Dynamic Trunking Protocol
The Dynamic Trunking Protocol (DTP) is a proprietary network protocol from Cisco Systems . It is used in LANs to on Ethernet -Left the VLAN - trunking (ie, whether the link into a trunk or to an access port, or inactive remains) and possibly the type of encapsulation ( ISL or IEEE 802.1q independent) to negotiate. To do this, the interface sends DTP frames to the multicast - MAC address 01-00-0C-CC-CC-CC, which is also used by other proprietary Cisco protocols such as B. VTP , PAgP or UDLD is used.
Trunking here means that several VLANs are transmitted over a single physical connection. The best-known standardized procedure for this is IEEE 802.1q . There are also some proprietary solutions such as B. Cisco's ISL.
DTP knows the five modes access , trunk , dot1q-tunnel , dynamic auto and dynamic desirable .
Negotiating trunking can save the network administrator time and work, but is viewed more critically from the point of view of network security. Most security experts therefore recommend disabling DTP.
The "Status" field indicates the DTP mode (on, off, desirable or auto), "dtptype" contains information about the type of trunking (Negotiate, ISL or IEEE 802.1q)
configuration
The DTP is configured under IOS in the interface configuration mode with the [no] switchport negotiate command .
This command generally switches DTP on or off on the relevant interface. However, this is only possible if either access or trunk mode has been configured on the interface.
On the other hand, the switchport mode command, which defines the respective mode, still has a major influence on the behavior of DTP:
- switchport mode access
- Interface is in permanent non-trunking mode, DTP frames are sent.
- switchport mode trunk
- Permanent trunking, DTP frames are sent.
- switchport mode dot1q tunnel
- Port becomes the tunnel port on which a single tunnel VLAN is configured. Several customer VLANs can be transported via this tunnel VLAN.
- The ingress switch inserts a second dot1q tag into the Ethernet frame before the actual VLAN tag. This is replaced by the egress switch and the packet is processed further as a normal tagged packet.
- switchport mode dynamic desirable
- (active) Port becomes trunk if its neighboring port at the other end is configured as trunk , dynamic desirable or dynamic auto . DTP frames are sent.
- switchport mode dynamic auto
- (passive) Port becomes trunk if the neighboring port at the other end is configured as trunk or dynamic desirable . DTP frames are not sent.
Output with show or debug command
The following show command shows whether DTP is switched on or off, as well as the configured DTP mode:
Switch#show interface FastEthernet 4/1 switchport … Administrative Mode: dynamic desirable Operational Mode: trunk … Negotiation of Trunking: On …
Debugging is also possible, as is usual with Cisco:
Switch# debug dtp states 01:15:37: DTP-state:Fa4/1:Starting state transition from state S3:NT-DTP, event 6b:TN EXP TRK ../dyntrk/dyntrk_fsm.c:631 01:15:37: DTP-state:Fa4/1:Executing action 8 ../dyntrk/dyntrk_fsm.c:816 01:15:37: DTP-state:Fa4/1:Ending state transition to state S6:TRUNK ../dyntrk/dyntrk_fsm.c:659
Web links
- cisco.com
- nsa.gov switch guide version (pdf; 663 kB)
Individual evidence
- ↑ a b Cisco Networking Academy's Introduction to VLANs. April 7, 2014, accessed October 7, 2018 .
- ↑ Catalyst 3750-X and 3560-X Switch Software Configuration Guide, Release 12.2 (55) SE. Chapter: Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling. Cisco, September 11, 2018, accessed April 21, 2017 .