Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
In a network of Cisco switches connected through IEEE 802.1Q trunks, the switches 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 switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco switch combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of the non-Cisco IEEE 802.1Q switch. However, spanning-tree information for each VLAN is maintained by Cisco switches separated by a cloud of non-Cisco IEEE 802.1Q switches. The non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches.
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
Dynamic Trunking Protocol (DTP) is not supported on private-VLAN ports or tunnel ports.
The switch does not support Layer 3 trunks; you cannot configure subinterfaces or use
the encapsulation keyword on Layer 3 interfaces. The switch
does support Layer 2 trunks and Layer 3 VLAN interfaces, which provide equivalent
capabilities.
Information About VLAN Trunks
Trunking Overview
A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device such as a router or a switch. 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:
Inter-Switch Link (ISL)—Cisco-proprietary trunking encapsulation.
Figure 1. Switches in an ISL Trunking Environment. You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle.
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.
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.
switchport mode dot1q-tunnel
Configures the interface as a tunnel (nontrunking) port to be connected in an asymmetric link with an IEEE 802.1Q trunk port. The IEEE 802.1Q tunneling is used to maintain customer VLAN integrity across a service provider network.
This table lists the Ethernet trunk encapsulation types and keywords.
Table 2 Ethernet Trunk Encapsulation Types and Keywords
Encapsulation
Function
switchport trunk encapsulation isl
Specifies ISL encapsulation on the trunk link.
switchport trunk encapsulation dot1q
Specifies IEEE 802.1Q encapsulation on the trunk link.
switchport trunk encapsulation negotiate
Specifies that the interface negotiate with the neighboring interface to become an ISL (preferred) or IEEE 802.1Q trunk, depending on the configuration and capabilities of the neighboring interface. This is the default for the switch.
This table lists the Ethernet trunk encapsulation types and keywords.
Table 3 Ethernet Trunk Encapsulation Types and Keywords
Encapsulation
Function
switchport trunk encapsulation dot1q
Specifies IEEE 802.1Q encapsulation on the trunk link.
switchport trunk encapsulation negotiate
Specifies that the interface negotiate with the neighboring interface to become an ISL (preferred) or IEEE 802.1Q trunk, depending on the configuration and capabilities of the neighboring interface. This is the default for the switch.
The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected interfaces decide whether a link becomes an ISL or IEEE 802.1Q trunk.
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.
Load sharing divides the bandwidth supplied by parallel trunks connecting switches. To avoid loops, STP normally blocks all but one parallel link between switches. 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 switch. For load sharing using STP path costs, each load-sharing link can be connected to the same switch or to two different switches.
When two ports on the same switch form a loop, the switch 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.
Figure 2. Load Sharing by Using STP Port Priorities.
This figure shows two trunks connecting supported switches.
VLANs 8 through 10 are assigned a port priority of 16 on Trunk 1.
VLANs 3 through 6 retain the default port priority of 128 on Trunk 1.
VLANs 3 through 6 are assigned a port priority of 16 on Trunk 2.
VLANs 8 through 10 retain the default port priority of 128 on Trunk 2.
Trunk 1 carries traffic for VLANs 8 through 10, and Trunk 2 carries traffic for VLANs 3 through 6. If the active trunk fails, the trunk with the lower priority takes over and carries the traffic for all of the VLANs. No duplication of traffic occurs over any trunk port.
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.
Figure 3. Load-Sharing Trunks with Traffic Distributed by Path Cost.
Trunk ports 1 and 2 are configured as 100BASE-T ports. These VLAN path costs are assigned:
VLANs 2 through 4 are assigned a path cost of 30 on Trunk port 1.
VLANs 8 through 10 retain the default 100BASE-T path cost on Trunk port 1 of 19.
VLANs 8 through 10 are assigned a path cost of 30 on Trunk port 2.
VLANs 2 through 4 retain the default 100BASE-T path cost on Trunk port 2 of 19.
Trunking interacts with other features in these ways:
A trunk port cannot be a secure port.
A trunk port cannot be a tunnel 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 switch 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.
Because trunk ports send and receive VTP advertisements, to use VTP you must ensure that at least one trunk port is configured on the switch and that this trunk port is connected to the trunk port of a second switch. Otherwise, the switch 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.
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 4
switchport access vlanvlan-id
Example:
Switch(config-if)# switchport access vlan 200
(Optional) Specifies the default VLAN, which is used if the interface stops trunking.
Switch# 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 8
show interfacesinterface-idtrunk
Example:
Switch# show interfaces gigabitethernet1/0/2 trunk
Displays the trunk configuration of the interface.
Step 9
copy running-config startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries in the configuration file.
VLAN 1 is the default VLAN on all trunk ports in all Cisco switches, 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.
(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 5
end
Example:
Switch(config)# end
Returns to privileged EXEC mode.
Step 6
show interfacesinterface-idswitchport
Example:
Switch# show interfaces gigabitethernet1/0/1
Verifies your entries in the Trunking VLANs Enabled field of the display.
Step 7
copy running-config startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries in the configuration file.
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.
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 4
end
Example:
Switch(config)# end
Returns to privileged EXEC mode.
Step 5
show interfacesinterface-idswitchport
Example:
Switch# show interfaces gigabitethernet2/0/1 switchport
Verifies your entries in the Pruning VLANs Enabled field of the display.
Step 6
copy running-config startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries in the configuration file.
Configuring the Native VLAN for Untagged Traffic
A trunk port configured with IEEE 802.1Q tagging can receive both tagged and untagged traffic. By default, the switch 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 switch sends the packet with a tag.
