The VLAN Trunking Protocol is organized into management domains. A Cisco VTP domain is one switch or several interconnected switches sharing the same VTP environment. A switch can be configured only in one VTP domain. Switches in different VTP domains do not share VTP information.
Switches in a VTP domain advertise many VLAN attributes to their VLAN VTP domain neighbors. These VLAN advertisements contain information about the VTP management domain. VTP revision number, available VLANs, and other VLAN parameters.
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Table of Contents
VTP Modes
A network switch, which is participating in VLAN Trunking Protocol, can have three different modes.
Server Mode
Client Mode
Transparent Mode
VLAN Trunking Protocol Server Mode
VTP server mode is the default VTP mode for all Crystal switches. At least one server is required in a VTP domain to propagate VLAN information within the VTP domain. We can create, add, or delete VLANs of a VTP domain in a switch that is in VTP server mode and change VLAN information in a VTP server. The changes made in a switch in server mode are advertised to the entire VTP domain.
VLAN Trunking Protocol Client Mode
VTP client mode switches listen to VTP advertisements from other switches and modify their VLAN configuration accordingly. A network switch in VTP client mode requires a server switch to inform it about the VLAN changes. We cannot create, add, or delete VLANs in a VTP client.
VLAN Trunking Protocol Transparent Mode
VTP transparent mode switches do not participate in the VTP domain, but VTP transparent mode switches can receive and forward VTP advertisements through the configured trunk links.
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VTP Advertisement message
Once the VTP is configured on the switches, the switches start advertising VTP information between them on their trunk’s ports. The main information which the switches advertise is management VTP Domain name, configuration revision number, and the configured VLANs. VTP advertisements are sent as multicast Ethernet frames and all neighbor devices receive the Ethernet frames.
There are three types of VTP advertisement messages;
Client advertisement request:
A client advertisement request message is a VTP message which a client generates for VLAN information to a server. Servers respond with both summary and subset advertisements.
Summary advertisement request
Summary advertisements are sent out every 300 seconds (5 minutes) by default or when a configuration change occurs, which is the summarized VLAN information.
Subset advertisement:
Subset advertisement is sent when a configuration change takes place on the server switch. Subset advertisements are VLAN specific and contain details about each VLAN.
VLAN Trunking Protocol Pruning
VTP is used to communicate VLAN information between switches in the same VTP domain. VTP pruning is a feature in Cisco switches, which stops VLAN update information traffic from being sent down trunk links if the updates are not needed. If the VLAN traffic is needed later, VTP will dynamically add the VLAN back to the trunk link.
In normal operation, a switch needs to floods broadcast frames, multicast frames, or unicast frames where the destination MAC address is unknown to all its ports. If the neighboring switch doesn’t have any active ports in the source VLAN, this broadcast is unnecessary, and excessive unwanted traffic is may create problems on the network.
VTP pruning helps in increasing the available bandwidth by reducing unnecessary flooded traffic. Broadcast frame, multicast frames, or unicast frames where the destination MAC address is unknown are forwarded over a trunk link only if the switch on the receiving end of the trunk link has ports in the source VLAN.
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