. This may cause communications between switches to fail or may result in entire VLANs being deleted.

The first indication of a VTP problem often occurs when a VLAN is created on a switch but the VLAN fails to propagate to all other switches within the domain. When this occurs the first thing to check is the VTP mode. Each switch can be placed into one of the following VTP modes:

- Server
- Client
- Transparent

Changes to the VLAN configuration within a VTP domain can only be made on a switch that is in the VTP server mode. Furthermore, VLAN information will only be propagated to switches in a VTP mode of Server or Client that are in the same VTP Domain. The VTP mode and domain can be verified using the show vtp status command.

This problem occurs when there is a large switched network where all switches are part of the same VTP domain, and another switch is added to the network.

Assume that this switch has been previously used, and already has an appropriate VTP domain name entered. The switch is configured as a VTP client, and then connected to the rest of the network. The instant the trunk link is brought up to the rest of the network, the whole network goes down. What could have happened?

The most likely explanation is that the configuration revision number of the inserted switch was higher than the configuration revision of the VTP domain. Therefore, the recently introduced switch with minimal VLAN information has erased all VLANs through the VTP domain.

This situation will happen whether the switch is a VTP client or a VTP server. A VTP client can erase VLAN information on a VTP server. This is evident when many of the ports in the network go into inactive state and are assigned to a non-existing VLAN.

Quickly reconfigure all of the VLANs on one of the VTP servers. Always make sure that the configuration revision of all switches inserted into the VTP domain is lower than the configuration revision of the switches already in the VTP domain.