Configuring Access and Trunk Interfaces

Information About Access and Trunk Interfaces

Understanding Access and Trunk Interfaces

Ethernet interfaces can be configured either as access ports or a trunk ports, as follows:

  • An access port can have only one VLAN configured on the interface; it can carry traffic for only one VLAN.

  • A trunk port can have two or more VLANs configured on the interface; it can carry traffic for several VLANs simultaneously.


Note

Cisco NX-OS supports only IEEE 802.1Q-type VLAN trunk encapsulation.

The following figure shows how you can use trunk ports in the network. The trunk port carries traffic for two or more VLANs.

Figure 1. Devices in a Trunking Environment

In order to correctly deliver the traffic on a trunk port with several VLANs, the device uses the IEEE 802.1Q encapsulation or tagging method.

To optimize the performance on access ports, you can configure the port as a host port. Once the port is configured as a host port, it is automatically set as an access port, and channel grouping is disabled. Use the host designation to decrease the time it takes the designated port to begin to forward packets.


Note

Only an end station can be set as a host port; you will receive an error message if you attempt to configure other ports as hosts.

If an access port receives a packet with an 802.1Q tag in the header other than the access VLAN value, that port drops the packet without learning its MAC source address.


Note

An Ethernet interface can function as either an access port or a trunk port; it cannot function as both port types simultaneously.

Understanding IEEE 802.1Q Encapsulation

A trunk is a point-to-point link between the device and another networking device. Trunks carry the traffic of multiple VLANs over a single link and allow you to extend VLANs across an entire network.

To correctly deliver the traffic on a trunk port with several VLANs, the device uses the IEEE 802.1Q encapsulation (tagging) method. This tag carries information about the specific VLAN to which the frame and packet belong. This method allows packets that are encapsulated for several different VLANs to traverse the same port and maintain traffic separation between the VLANs. The encapsulated VLAN tag also allows the trunk to move traffic end-to-end through the network on the same VLAN.

Figure 2. Header Without and With 802.1Q Tag Included

Understanding Access VLANs

When you configure a port in access mode, you can specify which VLAN will carry the traffic for that interface. If you do not configure the VLAN for a port in access mode, or an access port, the interface carries traffic for the default VLAN (VLAN1).

You can change the access port membership in a VLAN by specifying the new VLAN. You must create the VLAN before you can assign it as an access VLAN for an access port. If you change the access VLAN on an access port to a VLAN that is not yet created, the system will shut that access port down.


Note

If you change the VLAN on an access port or a trunk port it will flap the interface. However, if the port is part of a vPC, then first change the native VLAN on the secondary vPC, and then to primary vPC.

If an access port receives a packet with an 802.1Q tag in the header other than the access VLAN value, that port drops the packet without learning its MAC source address.

Understanding the Native VLAN ID for Trunk Ports

A trunk port can carry untagged packets simultaneously with the 802.1Q tagged packets. When you assign a default port VLAN ID to the trunk port, all untagged traffic travels on the default port VLAN ID for the trunk port, and all untagged traffic is assumed to belong to this VLAN. This VLAN is referred to as the native VLAN ID for a trunk port. The native VLAN ID is the VLAN that carries untagged traffic on trunk ports.

The trunk port sends an egressing packet with a VLAN that is equal to the default port VLAN ID as untagged; all the other egressing packets are tagged by the trunk port. If you do not configure a native VLAN ID, the trunk port uses the default VLAN.


Note

Native VLAN ID numbers must match on both ends of the trunk.

Understanding Allowed VLANs

By default, a trunk port sends traffic to and receives traffic from all VLANs. All VLAN IDs are allowed on each trunk. However, you can remove VLANs from this inclusive list to prevent traffic from the specified VLANs from passing over the trunk. You can add any specific VLANs later that you may want the trunk to carry traffic for back to the list.

To partition the Spanning Tree Protocol (STP) topology for the default VLAN, you can remove VLAN1 from the list of allowed VLANs. Otherwise, VLAN1, which is enabled on all ports by default, will have a very big STP topology, which can result in problems during STP convergence. When you remove VLAN1, all data traffic for VLAN1 on this port is blocked, but the control traffic continues to move on the port.

Understanding Native 802.1Q VLANs

To provide additional security for traffic passing through an 802.1Q trunk port, the vlan dot1q tag native command was introduced. This feature provides a means to ensure that all packets going out of a 802.1Q trunk port are tagged and to prevent reception of untagged packets on the 802.1Q trunk port.