SUMMARY STEPS
1.configureterminal
2.interfaceinterface-id
3.switchport trunk native vlanvlan-id
4.end
5.show interfacesinterface-idswitchport
6.copy running-config startup-config
DETAILED STEPS
Command or Action
Purpose
Step 1
configureterminal
Example:
Switch# configure terminal
Enters the global configuration mode.
Step 2
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/2
Defines the interface that is configured as the IEEE 802.1Q trunk, and enters interface configuration mode.
Configures the VLAN that is sending and receiving untagged traffic on the trunk port.
For vlan-id, the range is 1 to 4094.
Step 4
end
Example:
Switch(config-if)# end
Returns to privileged EXEC mode.
Step 5
show interfacesinterface-idswitchport
Example:
Switch# show interfaces gigabitethernet1/0/2 switchport
Verifies your entries in the Trunking Native Mode VLAN field.
Step 6
copy running-config startup-config
Example:
Switch# 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
If your switch is a member of a switch stack, you must use the
spanning-tree [vlanvlan-id] costcost interface configuration command instead of the
spanning-tree [vlanvlan-id] port-prioritypriority 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.configure terminal
2.vtp domaindomain-name
3.vtp mode server
4.end
5.show vtp status
6.show vlan
7.configure terminal
8.interfaceinterface-id
9.switchport mode trunk
10.end
11.show interfacesinterface-idswitchport
12.Repeat Steps 7 through 10 on Switch A for a second port in the switch or switch
stack.
13.Repeat Steps 7 through 10 on Switch B to configure the trunk ports that connect to
the trunk ports configured on Switch A.
Verifies the VTP configuration on both Switch A and Switch B.
In the display, check the VTP Operating Mode and the VTP Domain Name fields.
Step 6
show vlan
Example:
Switch# show vlan
Verifies that the VLANs exist in the database on Switch A.
Step 7
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 8
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/1
Defines the interface to be configured as a trunk, and enter interface configuration mode.
Step 9
switchport mode trunk
Example:
Switch(config-if)# switchport mode trunk
Configures the port as a trunk port.
Step 10
end
Example:
Switch(config-if)# end
Returns to privileged EXEC mode.
Step 11
show interfacesinterface-idswitchport
Example:
Switch# show interfaces gigabitethernet1/0/1
Verifies the VLAN configuration.
Step 12
Repeat Steps 7 through 10 on Switch A for a second port in the switch or switch
stack.
Step 13
Repeat Steps 7 through 10 on Switch B to configure the trunk ports that connect to
the trunk ports configured on Switch A.
Step 14
show vlan
Example:
Switch# show vlan
When the trunk links come up, VTP passes the VTP and VLAN information to Switch B. This command verifies that Switch B has learned the VLAN configuration.
Step 15
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode on Switch A.
Step 16
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/1
Defines the interface to set the STP port priority, and enter interface configuration mode.
These steps describe how to configure a network with load sharing using STP path costs.
SUMMARY STEPS
1.configure terminal
2.interfaceinterface-id
3.switchport mode trunk
4.exit
5.Repeat Steps 2 through 4 on a second interface in Switch A or in Switch A
stack.
6.end
7.show running-config
8.show vlan
9.configure terminal
10.interfaceinterface-id
11.spanning-tree vlanvlan-rangecostcost-value
12.end
13.Repeat Steps 9 through 13 on the other configured trunk interface on Switch A, and
set the spanning-tree path cost to 30 for VLANs 8, 9, and 10.
14.exit
15.show running-config
16.copy running-config startup-config
DETAILED STEPS
Command or Action
Purpose
Step 1
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode on Switch A.
Step 2
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/1
Defines the interface to be configured as a trunk, and enter interface configuration mode.
Step 3
switchport mode trunk
Example:
Switch(config-if)# switchport mode trunk
Configures the port as a trunk port.
Step 4
exit
Example:
Switch(config-if)# exit
Returns to global configuration mode.
Step 5
Repeat Steps 2 through 4 on a second interface in Switch A or in Switch A
stack.
Step 6
end
Example:
Switch(config)# end
Returns to privileged EXEC mode.
Step 7
show running-config
Example:
Switch# show running-config
Verifies your entries. In the display, make sure that the interfaces are configured as trunk ports.
Step 8
show vlan
Example:
Switch# show vlan
When the trunk links come up, Switch A receives the VTP information from the other switches. This command verifies that Switch A has learned the VLAN configuration.
Step 9
configure terminal
Example:
Switch# configure terminal
Enters global configuration mode.
Step 10
interfaceinterface-id
Example:
Switch(config)# interface gigabitethernet1/0/1
Defines the interface on which to set the STP cost, and enters interface configuration mode.
Step 11
spanning-tree vlanvlan-rangecostcost-value
Example:
Switch(config-if)# spanning-tree vlan 2-4 cost 30
Sets the spanning-tree path cost to 30 for VLANs 2 through 4.
Step 12
end
Example:
Switch(config-if)# end
Returns to global configuration mode.
Step 13
Repeat Steps 9 through 13 on the other configured trunk interface on Switch A, and
set the spanning-tree path cost to 30 for VLANs 8, 9, and 10.
Step 14
exit
Example:
Switch(config)# exit
Returns to privileged EXEC mode.
Step 15
show running-config
Example:
Switch# show running-config
Verifies your entries. In the display, verify that the path costs are set correctly for both trunk interfaces.
Step 16
copy running-config startup-config
Example:
Switch# copy running-config startup-config
(Optional) Saves your entries in the configuration file.
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.
A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking device such as a router or a switch. 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: