Configuring VLAN Trunks

Finding Feature Information

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Prerequisites for VLAN Trunks

The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:

  • In a network of Cisco devices connected through IEEE 802.1Q trunks, the devices maintain one spanning-tree instance for each VLAN allowed on the trunks. Non-Cisco devices might support one spanning-tree instance for all VLANs.

    When you connect a Cisco device to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco device combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of the non-Cisco IEEE 802.1Q device. However, spanning-tree information for each VLAN is maintained by Cisco devices separated by a cloud of non-Cisco IEEE 802.1Q devices. The non-Cisco IEEE 802.1Q cloud separating the Cisco devices is treated as a single trunk link between the devices.

  • Make sure the native VLAN for an IEEE 802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk is different from the native VLAN on the other end, spanning-tree loops might result.

  • Disabling spanning tree on the native VLAN of an IEEE 802.1Q trunk without disabling spanning tree on every VLAN in the network can potentially cause spanning-tree loops. We recommend that you leave spanning tree enabled on the native VLAN of an IEEE 802.1Q trunk or disable spanning tree on every VLAN in the network. Make sure your network is loop-free before disabling spanning tree.

Restrictions for VLAN Trunks

The following are restrictions for VLAN trunks:

  • A trunk port cannot be a secure port.

  • Trunk ports can be grouped into EtherChannel port groups, but all trunks in the group must have the same configuration. When a group is first created, all ports follow the parameters set for the first port to be added to the group. If you change the configuration of one of these parameters, the device propagates the setting that you entered to all ports in the group:

    • Allowed-VLAN list.

    • STP port priority for each VLAN.

    • STP Port Fast setting.

    • Trunk status:

      If one port in a port group ceases to be a trunk, all ports cease to be trunks.

  • We recommend that you configure no more than 24 trunk ports in Per VLAN Spanning Tree (PVST) mode and no more than 40 trunk ports in Multiple Spanning Tree (MST) mode.

  • If you try to enable IEEE 802.1x on a trunk port, an error message appears, and IEEE 802.1x is not enabled. If you try to change the mode of an IEEE 802.1x-enabled port to trunk, the port mode is not changed.

  • A port in dynamic mode can negotiate with its neighbor to become a trunk port. If you try to enable IEEE 802.1x on a dynamic port, an error message appears, and IEEE 802.1x is not enabled. If you try to change the mode of an IEEE 802.1x-enabled port to dynamic, the port mode is not changed.

  • Dynamic Trunking Protocol (DTP) is not supported on tunnel ports.

  • The device does not support Layer 3 trunks; you cannot configure subinterfaces or use the encapsulation keyword on Layer 3 interfaces. The device does support Layer 2 trunks and Layer 3 VLAN interfaces, which provide equivalent capabilities.

  • You cannot have a switch stack containing a mix of Catalyst 3850 and Catalyst 3650 switches.

Information About VLAN Trunks

Trunking Overview

A trunk is a point-to-point link between one or more Ethernet device interfaces and another networking device such as a router or a device. Ethernet trunks carry the traffic of multiple VLANs over a single link, and you can extend the VLANs across an entire network.

The following trunking encapsulations are available on all Ethernet interfaces:

  • IEEE 802.1Q— Industry-standard trunking encapsulation.

Trunking Modes

Ethernet trunk interfaces support different trunking modes. You can set an interface as trunking or nontrunking or to negotiate trunking with the neighboring interface. To autonegotiate trunking, the interfaces must be in the same VTP domain.

Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Point-to-Point Protocol (PPP). However, some internetworking devices might forward DTP frames improperly, which could cause misconfigurations.

