Content

| 2.7 |  | VLAN Trunking Protocol (VTP) |
| 2.7.1 |  | VTP benefits |

The role of the VLAN Trunking Protocol (VTP) is to maintain VLAN configuration consistency across the entire network. VTP is a messaging protocol that uses Layer 2 trunk frames to manage the addition, deletion, and renaming of VLANs on a network-wide basis from a centralized switch that is in the VTP server mode. VTP is responsible for synchronizing VLAN information within a VTP domain. This reduces the need to configure the same VLAN information on each switch.

VTP minimizes the possible configuration inconsistencies that arise when changes are made. These inconsistencies can result in security violations, because VLANs can crossconnect when duplicate names are used. They also could become internally disconnected when they are mapped from one LAN type to another, for example, Ethernet to ATM LANE ELANs or FDDI 802.10 VLANs. VTP provides a mapping scheme that enables seamless trunking within a network employing mixed-media technologies.

VTP provides the following benefits:

- VLAN configuration consistency across the network
- Mapping scheme that allows a VLAN to be trunked over mixed media
- Accurate tracking and monitoring of VLANs
- Dynamic reporting of added VLANs across the network
- Plug-and-play configuration when adding new VLANs

As beneficial as VTP can be, it does have disadvantages that are normally related to the Spanning-Tree Protocol (STP). The greatest risk is a bridging loop propagating throughout the entire campus network. Since Cisco switches run an instance of STP for each VLAN, and since VTP propagates VLANs across the campus LAN, VTP effectively creates more opportunities for a bridging loop to occur. Network designers and administrators must balance the ease of administration made possible by VTP with the risk of a large, potentially unstable STP domain.

Before creating VLANs on the switch that will be propagated via VTP, A VTP domain must first be set up. A VTP domain for a network is a set of all contiguously trunked switches with the same VTP domain name. All switches in the same management domain share their VLAN information with each other, and a switch can participate in only one VTP management domain. Switches in different domains do not share VTP information.

Using VTP, each Catalyst Family Switch advertises the following on its trunk ports:

- Management domain
- Configuration revision number
- Known VLANs and their specific parameters