Without this feature, all tagged ingress frames received on a 802.1Q trunk port are accepted as long as they fall inside the allowed VLAN list and their tags are preserved. Untagged frames are tagged with the native VLAN ID of the trunk port before further processing. Only those egress frames whose VLAN tags are inside the allowed range for that 802.1Q trunk port are received. If the VLAN tag on a frame happens to match that of the native VLAN on the trunk port, the tag is stripped off and the frame is sent untagged.

This behavior could potentially be exploited to introduce "VLAN hopping" in which a hacker could try and have a frame jump to a different VLAN. It is also possible for traffic to become part of the native VLAN by sending untagged packets into an 802.1Q trunk port.

To address the above issues, the vlan dot1q tag native command performs the following functions:

  • On the ingress side, all untagged data traffic is dropped.

  • On the egress side, all traffic is tagged. If traffic belongs to native VLAN it is tagged with the native VLAN ID.

This feature is supported on all the directly connected Ethernet and Port Channel interfaces.


Note

You can enable the vlan dot1q tag native command by entering the command in the global configuration mode.

Configuring Access and Trunk Interfaces

Configuring a LAN Interface as an Ethernet Access Port

You can configure an Ethernet interface as an access port. An access port transmits packets on only one, untagged VLAN. You specify which VLAN traffic that the interface carries. If you do not specify a VLAN for an access port, the interface carries traffic only on the default VLAN. The default VLAN is VLAN1.

The VLAN must exist before you can specify that VLAN as an access VLAN. The system shuts down an access port that is assigned to an access VLAN that does not exist.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface {{type slot/port} | {port-channel number}}
  3. switch(config-if)# switchport mode {access | trunk}
  4. switch(config-if)# switchport access vlan vlan-id

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface {{type slot/port} | {port-channel number}}

Specifies an interface to configure, and enters interface configuration mode.

Step 3

switch(config-if)# switchport mode {access | trunk}

Sets the interface as a nontrunking nontagged single-VLAN Ethernet interface. An access port can carry traffic in one VLAN only. By default, an access port carries traffic for VLAN1; to set the access port to carry traffic for a different VLAN, use the switchport access vlan command.

Step 4

switch(config-if)# switchport access vlan vlan-id

Specifies the VLAN for which this access port will carry traffic. If you do not enter this command, the access port carries traffic on VLAN1 only; use this command to change the VLAN for which the access port carries traffic.

Example

This example shows how to set an interface as an Ethernet access port that carries traffic for a specific VLAN only: 

switch# configure terminal
switch(config)# interface ethernet 1/10
switch(config-if)# switchport mode access
switch(config-if)# switchport access vlan 5

Configuring Access Host Ports

By using a switchport host, you can make an access port a spanning-tree edge port, and enable BPDU Filtering and BPDU Guard at the same time.

Before you begin

Ensure that you are configuring the correct interface; it must be an interface that is connected to an end station.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface type slot/port
  3. switch(config-if)# switchport host

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface type slot/port

Specifies an interface to configure, and enters interface configuration mode.

Step 3

switch(config-if)# switchport host

Sets the interface to spanning-tree port type edge, turns on BPDU Filtering and BPDU Guard.

Note

 

Apply this command only to switchports that connect to hosts.

Example

This example shows how to set an interface as an Ethernet access host port with EtherChannel disabled: 

switch# configure terminal
switch(config)# interface ethernet 1/10
switch(config-if)# switchport host

Configuring Trunk Ports

You can configure an Ethernet port as a trunk port; a trunk port transmits untagged packets for the native VLAN plus encapsulated, tagged, packets for multiple VLANs.


Note

Cisco NX-OS supports only 802.1Q encapsulation.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface {type slot/port | port-channel number}
  3. switch(config-if)# switchport mode {access | trunk}

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface {type slot/port | port-channel number}

Specifies an interface to configure, and enters interface configuration mode.

Step 3

switch(config-if)# switchport mode {access | trunk}

Sets the interface as an Ethernet trunk port. A trunk port can carry traffic in one or more VLANs on the same physical link (VLANs are based on the trunk-allowed VLANs list). By default, a trunk interface can carry traffic for all VLANs. To specify that only certain VLANs are allowed on the specified trunk, use the switchport trunk allowed vlan command.

