Difference Between DTP "dynamic desirable" and "dynamic auto" Modes
Dynamic Trunking Protocol (DTP) is a Cisco proprietary trunking protocol used for negotiating trunking on a link between two Cisco Switches. Dynamic Trunking Protocol (DTP) can also be used for negotiating the encapsulation type of either 802.1q or Cisco ISL (Inter-Switch Link). Please note that Cisco ISL (Inter-Switch Link) is no more a widely accepted trunking standard. Many of the recent Cisco switches only support IEEE 802.1Q trunking standard.
switchport mode dynamic auto
A switch's interface configured with "dynamic auto" mode will not actively try to convert the other side switch's interface to form a trunk link. "dynamic auto" interface becomes a trunk interface only if the other side switch's interface is configured to trunk mode or "dynamic desirable" mode.
When the interface is configured as "dynamic auto" mode, the interface passively listens for Dynamic Trunking Protocol (DTP) messages from the other side switch's interface. If a "dynamic auto" interface receives a Dynamic Trunking Protocol (DTP) message from the other side switch's interface a trunk link is formed and the interface starts tagging frames. No trunk link is formed if no message is arrived from other side switch's interface or the trunking capabilities of two switches are different.
For example, if both switch's interface are configured with "dynamic auto" mode, they will never generate Dynamic Trunking Protocol (DTP) messages and the link will be an access link.
switchport mode dynamic desirable
The interface which is configured as Dynamic Trunking Protocol (DTP) dynamic desirable mode will generate Dynamic Trunking Protocol (DTP) messages on the interface, and actively ttry to convert the other side switch's interface to form a trunk.
A trunk link will be formed if the other side switch's interface is configured with "dynamic desirable", "dynamic auto" or "trunk" mode.
Case 1: When both sides are "dynamic desirable"
Case 2: When one side is "dynamic desirable" and other side is "dynamic auto"
Case 3: When one side is "trunk" and other side is "dynamic desirable"
