0 Replies Latest reply: Sep 19, 2009 9:53 AM by Usman Ali Butt RSS

    VTP and DTP dependency

    Usman Ali Butt

      is there anybody who can explain and make it easy for me to understand VTP and DTP dependency

      Thanks in advance.

        • 1. Re: VTP and DTP dependency
          Cristian Matei - CCIEx2

          Hi,

           

                    Basically, there is no dependency between VTP and DTP; VTP is Virtual Trunking Protocol which helps advertising VLANs within the same domain; VTP messages are exchaned only on trunk links.

                    DTP, Dynamic Trunking Protocol(enaabled by default on all switch ports) helps create a trunk link betwwen switches; most of the time it succeeds. I say most of the time becase there are 2 states from the DTP perspective, which depend on the platform model:

            • dynamic desirable, which means it initiates the trunking formation
            • dynamic auto, which means it does NOT initiate but it responds to dynamic trunk negotiation.

          So for a trunk to form, you need:

            • dynamic desirable on both ends
            • dynamic desirable on one end and dynamic auto on the other end
            • of course, manually setting the trunk on both ends
            • dynamci desirable on one end and manual trunk on the other end(with DTP enabled)

          Trunk  will not be formed in these situations:

            • dynamic auto on both ends
            • manual trunk on one end and dynamci auto on the other end

              

           

          So the ONLY correlation betwwen DTP and VTP is that once DTP negotiates trunk, VTP will run over the trunk.

           

          Regards,

          Cristian.

          • 2. Re: VTP and DTP dependency
            Michael Antonakis

            Hello,

             

            Actually there is a dependency, check the link to the cisco document bellow.

            http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml

             

            Dynamic Trunking Protocol (DTP) sends the VTP domain name in a DTP packet. Therefore, if you have two ends of a link that belong to different VTP  domains, the trunk does not come up if you use DTP. In this special case, you must configure the trunk mode as on or nonegotiate, on both sides, in order to allow the trunk to come up without DTP negotiation agreement.

             

            Yours Sincerely,

            Michael

            • 3. Re: VTP and DTP dependency
              mongolio

              Yes, Michael`s link is rather good describes situation with VTP and DTP interaction. But there are few things to consider about VTP in its impact on DTP negotiation:

               

              1) If one side of link belongs to some VTP domain but other side have default configuration and belongs to NULL domain - trunk all the same will be negotiated

               

              2) There is no requirement that switches have ability to exchange VTP information - if one side have password configured and other not, but both have same VTP domain name than vtp  information cannot be exchanged but despite on that trunk via DTP will be negotiated (helpfull when building trunk with switch belonged to other organization or smth else)

              • 4. Re: VTP and DTP dependency
                Marko Milivojevic

                Actually... in #1, the switch will NULL domain will set its domain to be the same as the one of the first VTP advertisement it hears.

                 

                --

                Marko Milivojevic - CCIE #18427 (SP R&S)

                Senior Technical Instructor - IPexpert

                • 5. Re: VTP and DTP dependency
                  Sanjay Singh

                  In Short

                   

                  VTP - used to centralized the management of vlans.

                  DTP - Used to negotiation for trunk between switches.

                   

                   

                  Hi Mongolio, Can you share any diagram where it can possible to configure the same as you are saying.

                   

                   

                  Sanjay

                  sanjay9110@gmail.com

                  • 6. Re: VTP and DTP dependency
                    Marko Milivojevic

                    Here it is:

                     

                    SW1------------------------------------------SW2

                    VTP domain: NULL                     VTP domain: cisco

                    Fa0/24                                       Fa0/24

                     

                    --

                    Marko Milivojevic - CCIE #18427 (SP R&S)

                    Senior Technical Instructor - IPexpert

                    • 7. Re: VTP and DTP dependency
                      mongolio

                      >>Actually... in #1, the switch will NULL domain will set its domain to be the same as the one of the first VTP advertisement it hears.

                       

                      To hear VTP advertisment trunk link must be already negotiated. 

                      • 8. Re: VTP and DTP dependency
                        Marko Milivojevic

                        In theory, there is no difference between theory and practice.

                         

                        Lab it up and see with your own eyes. I could do it if you don't have access to switches, but I know what happens - I do it in the class every two weeks when I hear the book answer like yours :-)

                         

                        --

                        Marko Milivojevic - CCIE #18427 (SP R&S)

                        Senior Technical Instructor - IPexpert

                        • 9. Re: VTP and DTP dependency
                          Marko Milivojevic

                          But yes - technically speaking, you're right! The trunk will first be negotiated and the NULL side will learn the domain.

                           

                          I just wanted to clarify that :-)

                           

                          --

                          Marko Milivojevic - CCIE #18427 (SP R&S)

                          Senior Technical Instructor - IPexpert

                          • 10. Re: VTP and DTP dependency
                            sg4rb0sss

                            I realise this is an old post, but I thought I'd update it in case someone else is searching this topic and wants clarification.

                             

                            I just labbed this up.  VTP domains do need to match in order for DTP to form a trunk. I labbed this using Sw1 port 23 ----------- Sw2 port 23.

                             

                             

                            SW1#sh vtp status
                            VTP Version capable             : 1 to 3
                            VTP version running             : 2
                            VTP Domain Name                 : CCIE
                            VTP Pruning Mode                : Disabled
                            VTP Traps Generation            : Disabled
                            Device ID                       : 0022.be79.de80
                            Configuration last modified by 0.0.0.0 at 3-1-93 10:59:55
                            Feature VLAN:
                            --------------
                            VTP Operating Mode                : Transparent
                            Maximum VLANs supported locally   : 1005
                            Number of existing VLANs          : 14
                            Configuration Revision            : 0
                            MD5 digest                        : 0x93 0x4E 0x2D 0xA7 0x6E 0x9D 0xBE 0xE0
                                                                0x7F 0xF6 0x34 0x26 0x0C 0xC0 0x74 0xCA

                             

                            SW1#sh vtp status

                            VTP Version capable             : 1 to 3

                            VTP version running             : 2

                            VTP Domain Name                 : CCIE

                            VTP Pruning Mode                : Disabled

                            VTP Traps Generation            : Disabled

                            Device ID                       : 0022.be79.de80

                            Configuration last modified by 0.0.0.0 at 3-1-93 10:59:55

                             

                            Feature VLAN:

                            --------------

                            VTP Operating Mode                : Transparent

                            Maximum VLANs supported locally   : 1005

                            Number of existing VLANs          : 14

                            Configuration Revision            : 0

                            MD5 digest                        : 0x93 0x4E 0x2D 0xA7 0x6E 0x9D 0xBE 0xE0

                                                                0x7F 0xF6 0x34 0x26 0x0C 0xC0 0x74 0xCA

                             

                            SW2#sh vtp status

                            VTP Version capable             : 1 to 3

                            VTP version running             : 2

                            VTP Domain Name                 : TEST

                            VTP Pruning Mode                : Disabled

                            VTP Traps Generation            : Disabled

                            Device ID                       : 0023.0443.0c80

                            Configuration last modified by 0.0.0.0 at 3-1-93 10:59:55

                             

                            Feature VLAN:

                            --------------

                            VTP Operating Mode                : Transparent

                            Maximum VLANs supported locally   : 1005

                            Number of existing VLANs          : 16

                            Configuration Revision            : 0

                            MD5 digest                        : 0xD4 0xF6 0x2F 0x29 0x8D 0x94 0x79 0x5D

                                                                0x08 0xE1 0xE6 0xD1 0x44 0xD4 0x03 0x51

                             

                            As soon as I bring the link up, I get the log message:

                            %DTP-5-DOMAINMISMATCH: Unable to perform trunk negotiation on port Fa0/23 because of VTP domain mismatch.

                             

                            The only real debug to catch this problem is #debug dtp events, which shows

                             

                            DTP-pkt:Fa0/23:TLV Domain name mismatch: "CCIE" ../dyntrk/dyntrk_core.c:1367

                             

                            If you didn't spot the log message when you  brought the trunk up, and you used a debug to find out why DTP isn't working, then this really isn't a very good debug message as it doesn't say that it's the vtp domain name that is the problem.  So you kind of have to just remember that the VTP domain name will have to match.  Another way this will work is if you don't configure a vtp domain name at all, as I will explain below.

                             

                            If you are only using transparent switches in your network, and you add a new transparent switch with the domain name unconfigured, it will not learn the domain name.  I tested this in the lab, with Sw1 fa0/23 in dynamic auto in vtp domain CCIE, and Sw2 fa0/23 in dynamic desirble in a blank (unconfigured) vtp domain. The trunk comes up, but the domain name stays null, as my results below show.

                             

                            Sw1#sh vtp status

                            VTP Version capable             : 1 to 3

                            VTP version running             : 1

                            VTP Domain Name                 :

                            VTP Pruning Mode                : Disabled

                            VTP Traps Generation            : Disabled

                            Device ID                       : 0023.0443.0c80

                            Configuration last modified by 0.0.0.0 at 0-0-00 00:00:00

                             

                            Feature VLAN:

                            --------------

                            VTP Operating Mode                : Transparent

                            Maximum VLANs supported locally   : 1005

                            Number of existing VLANs          : 5

                            Configuration Revision            : 0

                            MD5 digest                        : 0x57 0xCD 0x40 0x65 0x63 0x59 0x47 0xBD

                                                                0x56 0x9D 0x4A 0x3E 0xA5 0x69 0x35 0xBC

                            Sw2#sh int fa0/23 switchport

                            Name: Fa0/23

                            Switchport: Enabled

                            Administrative Mode: dynamic desirable

                            Operational Mode: trunk

                            Administrative Trunking Encapsulation: negotiate

                            Operational Trunking Encapsulation: isl

                            Negotiation of Trunking: On

                            Access Mode VLAN: 1 (default)

                            Trunking Native Mode VLAN: 1 (default)

                            Administrative Native VLAN tagging: enabled

                            Voice VLAN: none

                            Administrative private-vlan host-association: none

                            Administrative private-vlan mapping: none

                            Administrative private-vlan trunk native VLAN: none

                            Administrative private-vlan trunk Native VLAN tagging: enabled

                            Administrative private-vlan trunk encapsulation: dot1q

                            Administrative private-vlan trunk normal VLANs: none

                            Administrative private-vlan trunk associations: none

                            Administrative private-vlan trunk mappings: none

                            Operational private-vlan: none

                            Trunking VLANs Enabled: ALL

                            Pruning VLANs Enabled: 2-1001

                            Capture Mode Disabled

                            Capture VLANs Allowed: ALL

                             

                            Protected: false

                            Unknown unicast blocked: disabled

                            Unknown multicast blocked: disabled

                            Appliance trust: none

                            Switch#sh int fa0/23 trunk


                            Port        Mode             Encapsulation Status        Native vlan

                            Fa0/23      desirable        n-isl         trunking      1

                             

                            Port        Vlans allowed on trunk

                            Fa0/23      1-4094

                             

                            Port        Vlans allowed and active in management domain

                            Fa0/23      1

                             

                            Port        Vlans in spanning tree forwarding state and not pruned

                            Fa0/23      1

                            Switch#

                            • 11. Re: VTP and DTP dependency
                              mongolio

                              Stephen wrote:

                               

                              I just labbed this up.  VTP domains do need to match in order for DTP to form a trunk.

                               

                              Indeed they do, and you prove it in your first test. BTW NULL domain name is as well ok to form trunk, as i wrote 4 years ago.

                              • 12. Re: VTP and DTP dependency
                                Networks_Ish

                                Thanks for labbing it, Stephen. Interesting point and worth keeping in mind if you utilise DTP/VTP in your networks.

                                • 13. Re: VTP and DTP dependency
                                  johncurtis1541

                                  Some people say that you need to know and feel what the router/switch is thinking before you can master routing-switching.

                                   

                                  And that is not enough, evident with this post.

                                   

                                  The thing is that you need to know what the programmer that programmed DTP/VTP was thinking before you can master routing-switching. And what was he/she thinking when he programmed DTP/VTP? He/she was thinking that closed source coding is terrible. But he/she has got to eat. And his/her boss asked him/her the progress of simplifying trunking configuration. Now what? Should he/she complain that VTP and DTP depending on each other creates a plethora of unpredictable negotiation results, and he/she got a job without becoming a CCIE first while others paying large sums of money to feed his/her Catalyst company? Or, should he/she consider that going down the VTP DTP dependency path creates endless jobs of troubleshooting 25 years, a quarter centuries, after the code was written. His/her boss never agreed to hire him/her, instead, the boss was somehow coerced to work with him/her. We all have worked as professional programmers one time or another, and we have seen programmers die leaving nasty things behind.

                                   

                                  The answer to this post's question is to disable all negotiation DTP, disable all VTP, disable ISL. The programmer that coded DTP/VTP has probably died in one of those 25 years since it was written. And mark this post answered.