Dynamic Trunking Protocol
DTP stands for dynamic trunking protocol. DTP allows Cisco switches to dynamically learn their interface status (access or trunk) without manual configuration. It means when one switch interface is configured, other switch automatically learn the status (doesn’t wait for manual configuration). For example two switches(S1 and S2) are connected together, S1 is configured as trunk port manually then S2 dynamically learn the status.
It is Cisco proprietary protocol.
By default, DTP is enabled on all Cisco switches. To disable DTP on an interface use switchport nonegotiate command.
In older switches, switchport mode dynamic desirable is the default administrative mode while in newer switches, switchport mode dynamic auto is the default mode.
DTP will not form a trunk with a router and PC. These switchport always will be in access mode.
Switchport dynamic modes
- Switchport mode dynamic desirable
- Switchport mode dynamic auto
Switchport mode dynamic desirable: A switchport in dynamic desirable mode will actively try to form a trunk with other Cisco switches.
Switchport mode dynamic auto: A switchport in dynamic auto mode will not actively try to form an access or trunk port. It checks the other interface status and then accordingly it will form a trunk or access. Like if other interface is trying to form a trunk, then it will become a trunk and if other interface is trying to form an access then it will become access port.
Switchport mode trunk: In this mode, switchport will always in trunk mode.
Switchport mode access: In this mode, switchport will always in access mode.
How DTP works?
Case1: S1 is in dynamic desirable mode
Let’s understand with below example.
You can see, here are two switches named as S1 and S2. S1 interface is in dynamic desirable mode. As we know, switchport in dynamic desirable mode will actively try to form a trunk. It will form a trunk if other interface is in trunk, dynamic desirable and dynamic auto mode.
It will not form a trunk when other interface is in static access mode.
S1 switchport mode | S2 switchport mode | Operational mode |
Dynamic desirable | trunk | trunk |
Dynamic desirable | Dynamic desirable | trunk |
Dynamic desirable | Dynamic auto | trunk |
Dynamic desirable | Access | access |
Case 2: S1 is in dynamic auto mode
S1 switchport mode | S2 switchport mode | Operational mode |
Dynamic auto | trunk | trunk |
Dynamic auto | Dynamic desirable | trunk |
Dynamic auto | Dynamic auto | access |
Dynamic auto | Access | access |
A switchport in dynamic auto will not actively try to form an access or trunk port, it works passively, first checks what other interface is trying to form and then accordingly it forms a trunk or access. In above table, S1 is in dynamic auto mode, and it will form trunk when other interface will be in trunk and dynamic desirable (both will try to actively form a trunk, hence dynamic auto will also form a trunk).
In last 2 rows S2 is in dynamic auto and access mode, in both cases it will form a access port.
Case 3: Manually configured trunk and access
You can see in above image, S1 interface is manually configured as trunk and S2 interface is manually configured as access port. But this doesn’t work, we can’t manually configure trunk and access together.
Summarization table:
