Valid Modes for VLAN Trunk Ports

BCD

When configuring a switch port as a VLAN trunk, there are several modes that can be used to establish the trunk link with another device. The valid modes for a switch port used as a VLAN trunk are:

A. Transparent B. Auto D. Desirable

Let's take a closer look at each of these modes:

A. Transparent mode: This mode allows the port to pass traffic for all VLANs, but it does not actually add or remove VLAN tags. Instead, it forwards the frames received on one VLAN unchanged to the other end of the trunk. This mode is typically used to interconnect two switches that have different native VLANs.

B. Auto mode: This mode allows the port to negotiate the trunking state with the other end of the link. If the other end of the link is set to desirable mode or on mode, the link becomes a trunk. If the other end of the link is set to access mode, the link becomes a non-trunk link. This mode is useful when you are uncertain about the configuration of the other end of the link.

C. On mode: This mode sets the port to always be a trunk, regardless of whether the other end of the link is a trunk or access port. If the other end of the link is an access port, the frames received on the trunk are dropped. This mode is useful when you want to ensure that the link always operates as a trunk.

D. Desirable mode: This mode actively tries to convert the link to a trunk link. It sends out DTP (Dynamic Trunking Protocol) messages to negotiate the trunking state with the other end of the link. If the other end of the link is set to auto mode or desirable mode, the link becomes a trunk. If the other end of the link is set to access mode, the link becomes a non-trunk link. This mode is useful when you want to actively negotiate the trunking state with the other end of the link.

E. Blocking mode: This is not a valid mode for a switch port used as a VLAN trunk. Blocking mode is used in spanning tree protocol to block redundant links and prevent loops.

F. Forwarding mode: This is not a valid mode for a switch port used as a VLAN trunk. Forwarding mode is the normal mode of operation for a switch port that is not blocked by spanning tree protocol.

In summary, the valid modes for a switch port used as a VLAN trunk are transparent, auto, and desirable.