Example

This example shows how to set an interface as an Ethernet trunk port: 

switch# configure terminal
switch(config)# interface ethernet 1/3
switch(config-if)# switchport mode trunk

Configuring the Native VLAN for 802.1Q Trunking Ports

If you do not configure this parameter, the trunk port uses the default VLAN as the native VLAN ID.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface {type slot/port | port-channel number}
  3. switch(config-if)# switchport trunk native vlan vlan-id

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface {type slot/port | port-channel number}

Specifies an interface to configure, and enters interface configuration mode.

Step 3

switch(config-if)# switchport trunk native vlan vlan-id

Sets the native VLAN for the 802.1Q trunk. Valid values are from 1 to 4094, except those VLANs reserved for internal use. The default value is VLAN1.

Example

This example shows how to set the native VLAN for an Ethernet trunk port: 

switch# configure terminal

switch(config)# interface ethernet 1/3

switch(config-if)# switchport trunk native vlan 5

Configuring the Allowed VLANs for Trunking Ports

You can specify the IDs for the VLANs that are allowed on the specific trunk port.

Before you configure the allowed VLANs for the specified trunk ports, ensure that you are configuring the correct interfaces and that the interfaces are trunks.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# interface {type slot/port | port-channel number}
  3. switch(config-if)# switchport trunk allowed vlan {vlan-list all | none [add |except | none | remove {vlan-list}]}

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# interface {type slot/port | port-channel number}

Specifies an interface to configure, and enters interface configuration mode.

Step 3

switch(config-if)# switchport trunk allowed vlan {vlan-list all | none [add |except | none | remove {vlan-list}]}

Sets allowed VLANs for the trunk interface. The default is to allow all VLANs on the trunk interface: 1 to 3967 and 4048 to 4094. VLANs 3968 to 4047 are the default VLANs reserved for internal use by default; this group of VLANs is configurable. By default, all VLANs are allowed on all trunk interfaces.

Note

 

You cannot add internally allocated VLANs as allowed VLANs on trunk ports. The system returns a message if you attempt to list an internally allocated VLAN as an allowed VLAN.

Example

This example shows how to add VLANs to the list of allowed VLANs on an Ethernet trunk port: 

switch# configure terminal

switch(config)# interface ethernet 1/3

switch(config-if)# switchport trunk allow vlan 15-20

Configuring Native 802.1Q VLANs

Typically, you configure 802.1Q trunks with a native VLAN ID, which strips tagging from all packets on that VLAN. This configuration allows all untagged traffic and control traffic to transit the Cisco Nexus device. Packets that enter the switch with 802.1Q tags that match the native VLAN ID value are similarly stripped of tagging.

To maintain the tagging on the native VLAN and drop untagged traffic, enter the vlan dot1q tag native command. The switch will tag the traffic received on the native VLAN and admit only 802.1Q-tagged frames, dropping any untagged traffic, including untagged traffic in the native VLAN.

Control traffic continues to be accepted untagged on the native VLAN on a trunked port, even when the vlan dot1q tag native command is enabled.


Note

The vlan dot1q tag native command is enabled on global basis.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# vlan dot1q tag native [tx-only]
  3. (Optional) switch(config)# no vlan dot1q tag native [tx-only]
  4. (Optional) switch# show vlan dot1q tag native

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# vlan dot1q tag native [tx-only]

Enables dot1q (IEEE 802.1Q) tagging for all native VLANs on all trunked ports on the Cisco Nexus device. By default, this feature is disabled.

Step 3

(Optional) switch(config)# no vlan dot1q tag native [tx-only]

(Optional)

Disables dot1q (IEEE 802.1Q) tagging for all native VLANs on all trunked ports on the switch.

Step 4

(Optional) switch# show vlan dot1q tag native

(Optional)

Displays the status of tagging on the native VLANs.

Example

This example shows how to enable 802.1Q tagging on the switch: 


switch# configure terminal
switch(config)# vlan dot1q tag native
switch(config)# exit
switch# show vlan dot1q tag native
vlan dot1q native tag is enabled

Verifying the Interface Configuration

Use the following commands to display access and trunk interface configuration information.

Command

Purpose

switch# show interface

Displays the interface configuration

switch# show interface switchport

Displays information for all Ethernet interfaces, including access and trunk interfaces.

switch# show interface brief

Displays interface configuration information.