9 Replies Latest reply: Oct 18, 2012 8:17 AM by JD RSS

    Full duplex required for trunking?


      Is full duplex a requirement for trunking between switches, or anywhere?

        • 1. Re: Full duplex required for trunking?

          Yes, in order to form an trunk, the link has to be PTP(point-to-point), from one L2 interface, ex. NIC, to one and one only L2 interface, not L1 or hub. Point-to-Point links also specify full-duplex, which simply means the two ends are able to transmit and receive at the same time. An L2 interface is present on PC's, swithches, routers, etc.. Therefore, when connecting an switch to an router for router-on-a-stick, you're connecting their L2 interfaces, which must be both PTP, and therefore, full-duplex as well. You can also form trunks from switches to PC's/servers with extra configuration required on the PC/server.



          • 2. Re: Full duplex required for trunking?
            Paul Stewart  -  CCIE Security

            Full duplex and trunking is two different concepts. I see no technical reason why you could not make a trunk half duplex. Full duplex

            basically simplifies the algorithm at the port itself by disabling colision detection (that is the cd part of csma/cd) and allowing the out queue to directly talk to the wire without consulting the cd algorithm. This really has nothing to do with tagging the frames that would already be done at this point.






            I don't think there is no reason that you would want to make a trunk anything other than full-duplex. I guess in theory, you could have a trunk connected through a dumb hub and that would not work very well unless you allowed it to do half duplex. That would be a very poor design. In practice a trunk should be a direct connection between two switches. Additionally, for best performance they should do full duplex. With a direct switch to switch connection, there is no reason not to.

            • 3. Re: Full duplex required for trunking?

              Hi Paul,


              Very insightful response. As you stated:'from an technical and theoretical standpoint full-duplex isn't required, but from an actual implementation standpoint, full-duplex is the only practical solution.'




              I appreciate your very deep insight into the architecture of the gear!





              • 4. Re: Full duplex required for trunking?


                Hi Vlan



                Here is an example




                interface FastEthernet0/18

                switchport access vlan 2

                switchport mode trunk

                duplex half




                Switch#show int trunk



                Port Mode Encapsulation Status Native vlan

                Fa0/18 on 802.1q trunking 1



                Port Vlans allowed on trunk

                Fa0/18 1-4094



                Regards Conwyn



                • 5. Re: Full duplex required for trunking?


                  Hi Conwyn,



                  Yeah, Paul Stewart set me straight. Thank you for posting an config. with verification though...much appreciated. I wrongly believed IOS and DTP, or any trunk, wanted to see an port duplex setting of full, in order to allow either ISL or dot1q.






                  • 6. Re: Full duplex required for trunking?
                    Mohamed Sobair








                    No full duplex is not required between trunks but its recommended to hardcode your interfaces to Full duplex if possible.












                    • 7. Re: Full duplex required for trunking?

                      Usually you setup trunks on Fiberport or Gigport. There you can´t choose HD on the Interface.

                      • 8. Re: Full duplex required for trunking?

                        I agree with your post. I am sitting here trying to think why you would want the switch to apply the cd algorithm to the trunking interface, especially considering that VTP is also running. This would allocate a ton of resources for an interface that already has a ton to do. I have found that many authors suggest that all trunking interface run in full duplex.


                        One should also pick an interface of atleast 100mbps. A bad design choice would be selecting a 10mbps interface as a trunking interface. I personally would choose fiber for a trunking interface if available.

                        • 9. Re: Full duplex required for trunking?

                          After reading the posts and the config, it's clear that full duplex is not fully necessary for trunking.


                          Thanks for the posts.