Related Tasks
Configuring a Trunk Port (CLI)
Related References
Layer 2 Interface Modes

Layer 2 Interface Modes

Table 1 Layer 2 Interface Modes

Mode

Function

switchport mode access

Puts the interface (access port) into permanent nontrunking mode and negotiates to convert the link into a nontrunk link. The interface becomes a nontrunk interface regardless of whether or not the neighboring interface is a trunk interface.

switchport mode dynamic auto

Makes the interface able to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk or desirable mode. The default switchport mode for all Ethernet interfaces is dynamic auto.

switchport mode dynamic desirable

Makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode.

switchport mode trunk

Puts the interface into permanent trunking mode and negotiates to convert the neighboring link into a trunk link. The interface becomes a trunk interface even if the neighboring interface is not a trunk interface.

switchport nonegotiate

Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link.

Related Concepts
Trunking Modes
Related Tasks
Configuring a Trunk Port (CLI)

Allowed VLANs on a Trunk

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs, 1 to 4094, are allowed on each trunk. However, you can remove VLANs from the allowed list, preventing traffic from those VLANs from passing over the trunk.

To reduce the risk of spanning-tree loops or storms, you can disable VLAN 1 on any individual VLAN trunk port by removing VLAN 1 from the allowed list. When you remove VLAN 1 from a trunk port, the interface continues to send and receive management traffic, for example, Cisco Discovery Protocol (CDP), Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), DTP, and VTP in VLAN 1.

If a trunk port with VLAN 1 disabled is converted to a nontrunk port, it is added to the access VLAN. If the access VLAN is set to 1, the port will be added to VLAN 1, regardless of the switchport trunk allowed setting. The same is true for any VLAN that has been disabled on the port.

A trunk port can become a member of a VLAN if the VLAN is enabled, if VTP knows of the VLAN, and if the VLAN is in the allowed list for the port. When VTP detects a newly enabled VLAN and the VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a member of the enabled VLAN. When VTP detects a new VLAN and the VLAN is not in the allowed list for a trunk port, the trunk port does not become a member of the new VLAN.

Related Tasks
Defining the Allowed VLANs on a Trunk (CLI)

Load Sharing on Trunk Ports

Load sharing divides the bandwidth supplied by parallel trunks connecting devices. To avoid loops, STP normally blocks all but one parallel link between devices. Using load sharing, you divide the traffic between the links according to which VLAN the traffic belongs.

You configure load sharing on trunk ports by using STP port priorities or STP path costs. For load sharing using STP port priorities, both load-sharing links must be connected to the same device. For load sharing using STP path costs, each load-sharing link can be connected to the same device or to two different devices.

Network Load Sharing Using STP Priorities

When two ports on the same device form a loop, the device uses the STP port priority to decide which port is enabled and which port is in a blocking state. You can set the priorities on a parallel trunk port so that the port carries all the traffic for a given VLAN. The trunk port with the higher priority (lower values) for a VLAN is forwarding traffic for that VLAN. The trunk port with the lower priority (higher values) for the same VLAN remains in a blocking state for that VLAN. One trunk port sends or receives all traffic for the VLAN.

Related Tasks
Configuring Load Sharing Using STP Port Priorities (CLI)

Network Load Sharing Using STP Path Cost

You can configure parallel trunks to share VLAN traffic by setting different path costs on a trunk and associating the path costs with different sets of VLANs, blocking different ports for different VLANs. The VLANs keep the traffic separate and maintain redundancy in the event of a lost link.

Related Tasks
Configuring Load Sharing Using STP Path Cost (CLI)

Feature Interactions

Trunking interacts with other features in these ways:

  • A trunk port cannot be a secure port.

  • Trunk ports can be grouped into EtherChannel port groups, but all trunks in the group must have the same configuration. When a group is first created, all ports follow the parameters set for the first port to be added to the group. If you change the configuration of one of these parameters, the device propagates the setting that you entered to all ports in the group:

    • Allowed-VLAN list.

    • STP port priority for each VLAN.

    • STP Port Fast setting.

    • Trunk status:

      If one port in a port group ceases to be a trunk, all ports cease to be trunks.

  • We recommend that you configure no more than 24 trunk ports in Per VLAN Spanning Tree (PVST) mode and no more than 40 trunk ports in Multiple Spanning Tree (MST) mode.

