CCIE Pursuit Blog

August 8, 2007

Disabling DTP

Filed under: Cisco,Cisco Certification,IOS,Switching — cciepursuit @ 11:18 am

Task:  Configure an ISL trunk between two 3560s without using DTP.

My first instinct is that this can be accomplished by hardsetting both side of the link to trunk with ISL encapsulation. 

This is a GREAT question because you need to explicitly disable DTP. The following solution will NOT work:

sw1#sh run int fa0/13
Building configuration…

Current configuration : 93 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk
end

sw2#sh run int fa0/13
Building configuration…

Current configuration : 93 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk

end

Even though trunking is not negotiated:
sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1

DTP is still ON:

sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On <-NOTE!!!
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)

sw2#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On  <-NOTE!!!
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)

To get the points for this question, you will need to completely disable DTP with the “nonegotiate” command:

sw1#sh run int fa0/13
Building configuration…

Current configuration : 117 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk
 switchport nonegotiate
end

sw2#sh run int fa0/13
Building configuration…

Current configuration : 117 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk
 switchport nonegotiate
end

sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1

sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Off <-note
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1

sw2#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Off <-note
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)

Further note that you must configure”switchport nonegotiate” on BOTH sides of the link to get the points (or ask the proctor to clarify).

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2 Comments »

  1. […] a trunk does not establish between two 3560s, trunking negotiation IS still on by default (see this post for details): Name: Po1 Switchport: Enabled Administrative Mode: dynamic auto Operational Mode: static access […]

    Pingback by Interpreting Tasks « CCIE Pursuit — August 11, 2007 @ 6:06 am | Reply

  2. Great explanation.

    Thank you,

    Mani

    Comment by Manisha — May 21, 2009 @ 2:07 am | Reply


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