VLAN Trunking Protocol (VTP) is a

Cisco Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...

proprietary protocol In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual. Intellectual property rights and enforcement Ownership by a single organization gives the owner the ability to place restricti ...

that propagates the definition of Virtual Local Area Networks (

VLAN A virtual local area network (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2).IEEE 802.1Q-2011, ''1.4 VLAN aims and benefits'' In this context, virtual, refers to a physi ...

) on the whole local area network.Understanding VLAN Trunk Protocol (VTP)
at Cisco. To do this, VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over

802.1Q IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual local area networking (VLANs) on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying pro ...

, and ISL trunks. VTP is available on most of the Cisco

Catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

Family products. Using VTP, each Catalyst Family Switch advertises the following on its trunk ports: * Management domain * Configuration revision number * Known VLANs and their specific parameters There are three versions of VTP, namely version 1, version 2, version 3. The comparable IEEE standard in use by other manufacturers is GVRP or the more recent

MVRP Multiple Registration Protocol (MRP), which replaced Generic Attribute Registration Protocol (GARP), is a generic registration framework defined by the IEEE 802.1ak amendment to the IEEE 802.1Q standard. MRP allows bridges, switches or other simila ...

Implementation details

On Cisco Devices, VTP (VLAN Trunking Protocol) maintains VLAN configuration consistency across a single Layer 2 network. VTP uses Layer 2 frames to manage the addition, deletion, and renaming of VLANs from switches in the VTP client mode. VTP is responsible for synchronizing VLAN information within a VTP domain and reduces the need to configure the same VLAN information on each switch thereby minimizing the possibility of configuration inconsistencies that arise when changes are made.

Upside

VTP provides the following benefits: * VLAN configuration consistency across the layer 2 network * Dynamic distribution of added VLANs across the network * Plug-and-play configuration when adding new VLANs

Downside

When a new switch is added to the network, by default it is configured with no VTP domain name or password, but in VTP server mode. If no VTP Domain Name has been configured, it assumes the one from the first VTP packet it receives. Since a new switch has a VTP configuration revision of 0, it will accept any revision number as newer and overwrite its VLAN information if the VTP passwords match. However, if you were to accidentally connect a switch to the network with the correct VTP domain name and password but a higher VTP revision number than what the network currently has (such as a switch that had been removed from the network for maintenance and returned with its VLAN information deleted) then the entire VTP Domain would adopt the VLAN configuration of the new switch which is likely to cause loss of VLAN information on all switches in the VTP Domain, leading to failures on the network. Since Cisco switches maintain VTP configuration information separately from the normal configuration, and since this particular issue occurs so frequently, it has become known colloquially as the "VTP Bomb". Before creating VLANs on the switch that will propagate 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 matching VTP settings (domain name, password and VTP version). All switches in the same VTP 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. Non-matching VTP settings might result in issues in negotiating VLAN trunks, port-channels or Virtual Port Channels.

References

{{reflist Cisco protocols Ethernet

Implementation details

Upside

Downside

See also

References