  • If you try to enable IEEE 802.1x on a trunk port, an error message appears, and IEEE 802.1x is not enabled. If you try to change the mode of an IEEE 802.1x-enabled port to trunk, the port mode is not changed.

  • A port in dynamic mode can negotiate with its neighbor to become a trunk port. If you try to enable IEEE 802.1x on a dynamic port, an error message appears, and IEEE 802.1x is not enabled. If you try to change the mode of an IEEE 802.1x-enabled port to dynamic, the port mode is not changed.

How to Configure VLAN Trunks

To avoid trunking misconfigurations, configure interfaces connected to devices that do not support DTP to not forward DTP frames, that is, to turn off DTP.

  • If you do not intend to trunk across those links, use the switchport mode access interface configuration command to disable trunking.

  • To enable trunking to a device that does not support DTP, use the switchport mode trunk and switchport nonegotiate interface configuration commands to cause the interface to become a trunk but to not generate DTP frames.

Configuring an Ethernet Interface as a Trunk Port

Configuring a Trunk Port (CLI)

Because trunk ports send and receive VTP advertisements, to use VTP you must ensure that at least one trunk port is configured on the device and that this trunk port is connected to the trunk port of a second device. Otherwise, the device cannot receive any VTP advertisements.

Before You Begin

By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode dynamic auto. If the neighboring interface supports trunking and is configured to allow trunking, the link is a Layer 2 trunk or, if the interface is in Layer 3 mode, it becomes a Layer 2 trunk when you enter the switchport interface configuration command.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    interface interface-id

    4.    switchport mode {dynamic {auto | desirable} | trunk}

    5.    switchport access vlan vlan-id

    6.    switchport trunk native vlan vlan-id

    7.    end

    8.    show interfaces interface-id switchport

    9.    show interfaces interface-id trunk

    10.    copy running-config startup-config


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example: 
    Device> enable
     

    Enables privileged EXEC mode. Enter your password if prompted.

     

    Step 2configure terminal


    Example: 
    
Device# configure terminal
     

    Enters the global configuration mode.

     
    Step 3interface interface-id


    Example: 
    Device(config)# interface gigabitethernet1/0/2
     

    Specifies the port to be configured for trunking, and enters interface configuration mode.

     
    Step 4switchport mode {dynamic {auto | desirable} | trunk}


    Example: 
    Device(config-if)# switchport mode dynamic desirable
     

    Configures the interface as a Layer 2 trunk (required only if the interface is a Layer 2 access port or tunnel port or to specify the trunking mode).

    • dynamic auto—Sets the interface to a trunk link if the neighboring interface is set to trunk or desirable mode. This is the default.

    • dynamic desirable—Sets the interface to a trunk link if the neighboring interface is set to trunk, desirable, or auto mode.

    • trunk—Sets the interface in permanent trunking mode and negotiate to convert the link to a trunk link even if the neighboring interface is not a trunk interface.

     
    Step 5switchport access vlan vlan-id


    Example: 
    Device(config-if)# switchport access vlan 200
     

    (Optional) Specifies the default VLAN, which is used if the interface stops trunking.

     
    Step 6switchport trunk native vlan vlan-id


    Example: 
    Device(config-if)# switchport trunk native vlan 200
     

    Specifies the native VLAN for IEEE 802.1Q trunks.

     
    Step 7end


    Example: 
    
Device(config)# end
     

    Returns to privileged EXEC mode.

     
    Step 8show interfaces interface-id switchport


    Example: 
    Device# show interfaces gigabitethernet1/0/2 switchport
     

    Displays the switch port configuration of the interface in the Administrative Mode and the Administrative Trunking Encapsulation fields of the display.

     
    Step 9show interfaces interface-id trunk


    Example: 
    Device# show interfaces gigabitethernet1/0/2 trunk
     

    Displays the trunk configuration of the interface.

     
    Step 10copy running-config startup-config


    Example: 
    Device# copy running-config startup-config
     

    (Optional) Saves your entries in the configuration file.

