VLAN Trunking Protocol (VTP) is a Cisco proprietary Layer 2 messaging protocol that manages the addition, deletion, and renaming of VLANs on a network-wide basis. Virtual Local Area Network (VLAN) Trunk Protocol (VTP) reduces administration in a switched network. When you configure a new VLAN on one VTP server, the VLAN is distributed through all switches in the domain. This reduces the need to configure the same VLAN everywhere. To do this VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over ISL, 802.1q, IEEE 802.10 and LANE trunks. VTP traffic is sent over the management VLAN (VLAN1), so all VLAN trunks must be configured to pass VLAN1. VTP is available on most of the Cisco Catalyst Family products.
The comparable IEEE standard in use by other manufacturers is GVRP.
VTP sends messages between trunked switches to maintain VLANs on these switches in order to properly trunk. VTP messages are exchanged between switches within a common VTP domain. If the domain name is different, the switch simply ignores the packet. If the name is the same then it checks by a revision number. If the revision number of an update received on a client or server VTP switch is higher than the previous revision, then the new configuration is applied. Otherwise, the configuration is ignored.
When new devices are added to a VTP domain, revision numbers should be reset on the entire domain to prevent conflicts. Utmost caution is advised when dealing with VTP topology changes, logical or physical. Exchanges of VTP information can be controlled by passwords. You need to put the same password on every switch for it to work.
|VTP Functionality||Support/Processing in Version 2|
|Token Ring||Token Ring Bridge Relay Function (TrBRF) and Token Ring Concentrator Relay Function (TrCRF) VLAN are supported|
|Unrecognized Type-Length-Value (TLV)||In V2, a server will propagate TLVs even those it does not understand. It also saves them in NVRAM when the switch is in VTP server mode. This could be useful if not all devices are at the same version or release level.|
|Version-Dependent Transparent Mode||Version 1 supports multiple domains while Version 2 supports only 1. Normal behavior for V1 would be to forward messages only if they match the destination domain name and version. VTPv2 does not do this check before forwarding.|
|Consistency Checks||VTPv1 does more consistency checking on messages, which can add overhead. As long as the MD5 digest on a message is correct, VTPv2 will forward it. VTPv2 will consistency-check new configuration information added through the configuration editor, Cluster Management Software or SNMP.|
VTP version 3: is a protocol that is only responsible for distributing a list of opaque databases over an administrative domain. When enabled, VTP version 3 provides the following enhancements to previous VTP versions:
|Step 1||Define the VTP domain name (Case sensitive)||vtp domain name|
|Step 2||Set which VTP version to run||vtp version #|
|Step 3||(Optional) Set a password for the VTP domain.||vtp password password|
|Step 4||Verify the VTP configuration.||show vtp status|
VTP can prune unneeded VLANs from trunk links. VTP maintains a map of VLANs and switches, enabling traffic to be directed only to those switches known to have ports on the intended VLAN. This enables more efficient use of trunk bandwidth.
Each switch will advertise which VLANs it has active to neighboring switches. The neighboring switches will then "prune" VLANs that are not active across that trunk, thus saving bandwidth. If a VLAN is then added to one of the switches, the switch will then re-advertise its active VLANs so that pruning can be updated by its neighbors. For this to work, VLAN pruning must be enabled on both ends of the trunk. It is easiest to enable VLAN pruning for an entire VTP management domain by simply enabling it on one of the VTP servers for that domain. To enable VLAN pruning for a VTP domain, enter the following command on a VTP server for that domain...
VTP_Server_Sw1(config)# vtp pruning
This will then propagate to all switches in the vtp domain.
|Step 1||Enable VTP pruning in the management domain.||set vtp pruning enable|
|Step 2||(Optional) Make specific VLANs pruning-ineligible on the device. (By default, VLANs 2-1000 are pruning-eligible.)||clear vtp pruneeligible vlan_range|
|Step 3||(Optional) Make specific VLANs pruning-eligible on the device.||set vtp pruneeligible vlan_range|
|Step 4||Verify the VTP pruning configuration.||show vtp domain|
|Step 5||Verify that the appropriate VLANs are being pruned on trunk ports.||show interface trunk|
Another problem can happen when you are inserting a switch with a different VTP domain name.
VTP can affect DTP (Dynamic Trunking Protocol) - switches will not form trunks unless they have matching VTP domain names.