Dynamic Trunking Protocol

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The Dynamic Trunking Protocol (DTP ) is a proprietary network protocol from Cisco Systems. It is used in LANs to Ethernet links on the VLAN trunking (ie, whether the link to a trunk or to an access port, or - in the worst case - remains inactive ) and possibly the type of encapsulation (ISL or IEEE 802.1q ) to independently negotiate. For this purpose, the interface sends DTP frames to the multicast MAC address 01 -00- 0C- CC -CC -CC, which is also used by other Cisco proprietary protocols such as VTP, PAgP, or UDLD.

Under trunking is understood here that multiple VLANs are transmitted over a single physical connection. The most common standardized method for this is IEEE 802.1q, but there are still some proprietary solutions such as Cisco 's ISL.

DTP knows the 5 modes Auto, On, Off, Desirable and No- Negotiate, see also configuration.

The negotiation of trunking can save time and work the network administrator while, but is considered more critical in terms of network security. Thus, most security experts advise to disable 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 configuration of DTP is taking in the IOS interface configuration mode on the one hand with the command [ no] switchport negotiate.

This command enables DTP on the corresponding interface generally on or off. This is only possible if either the mode "access" or "trunk " was configured on the interface.

On the other hand, also has the switchport mode command, which specifies the particular mode, still wields great influence on the behavior of DTP:

  • Switchport mode trunk
  • Switchport mode dynamic desirable
  • Switchport mode dynamic auto

Issue with show or debug command

Show the following command indicates whether DTP is turned on or off, as well as the configured DTP mode:

Switch # show interface Fast Ethernet 4/1 switchport ... Administrative Mode: dynamic desirable Operational Mode: trunk ... Negotiation of Trunking: On ... A debugging, as with Cisco common possible:

Switch # debug dtp states 01:15:37: DTP -state: Fa4 / 1: Starting state transition from state S3: NT desktop publishing, 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
  • See also VTP
  • Network protocol (data link layer )
  • Network protocol of Cisco Systems
Frame (networking) Port Aggregation Protocol
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