     
    Related Concepts
    Trunking Modes
    Related References
    Layer 2 Interface Modes

    Defining the Allowed VLANs on a Trunk (CLI)

    VLAN 1 is the default VLAN on all trunk ports in all Cisco devices, and it has previously been a requirement that VLAN 1 always be enabled on every trunk link. You can use the VLAN 1 minimization feature to disable VLAN 1 on any individual VLAN trunk link so that no user traffic (including spanning-tree advertisements) is sent or received on VLAN 1.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    interface interface-id

      4.    switchport mode trunk

      5.    switchport trunk allowed vlan { word | add | all | except | none | remove} vlan-list

      6.    end

      7.    show interfaces interface-id switchport

      8.    copy running-config startup-config


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example: 
      Device> enable
       

      Enables privileged EXEC mode. Enter your password if prompted.

       

      Step 2configure terminal


      Example: 
      
Device# configure terminal
       

      Enters the global configuration mode.

       
      Step 3interface interface-id


      Example: 
      Device(config)# interface gigabitethernet1/0/1
       

      Specifies the port to be configured, and enters interface configuration mode.

       
      Step 4switchport mode trunk


      Example: 
      Device(config-if)# switchport mode trunk
       

      Configures the interface as a VLAN trunk port.

       
      Step 5switchport trunk allowed vlan { word | add | all | except | none | remove} vlan-list


      Example: 
      Device(config-if)# switchport trunk allowed vlan remove 2
       

      (Optional) Configures the list of VLANs allowed on the trunk.

      The vlan-list parameter is either a single VLAN number from 1 to 4094 or a range of VLANs described by two VLAN numbers, the lower one first, separated by a hyphen. Do not enter any spaces between comma-separated VLAN parameters or in hyphen-specified ranges.

      All VLANs are allowed by default.

       
      Step 6end


      Example: 
      
Device(config)# end
       

      Returns to privileged EXEC mode.

       
      Step 7show interfaces interface-id switchport


      Example: 
      Device# show interfaces gigabitethernet1/0/1 switchport
       

      Verifies your entries in the Trunking VLANs Enabled field of the display.

       
      Step 8copy running-config startup-config


      Example: 
      Device# copy running-config startup-config
       

      (Optional) Saves your entries in the configuration file.

       
      Related Concepts
      Allowed VLANs on a Trunk

      Changing the Pruning-Eligible List (CLI)

      The pruning-eligible list applies only to trunk ports. Each trunk port has its own eligibility list. VTP pruning must be enabled for this procedure to take effect.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    interface interface-id

        4.    switchport trunk pruning vlan {add | except | none | remove} vlan-list [,vlan [,vlan [,,,]]

        5.    end

        6.    show interfaces interface-id switchport

        7.    copy running-config startup-config


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example: 
        Device> enable
         

        Enables privileged EXEC mode. Enter your password if prompted.

         

        Step 2configure terminal


        Example: 
        
Device# configure terminal
         

        Enters the global configuration mode.

         
        Step 3interface interface-id


        Example: 
        Device(config)# interface gigabitethernet2/0/1
         

        Selects the trunk port for which VLANs should be pruned, and enters interface configuration mode.

         
        Step 4switchport trunk pruning vlan {add | except | none | remove} vlan-list [,vlan [,vlan [,,,]]
         

        Configures the list of VLANs allowed to be pruned from the trunk.

        For explanations about using the add, except, none, and remove keywords, see the command reference for this release.

        Separate non-consecutive VLAN IDs with a comma and no spaces; use a hyphen to designate a range of IDs. Valid IDs are 2 to 1001. Extended-range VLANs (VLAN IDs 1006 to 4094) cannot be pruned.

        VLANs that are pruning-ineligible receive flooded traffic.

        The default list of VLANs allowed to be pruned contains VLANs 2 to 1001.

         
        Step 5end


        Example: 
        
Device(config)# end
         

        Returns to privileged EXEC mode.

