CCIE Pursuit Blog

August 8, 2007

Changing DTP Settings Can Reset An Interface

Filed under: Cisco,Cisco Certification,IOS,Switching — cciepursuit @ 8:54 am

This is more of a real world issue than a lab issue.  There are a number of seemingly innocuous, small changes that you can make in IOS that can come back to bite you in the butt.  It only took one speed/duplex change on a trunk link briefly dropping connectivity to an IDF at my old job to ensure that ANY future speed/duplex changes would require a full-blown change control request.  I have found that there is a similar danger with changing the DTP mode of a trunk link.

Imagine the scenario: You have telnetted to a 3560 and decide to change the DTP settings on a trunk link from the default of “dynamic auto” to “dynamic desirable”:

sw1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
sw1(config)#int fa0/1
sw1(config-if)#switch mode dyn des

Before you can hit “control + z” you see the trunk drop and then restore:

*Mar  1 02:52:37: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
*Mar  1 02:52:40: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

Oh crap!  The traffic on that trunk link drops briefly.  Well, maybe no one will notice.  Then you get the call from the NOC, “We just saw the uplink to the Financial Services LAN go down.  When we looked at the log we saw that the last change was attributed to your TACACS ID.  What were you doing?”   From there your day just goes further down the toilet as your manager and the Financial Services manager take you to a dark conference room and make you cry like you haven’t cried since you were five years old.  🙂

While there are very few reasons for changing DTP settings on working trunks AND only doing DTP changes afterhours is probably the best strategy – you can make all but two types of DTP changes without dropping the trunk link. 

Huge Caveat: I tested these in a lab with two 3560s.  Your mileage may vary.  Again, it’s best to make DTP changes afterhours.  🙂

I labbed up the following scenarios and noted whether or not the link drops:

Switch 1 Switch 2 Link Drops?
Dynamic Auto Dynamic Desirable Yes
Dynamic Auto Access No
Dynamic Auto Dynamic Auto No
Dynamic Desirable Dynamic Auto No
Dynamic Desirable Access No
Dynamic Desirable Dynamic Desirable No
Access Dynamic Auto No
Access Dynamic Desirable Yes
Access Access No


  1. Hello,

    You haven’t hit the enter key and the iface dropped ?


    Comment by fropert — August 8, 2007 @ 9:20 am | Reply

  2. Good point fropert. You do have to hit enter before the DTP change takes place (and then the interface will drop). I can see where this is confusing in the output.

    Comment by cciepursuit — August 8, 2007 @ 1:26 pm | Reply

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