what is a trunk prot ? | Dynamic Trunking Protocol (DTP) | explained concepts

Описание: A VLAN trunk is an Ethernet point-to-point link, between an Ethernet switch interface and an Ethernet interface on another networking device, such as a router or a switch, carrying the traffic of multiple VLANs over the singular link.
A VLAN trunk enables you to extend the VLANs across an entire network. A VLAN trunk does not belong to a specific VLAN; instead, it serves as a conduit for VLANs between switches. This diagram shows a small switched network with a trunk link between S1 and S2 carrying multiple VLAN traffic.
When a frame is placed on a trunk link, information about the VLAN it belongs to, must be added to the frame. This is accomplished by using IEEE 802.1Q frame tagging.
The VLAN tag field consists of a 16-bits Type field, called the EtherType field, and a Tag control information field. The EtherType field is set to the this hexadecimal value 0x8100. This value is called the tag protocol ID (TPID) value. With the EtherType field set to the TPID value, the switch receiving the frame knows to look for information in the Tag control information field. The Tag control information field contains.
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DTP is a Cisco-proprietary protocol. DTP manages trunk negotiation only, if the port on the other switch is configured in a trunk mode, that supports DTP.
Trunk ports are ports that are used to carry the traffic of more than one VLAN. The port which connects two different switches and the switches have more than one VLAN configured, then that port should be made trunk.
A switch port on a Cisco Catalyst switch supports a number of trunking modes. The trunking mode defines how the port negotiates using DTP to set up a trunk link with its peer port.
This table summarizes the results of DTP negotiations based on the different DTP configuration commands on local and remote ports.

■ If the switch is configured with the switch port mode trunk dynamic auto command, the local switch port advertises to the remote switch port, that it is able to trunk but does not request to go to the trunking state.
After a DTP negotiation, the local port ends up in the trunking state only, if the remote port trunk mode has been configured, so that the status is on or desirable.
If both ports on the switches are set to auto, they do not negotiate to be in a trunking state. They negotiate to be in the access mode state.
■ If the switch is configured with the switch port mode dynamic desirable command, the local switch port advertises to the remote switch port, that it is able to trunk, and asks the remote switch port to go to the trunking state. If the local port detects that the remote port has been configured as on, desirable, or auto mode, the local port ends up in the trunking state.
If the remote switch port is in the nonegotiate mode, the local switch port remains as a nontrunking port.
■ If the switch is configured with the switchport mode trunk command, the switch port periodically sends DTP messages to the remote port, advertising that it is in an unconditional trunking state.
■ If the switch is configured with the switchport nonegotiate command, the local port is considered to be in an unconditional trunking state.
Use this feature, when you need to configure a trunk with a switch from another switch vendor.A VLAN trunk enables you to extend the VLANs across an entire network. A VLAN trunk does not belong to a specific VLAN; instead, it serves as a conduit for VLANs between switches.