         
        Step 6show interfaces interface-id switchport


        Example: 
        Device# show interfaces gigabitethernet2/0/1 switchport
         

        Verifies your entries in the Pruning VLANs Enabled field of the display.

         
        Step 7copy running-config startup-config


        Example: 
        Device# copy running-config startup-config
         

        (Optional) Saves your entries in the configuration file.

         

        Configuring the Native VLAN for Untagged Traffic (CLI)

        A trunk port configured with IEEE 802.1Q tagging can receive both tagged and untagged traffic. By default, the device forwards untagged traffic in the native VLAN configured for the port. The native VLAN is VLAN 1 by default.

        The native VLAN can be assigned any VLAN ID.

        If a packet has a VLAN ID that is the same as the outgoing port native VLAN ID, the packet is sent untagged; otherwise, the device sends the packet with a tag.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    interface interface-id

          4.    switchport trunk native vlan vlan-id

          5.    end

          6.    show interfaces interface-id switchport

          7.    copy running-config startup-config


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example: 
          Device> enable
           

          Enables privileged EXEC mode. Enter your password if prompted.

           

          Step 2configure terminal


          Example: 
          
Device# configure terminal
           

          Enters the global configuration mode.

           
          Step 3interface interface-id


          Example: 
          Device(config)# interface gigabitethernet1/0/2
           

          Defines the interface that is configured as the IEEE 802.1Q trunk, and enters interface configuration mode.

           
          Step 4switchport trunk native vlan vlan-id


          Example: 
          Device(config-if)# switchport trunk native vlan 12
           

          Configures the VLAN that is sending and receiving untagged traffic on the trunk port.

          For vlan-id, the range is 1 to 4094.

           
          Step 5end


          Example: 
          Device(config-if)# end
           

          Returns to privileged EXEC mode.

           
          Step 6show interfaces interface-id switchport


          Example: 
          Device# show interfaces gigabitethernet1/0/2 switchport
           

          Verifies your entries in the Trunking Native Mode VLAN field.

           
          Step 7copy running-config startup-config


          Example: 
          Device# copy running-config startup-config
           

          (Optional) Saves your entries in the configuration file.

           

          Configuring Trunk Ports for Load Sharing

          Configuring Load Sharing Using STP Port Priorities (CLI)

          If your device is a member of a device stack, you must use the spanning-tree [vlan vlan-id] cost cost interface configuration command instead of the spanning-tree [vlan vlan-id] port-priority priority interface configuration command to select an interface to put in the forwarding state. Assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last.

          These steps describe how to configure a network with load sharing using STP port priorities.

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    vtp domain domain-name

            4.    vtp mode server

            5.    end

            6.    show vtp status

            7.    show vlan

            8.    configure terminal

            9.    interface interface-id

            10.    switchport mode trunk

            11.    end

            12.    show interfaces interface-id switchport

            13.    Repeat the above steps on Device A for a second port in the device or device stack.

            14.    Repeat the above steps on Device B to configure the trunk ports that connect to the trunk ports configured on Device A.

            15.    show vlan

            16.    configure terminal

            17.    interface interface-id

            18.    spanning-tree vlan vlan-range port-priority priority-value

            19.    exit

            20.    interface interface-id

            21.    spanning-tree vlan vlan-range port-priority priority-value

            22.    end

            23.    show running-config

            24.    copy running-config startup-config


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 enable


            Example: 
            Device> enable
             

            Enables privileged EXEC mode. Enter your password if prompted.

             

            Step 2configure terminal


            Example: 
            Device# configure terminal
             

            Enters global configuration mode on Device A.

             
            Step 3vtp domain domain-name


            Example: 
            Device(config)# vtp domain workdomain
             

            Configures a VTP administrative domain.

            The domain name can be 1 to 32 characters.

             
            Step 4vtp mode server


            Example: 
            Device(config)# vtp mode server
             

            Configures Device A as the VTP server.

