VLAN TRUNKING PROTOCOL is a Cisco proprietary layer 2 messaging Protocol that manages the addition, deletion, and renaming of Virtual Local Area Networks (VLAN) on a network-wide basis. Cisco's Vlan Trunk protocol reduces administration in a switched-network. when a new VLAN is configured 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.
Switches within a VTP management domain synchronize their VLAN database by sending and receiving VTP advertisements over trunk link. VTP advertisement are flooded throughout the management domain by switches that are running in specific modes of operation. Advertisements are sent every 5 minutes or whenever there is a change in VLAN configuration. VTP advertisements are transmitted over VLAN1, using a layer 2 Multicast frame. VLAN advertisements are not propagated from a switch until a management domain name is specified or learned. VTP advertisement 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 will advertise VLANs 1-1005 only. VTP update are exchanged only across trunk links. Each switch operates in a given VTP mode that determines how VTP updates are sent from and received by that switch.
A device that receives a VTP advertisement will check that the VTP management domain name and password in the advertisement match those configured in the local switch. If a match is found, a switch further inspects the VTP update to see the configuration revision number. If the configuration revision number of the message is greater than the number currently in use, and the switch is running in VTP server or client mode, the switch overwrites its current VLAN information with that in the received update.
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