CCIE Pursuit Blog

June 12, 2007

Cisco 3550/3560 DTP Defaults

Filed under: CCIE Lab Workbooks,Home Lab,IOS,Tech Tips — cciepursuit @ 1:02 pm

The Internetwork Workbook uses a lab that includes 4 switches.  Two Cisco 3550 switches and two Cisco 3560 switches – all running IOS version 12.2(25)SEE2.  One very important difference between these two models of switches is the default DTP (Dynamic Trunking Protocol) settings.  The 3550 default DTP setting is “switchport mode dynamic desirable” and the 3560 default DTP setting is “switchport mode dynamic auto”:

From Catalyst 3550 Multilayer Switch Command Reference, Rel. 12.2(25)SEE
switchport mode

Defaults The default mode is dynamic desirable.

From Catalyst 3560 Switch Command Reference, Rel. 12.2(25)SEE
switchport mode

Defaults The default mode is dynamic auto.

Basically this means that a default 3550 port will create a trunk when connected to any other port except one that is configured as a access port (“switchport mode access” – no trunking).  A default 3560 port will create a trunk when connected to any other port except one that is configured as a access port OR a port configured as “switchport mode dynamic auto.”  The biggest “gotcha” with these defaults is that a when connecting two 3560s (with default ports) a trunk will NOT be created.

DTP Grid:

Switchport Mode Access Dynamic Desirable Dynamic Auto Trunk
Access No Trunk No Trunk No Trunk No Trunk
Dynamic Auto No Trunk Trunk No Trunk Trunk
Dynamic Desirable No Trunk Trunk Trunk Trunk
Trunk No Trunk Trunk Trunk Trunk

DTP configurations:

Access – switchport mode access
Dynamic Auto – switchport mode dynamic auto
Dynamic Desirable – switchport mode dynamic desirable
Trunk – switchport mode trunk

Cisco Documentation:

Cisco Catalyst 3550 Series Switches Command References

Catalyst 3550 Multilayer Switch Command Reference, Rel. 12.2(25)SEE

Catalyst 3560 Series Switches Command References

Catalyst 3560 Switch Command Reference, Rel. 12.2(25)SEE

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7 Comments »

  1. […] Feature Comparison I recently blogged about the difference in the default DTP settings on the 3550 versus 3560 switches.  Internetwork Expert has a webpage detailing the differences […]

    Pingback by 3550/3560 Feature Comparison « CCIE Pursuit — June 23, 2007 @ 10:47 am | Reply

  2. […] (Dynamic Trunking Protocol) had hooked me up with trunks between these two 3560s.  Wrong!!!  As I mentioned in an earlier post, the default DTP configuration on the 3560 is “switchport mode dynamic auto”.  A […]

    Pingback by LFU2 - Know Your Aconyms « CCIE Pursuit — June 28, 2007 @ 10:03 pm | Reply

  3. […] The default on the 3560s is “switchport mode dynamic auto”, so they will only dynamically trunk if the other side is NOT set to auto.  We can simply configure “switchport mode dynamic desirable” on one switch (let’s use sw1) sw1(config)#int range fa0/13 – 15 sw1(config-if-range)#switch mode dyn des sw1(config-if-range)#no sh […]

    Pingback by Internetwork Expert Volume II: Lab 3 - Section 1 « CCIE Pursuit — December 18, 2007 @ 1:04 pm | Reply

  4. i faced a problem in my lab where i tried to manually set the mode of the interface to trunk (not dynamic), and i would get a message that i can not do that while the port is in dynamic autp mode, and that happened even when the port in shut.

    Comment by Bassem — January 19, 2008 @ 6:20 am | Reply

  5. @Bassem

    Did you configure your trunking encapsulation? If you don’t do that first, you’ll get the following error:

    sw1(config)#int fa0/6
    sw1(config-if)#switch mode trunk
    Command rejected: An interface whose trunk encapsulation is “Auto” can not be configured to “trunk” mode.

    You’ll need to specify a trunking encapsulation before hard-coding a port to be a trunk:

    sw1(config-if)#switchport trunk encapsulation ?
    dot1q Interface uses only 802.1q trunking encapsulation when trunking
    isl Interface uses only ISL trunking encapsulation when trunking
    negotiate Device will negotiate trunking encapsulation with peer on
    interface

    sw1(config-if)#switchport trunk encapsulation dot1q
    sw1(config-if)#switchport mode trunk
    sw1(config-if)#do sh int fa0/6 switchport
    Name: Fa0/6
    Switchport: Enabled
    Administrative Mode: trunk
    Operational Mode: down
    Administrative Trunking Encapsulation: dot1q
    Negotiation of Trunking: On
    Access Mode VLAN: 1 (default)
    Trunking Native Mode VLAN: 1 (default)
    Administrative Native VLAN tagging: enabled

    Comment by cciepursuit — January 19, 2008 @ 8:09 am | Reply

  6. […] going to try and know it because it’s easy enough to make a table and just learn; check out CCIE Pursuit for a table on the 3550/3560 routers and bitmindframes for more info on each DTP config […]

    Pingback by VLAN Configuration Errors at Network Ninja — July 8, 2008 @ 1:17 am | Reply

  7. […] the same VTP domain. (VTP is discussed Here.) You can find more info on catalyst switch defaults @ CCIEpursuit Switches have two “non-dynamic” options: Switch1(config-if)#switchport mode access An […]

    Pingback by DTP dynamic trunking protocol | bitmindframes — July 22, 2008 @ 2:25 pm | Reply


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