             
            Step 5end


            Example: 
            
Device(config)# end
             

            Returns to privileged EXEC mode.

             
            Step 6show vtp status


            Example: 
            Device# show vtp status
             

            Verifies the VTP configuration on both Device A and Device B.

            In the display, check the VTP Operating Mode and the VTP Domain Name fields.

             
            Step 7show vlan


            Example: 
            Device# show vlan
             

            Verifies that the VLANs exist in the database on Device A.

             
            Step 8configure terminal


            Example: 
            Device# configure terminal
             

            Enters global configuration mode.

             
            Step 9interface interface-id


            Example: 
            Device(config)# interface gigabitethernet1/0/1
             

            Defines the interface to be configured as a trunk, and enters interface configuration mode.

             
            Step 10switchport mode trunk


            Example: 
            Device(config-if)# switchport mode trunk
             

            Configures the port as a trunk port.

             
            Step 11end


            Example: 
            Device(config-if)# end
             

            Returns to privileged EXEC mode.

             
            Step 12show interfaces interface-id switchport


            Example: 
            Device# show interfaces gigabitethernet1/0/1 switchport
             

            Verifies the VLAN configuration.

             
            Step 13Repeat the above steps on Device A for a second port in the device or device stack.    
            Step 14Repeat the above steps on Device B to configure the trunk ports that connect to the trunk ports configured on Device A.    
            Step 15show vlan


            Example: 
            Device# show vlan
             

            When the trunk links come up, VTP passes the VTP and VLAN information to Device B. This command verifies that Device B has learned the VLAN configuration.

             
            Step 16configure terminal


            Example: 
            Device# configure terminal
             

            Enters global configuration mode on Device A.

             
            Step 17interface interface-id


            Example: 
            Device(config)# interface gigabitethernet1/0/1
             

            Defines the interface to set the STP port priority, and enters interface configuration mode.

             
            Step 18spanning-tree vlan vlan-range port-priority priority-value


            Example: 
            Device(config-if)# spanning-tree vlan 8-10 port-priority 16
             

            Assigns the port priority for the VLAN range specified. Enter a port priority value from 0 to 240. Port priority values increment by 16.

             
            Step 19exit


            Example: 
            Device(config-if)# exit
             

            Returns to global configuration mode.

             
            Step 20interface interface-id


            Example: 
            Device(config)# interface gigabitethernet1/0/2
             

            Defines the interface to set the STP port priority, and enters interface configuration mode.

             
            Step 21spanning-tree vlan vlan-range port-priority priority-value


            Example: 
            Device(config-if)# spanning-tree vlan 3-6 port-priority 16
             

            Assigns the port priority for the VLAN range specified. Enter a port priority value from 0 to 240. Port priority values increment by 16.

             
            Step 22end


            Example: 
            Device(config-if)# end
             

            Returns to privileged EXEC mode.

             
            Step 23show running-config


            Example: 
            Device# show running-config
             

            Verifies your entries.

             
            Step 24copy running-config startup-config


            Example: 
            Device# copy running-config startup-config
             

            (Optional) Saves your entries in the configuration file.

             
            Related Concepts
            Network Load Sharing Using STP Priorities

            Configuring Load Sharing Using STP Path Cost (CLI)

            These steps describe how to configure a network with load sharing using STP path costs.

            SUMMARY STEPS

              1.    enable

              2.    configure terminal

              3.    interface interface-id

              4.    switchport mode trunk

              5.    exit

              6.    Repeat Steps 2 through 4 on a second interface in Device A or in Device A stack.

              7.    end

              8.    show running-config

              9.    show vlan

              10.    configure terminal

              11.    interface interface-id

              12.    spanning-tree vlan vlan-range cost cost-value

              13.    end

              14.    Repeat Steps 9 through 13 on the other configured trunk interface on Device A, and set the spanning-tree path cost to 30 for VLANs 8, 9, and 10.

              15.    exit

              16.    show running-config

              17.    copy running-config startup-config


            DETAILED STEPS
               Command or ActionPurpose
              Step 1 enable


              Example: 
              Device> enable
               

              Enables privileged EXEC mode. Enter your password if prompted.

               

              Step 2configure terminal


              Example: 
              Device# configure terminal
               

              Enters global configuration mode on Device A.

               
              Step 3interface interface-id


              Example: 
              Device(config)# interface gigabitethernet1/0/1
               

              Defines the interface to be configured as a trunk, and enters interface configuration mode.

               
              Step 4switchport mode trunk


              Example: 
              Device(config-if)# switchport mode trunk
               

              Configures the port as a trunk port.

               
              Step 5exit


              Example: 
              Device(config-if)# exit
               

              Returns to global configuration mode.

               
              Step 6Repeat Steps 2 through 4 on a second interface in Device A or in Device A stack.    
              Step 7end


              Example: 
              
Device(config)# end
               

              Returns to privileged EXEC mode.

               
              Step 8show running-config


              Example: 
              Device# show running-config
               

              Verifies your entries. In the display, make sure that the interfaces are configured as trunk ports.

               
              Step 9show vlan


              Example: 
              Device# show vlan
               

              When the trunk links come up, Device A receives the VTP information from the other devices. This command verifies that Device A has learned the VLAN configuration.

               
              Step 10configure terminal


              Example: 
              Device# configure terminal
               

              Enters global configuration mode.

               
              Step 11interface interface-id


              Example: 
              Device(config)# interface gigabitethernet1/0/1
               

              Defines the interface on which to set the STP cost, and enters interface configuration mode.

               
              Step 12spanning-tree vlan vlan-range cost cost-value


              Example: 
              Device(config-if)# spanning-tree vlan 2-4 cost 30
               

              Sets the spanning-tree path cost to 30 for VLANs 2 through 4.

               
              Step 13end


              Example: 
              Device(config-if)# end
               

              Returns to global configuration mode.

               
              Step 14Repeat Steps 9 through 13 on the other configured trunk interface on Device A, and set the spanning-tree path cost to 30 for VLANs 8, 9, and 10.    
              Step 15exit


              Example: 
              Device(config)# exit
               

              Returns to privileged EXEC mode.

               
              Step 16show running-config


              Example: 
              Device# show running-config
               

              Verifies your entries. In the display, verify that the path costs are set correctly for both trunk interfaces.

               
              Step 17copy running-config startup-config


              Example: 
              Device# copy running-config startup-config
               

              (Optional) Saves your entries in the configuration file.

               
              Related Concepts
              Network Load Sharing Using STP Path Cost

              Where to Go Next

              After configuring VLAN trunks, you can configure the following:

              • VLANs

              • VLAN groups

              • Voice VLANs

              Additional References

              Related Documents

              Related Topic Document Title

              For complete syntax and usage information for the commands used in this chapter.

              VLAN Command Reference (Catalyst 3850 Switches)

              Layer 2/3 Command Reference (Catalyst 3850 Switches)

              Spanning Tree Protocol (STP)

              Network Management Command Reference (Catalyst 3850 Switches)

              Network Management Configuration Guide (Catalyst 3850 Switches)

              Error Message Decoder

              Description Link

              To help you research and resolve system error messages in this release, use the Error Message Decoder tool.

              https:/​/​www.cisco.com/​cgi-bin/​Support/​Errordecoder/​index.cgi

              Standards and RFCs

              Standard/RFC Title

              RFC 1573

              Evolution of the Interfaces Group of MIB-II

              RFC 1757

              Remote Network Monitoring Management

              RFC 2021

              SNMPv2 Management Information Base for the Transmission Control Protocol using SMIv2

              MIBs

              MIB MIBs Link

              All supported MIBs for this release.

              To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

              http:/​/​www.cisco.com/​go/​mibs

              Technical Assistance

              Description Link

              The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

              To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

              Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

              http:/​/​www.cisco.com/​support

              Feature History and Information for VLAN Trunks

              Release

              Modification