Catalyst 3550 Multilayer Switch Software Configuration Guide, 12.1(13)EA1 - Configuring VLANs [Cisco Catalyst 3550 Series Switches]

Table Of Contents

Configuring VLANs

Understanding VLANs

Supported VLANs

VLAN Port Membership Modes

Configuring Normal-Range VLANs

Token Ring VLANs

Normal-Range VLAN Configuration Guidelines

VLAN Configuration Mode Options

VLAN Configuration in config-vlan Mode

VLAN Configuration in VLAN Configuration Mode

Saving VLAN Configuration

Default Ethernet VLAN Configuration

Creating or Modifying an Ethernet VLAN

Deleting a VLAN

Assigning Static-Access Ports to a VLAN

Configuring Extended-Range VLANs

Default VLAN Configuration

Extended-Range VLAN Configuration Guidelines

Creating an Extended-Range VLAN

Creating an Extended-Range VLAN with an Internal VLAN ID

Displaying VLANs

Configuring VLAN Trunks

Trunking Overview

Encapsulation Types

802.1Q Configuration Considerations

Default Layer 2 Ethernet Interface VLAN Configuration

Configuring an Ethernet Interface as a Trunk Port

Interaction with Other Features

Configuring a Trunk Port

Defining the Allowed VLANs on a Trunk

Changing the Pruning-Eligible List

Configuring the Native VLAN for Untagged Traffic

Load Sharing Using STP

Load Sharing Using STP Port Priorities

Load Sharing Using STP Path Cost

Configuring VMPS

Understanding VMPS

Dynamic Port VLAN Membership

VMPS Database Configuration File

Default VMPS Configuration

VMPS Configuration Guidelines

Configuring the VMPS Client

Entering the IP Address of the VMPS

Configuring Dynamic Access Ports on VMPS Clients

Reconfirming VLAN Memberships

Changing the Reconfirmation Interval

Changing the Retry Count

Monitoring the VMPS

Troubleshooting Dynamic Port VLAN Membership

VMPS Configuration Example

Configuring VLANs

This chapter describes how to configure normal-range VLANs (VLAN IDs 1 to 1005) and extended-range VLANs (VLAN IDs 1006 to 4094) on your Catalyst 3550 switch. It includes information about VLAN modes and the VLAN Membership Policy Server (VMPS).

Note For complete syntax and usage information for the commands used in this chapter, refer to the command reference for this release.

The chapter includes these sections:

•Understanding VLANs

•Configuring Normal-Range VLANs

•Configuring Extended-Range VLANs

•Displaying VLANs

•Configuring VLAN Trunks

•Configuring VMPS

Understanding VLANs

A VLAN is a switched network that is logically segmented by function, project team, or application, without regard to the physical locations of the users. VLANs have the same attributes as physical LANs, but you can group end stations even if they are not physically located on the same LAN segment. Any switch port can belong to a VLAN, and unicast, broadcast, and multicast packets are forwarded and flooded only to end stations in the VLAN. Each VLAN is considered a logical network, and packets destined for stations that do not belong to the VLAN must be forwarded through a router or bridge as shown in Figure 11-1. Because a VLAN is considered a separate logical network, it contains its own bridge Management Information Base (MIB) information and can support its own implementation of spanning tree. See "Configuring STP" and "Configuring RSTP and MSTP."

Note Before you create VLANs, you must decide whether to use VLAN Trunking Protocol (VTP) to maintain global VLAN configuration for your network. For more information on VTP, see "Configuring VTP."

Figure 11-1 shows an example of VLANs segmented into logically defined networks.

Figure 11-1 VLANs as Logically Defined Networks

VLANs are often associated with IP subnetworks. For example, all the end stations in a particular IP subnet belong to the same VLAN. Interface VLAN membership on the switch is assigned manually on an interface-by-interface basis. When you assign switch interfaces to VLANs by using this method, it is known as interface-based, or static, VLAN membership.

Traffic between VLANs must be routed or fallback bridged. A Catalyst 3550 switch can route traffic between VLANs by using switch virtual interfaces (SVIs). An SVI must be explicitly configured and assigned an IP address to route traffic between VLANs. For more information, see the "Switch Virtual Interfaces" section and the "Configuring Layer 3 Interfaces" section.

Supported VLANs

The Catalyst 3550 switch supports 1005 VLANs in VTP client, server, and transparent modes. VLANs are identified with a number from 1 to 4094. VLAN IDs 1002 through 1005 are reserved for Token Ring and FDDI VLANs. VTP only learns normal-range VLANs, with VLAN IDs 1 to 1005; VLAN IDs greater than 1005 are extended-range VLANs and are not stored in the VLAN database. The switch must be in VTP transparent mode when you create VLAN IDs from 1006 to 4094.

The switch supports per-VLAN spanning tree (PVST) and per-VLAN rapid spanning tree (PVRST) with a maximum of 128 spanning-tree instances. One spanning-tree instance is allowed per VLAN. See the "Normal-Range VLAN Configuration Guidelines" section for more information about the number of spanning-tree instances and the number of VLANs. The switch supports both Inter-Switch Link (ISL) and IEEE 802.1Q trunking methods for sending VLAN traffic over Ethernet ports.

VLAN Port Membership Modes

You configure a port to belong to a VLAN by assigning a membership mode that determines the kind of traffic the port carries and the number of VLANs to which it can belong. Table 11-1 lists the membership modes and membership and VTP characteristics.

Table 11-1 Port Membership Modes

Membership Mode

VLAN Membership Characteristics

VTP Characteristics

Static-access

A static-access port can belong to one VLAN and is manually assigned to that VLAN. For more information, see the "Assigning Static-Access Ports to a VLAN" section.

VTP is not required. If you do not want VTP to globally propagate information, set the VTP mode to transparent to disable VTP. To participate in VTP, there must be at least one trunk port on the switch connected to a trunk port of a second switch.

Trunk (ISL or
IEEE 802.1Q)

A trunk port is a member of all VLANs by default, including extended-range VLANs, but membership can be limited by configuring the allowed-VLAN list. You can also modify the pruning-eligible list to block flooded traffic to VLANs on trunk ports that are included in the list. For information about configuring trunk ports, see the "Configuring an Ethernet Interface as a Trunk Port" section.

VTP is recommended but not required. VTP maintains VLAN configuration consistency by managing the addition, deletion, and renaming of VLANs on a network-wide basis. VTP exchanges VLAN configuration messages with other switches over trunk links.

Dynamic access

A dynamic-access port can belong to one normal-range VLAN (VLAN ID 1 to 1005) and is dynamically assigned by a VMPS. The VMPS can be a Catalyst 5000 or Catalyst 6000 series switch, for example, but never a Catalyst 3550 switch.

You can have dynamic-access ports and trunk ports on the same switch, but you must connect the dynamic-access port to an end station and not to another switch.

For configuration information, see the "Configuring Dynamic Access Ports on VMPS Clients" section.

VTP is required.

Configure the VMPS and the client with the same VTP domain name.

You can change the reconfirmation interval and retry count on the VMPS client switch.

Voice VLAN

A voice VLAN port is an access port attached to a Cisco IP Phone, configured to use one VLAN for voice traffic and another VLAN for data traffic from a device attached to the phone. For more information about voice VLAN ports, see "Configuring Voice VLAN."

VTP is not required; it has no affect on voice VLAN.

Tunnel (dot1q-tunnel)

Tunnel ports are used for 802.1Q tunneling to maintain customer VLAN integrity across a service provider network. You configure a tunnel port on an edge switch in the service provider network and connect it to an 802.1Q trunk port on a customer interface, creating an asymmetric link. A tunnel port belongs to a single VLAN that is dedicated to tunneling.

For more information about tunnel ports, see "Configuring 802.1Q and Layer 2 Protocol Tunneling."

VTP is not required. You manually assign the tunnel port to a VLAN by using the switchport access vlan interface configuration command.

For more detailed definitions of the modes and their functions, see Table 11-4.

When a port belongs to a VLAN, the switch learns and manages the addresses associated with the port on a per-VLAN basis. For more information, see the "Managing the MAC Address Table" section.

Configuring Normal-Range VLANs

Normal-range VLANs are VLANs with VLAN IDs 1 to 1005. If the switch is in VTP server or transparent mode, you can add, modify or remove configurations for VLANs 2 to 1001 in the VLAN database. (VLAN IDs 1 and 1002 to 1005 are automatically created and cannot be removed.)

Note When the switch is in VTP transparent mode, you can also create extended-range VLANs (VLANs with IDs from 1006 to 4094), but these VLANs are not saved in the VLAN database. See the "Configuring Extended-Range VLANs" section.

Configurations for VLAN IDs 1 to 1005 are written to the file vlan.dat (VLAN database), and you can display them by entering the show vlan privileged EXEC command. The vlan.dat file is stored in nonvolatile RAM (NVRAM).

Caution You can cause inconsistency in the VLAN database if you attempt to manually delete the vlan.dat file. If you want to modify the VLAN configuration, use the commands described in these sections and in the command reference for this release. To change the VTP configuration, see "Configuring VTP."

You use the interface configuration mode to define the port membership mode and to add and remove ports from VLANs. The results of these commands are written to the running-configuration file, and you can display the file by entering the show running-config privileged EXEC command.

You can set these parameters when you create a new normal-range VLAN or modify an existing VLAN in the VLAN database:

•VLAN ID

•VLAN name

•VLAN type (Ethernet, Fiber Distributed Data Interface [FDDI], FDDI network entity title [NET], TrBRF, or TrCRF, Token Ring, Token Ring-Net)

•VLAN state (active or suspended)

•Maximum transmission unit (MTU) for the VLAN

•Security Association Identifier (SAID)

•Bridge identification number for TrBRF VLANs

•Ring number for FDDI and TrCRF VLANs

•Parent VLAN number for TrCRF VLANs

•Spanning Tree Protocol (STP) type for TrCRF VLANs

•VLAN number to use when translating from one VLAN type to another

Note This section does not provide configuration details for most of these parameters. For complete information on the commands and parameters that control VLAN configuration, refer to the command reference for this release.

This section includes information about these topics about normal-range VLANs:

•Token Ring VLANs

•Normal-Range VLAN Configuration Guidelines

•VLAN Configuration Mode Options

•Saving VLAN Configuration

•Default Ethernet VLAN Configuration

•Creating or Modifying an Ethernet VLAN

•Deleting a VLAN

•Assigning Static-Access Ports to a VLAN

Token Ring VLANs

Although the switch does not support Token Ring connections, a remote device such as a Catalyst 5000 series switch with Token Ring connections could be managed from one of the supported switches. Switches running VTP version 2 advertise information about these Token Ring VLANs:

•Token Ring TrBRF VLANs

•Token Ring TrCRF VLANs

For more information on configuring Token Ring VLANs, refer to the Catalyst 5000 Series Software Configuration Guide.

Normal-Range VLAN Configuration Guidelines

Follow these guidelines when creating and modifying normal-range VLANs in your network:

•The switch supports 1005 VLANs in VTP client, server, and transparent modes. Normal-range VLANs are identified with a number between 1 and 1001. VLAN numbers 1002 through 1005 are reserved for Token Ring and FDDI VLANs.

•VLAN configuration for VLANs 1 to 1005 are always saved in the VLAN database. If VTP mode is transparent, VTP and VLAN configuration is also saved in the switch running configuration file.

•The switch also supports VLAN IDs 1006 through 4094 in VTP transparent mode (VTP disabled). These are extended-range VLANs and configuration options are limited. Extended-range VLANs are not saved in the VLAN database. See the "Configuring Extended-Range VLANs" section.

•Before you can create a VLAN, the switch must be in VTP server mode or VTP transparent mode. If the switch is a VTP server, you must define a VTP domain or VTP will not function.

•The switch does not support Token Ring or FDDI media. The switch does not forward FDDI, FDDI-Net, TrCRF, or TrBRF traffic, but it does propagate the VLAN configuration through VTP.

•The switch supports 128 spanning-tree instances. If a switch has more active VLANs than supported spanning-tree instances, spanning tree can be enabled on 128 VLANs and is disabled on the remaining VLANs. If you have already used all available spanning-tree instances on a switch, adding another VLAN anywhere in the VTP domain creates a VLAN on that switch that is not running spanning-tree. If you have the default allowed list on the trunk ports of that switch (which is to allow all VLANs), the new VLAN is carried on all trunk ports. Depending on the topology of the network, this could create a loop in the new VLAN that would not be broken, particularly if there are several adjacent switches that all have run out of spanning-tree instances. You can prevent this possibility by setting allowed lists on the trunk ports of switches that have used up their allocation of spanning-tree instances.

If the number of VLANs on the switch exceeds the number of supported spanning tree instances, we recommend that you configure the IEEE 802.1S Multiple STP (MSTP) on your switch to map multiple VLANs to a single STP instance. For more information about MSTP, see "Configuring RSTP and MSTP."

VLAN Configuration Mode Options

You can configure normal-range VLANs (with VLAN IDs 1 to 1005) by using these two configuration modes:

•VLAN Configuration in config-vlan Mode

You access config-vlan mode by entering the vlan vlan-id global configuration command.

•VLAN Configuration in VLAN Configuration Mode

You access VLAN database configuration mode by entering the vlan database privileged EXEC command.

VLAN Configuration in config-vlan Mode

To access config-vlan mode, enter the vlan global configuration command with a VLAN ID. Enter a new VLAN ID to create a VLAN or with an existing VLAN ID to modify the VLAN. You can use the default VLAN configuration (Table 11-2) or enter multiple commands to configure the VLAN. For more information about commands available in this mode, refer to the vlan global configuration command description in the command reference for this release. When you have finished the configuration, you must exit config-vlan mode for the configuration to take effect. To display the VLAN configuration, enter the show vlan privileged EXEC command.

You must use this config-vlan mode when creating extended-range VLANs (VLAN IDs greater than 1005). See the "Configuring Extended-Range VLANs" section.

VLAN Configuration in VLAN Configuration Mode

To access VLAN configuration mode, enter the vlan database privileged EXEC command. Then enter the vlan command with a new VLAN ID to create a VLAN or with an existing VLAN ID to modify the VLAN. You can use the default VLAN configuration (Table 11-2) or enter multiple commands to configure the VLAN. For more information about keywords available in this mode, refer to the vlan VLAN configuration command description in the command reference for this release. When you have finished the configuration, you must enter apply or exit for the configuration to take effect. When you enter the exit command, it applies all commands and updates the VLAN database. VTP messages are sent to other switches in the VTP domain, and the privileged EXEC mode prompt appears.

Saving VLAN Configuration

The configurations of VLAN IDs 1 to 1005 are always saved in the VLAN database (vlan.dat file). If VTP mode is transparent, they are also saved in the switch running configuration file and you can enter the copy running-config startup-config privileged EXEC command to save the configuration in the startup configuration file. You can use the show running-config vlan privileged EXEC command to display the switch running configuration file. To display the VLAN configuration, enter the show vlan privileged EXEC command.

When you save VLAN and VTP information (including extended-range VLAN configuration information) in the startup configuration file and reboot the switch, the switch configuration is determined as follows:

•If the VTP mode is transparent in the startup configuration, and the VLAN database and the VTP domain name from the VLAN database matches that in the startup configuration file, the VLAN database is ignored (cleared), and the VTP and VLAN configurations in the startup configuration file are used. The VLAN database revision number remains unchanged in the VLAN database.

•If the VTP mode or domain name in the startup configuration does not match the VLAN database, the domain name and VTP mode and configuration for the first 1005 VLANs use the VLAN database information.

•If VTP mode is server, the domain name and VLAN configuration for the first 1005 VLANs use the VLAN database information

•If the switch is running IOS Release 12.1(9)EA1 or later and you use an older startup configuration file to boot up the switch, the configuration file does not contain VTP or VLAN information, and the switch uses the VLAN database configurations.

•If the switch is running an IOS release earlier than 12.1(9)EA1 and you use a startup configuration file from IOS Release 12.1(9)EA1 or later to boot up the switch, the image on the switch does not recognize the VLAN and VTP configurations in the startup configuration file, so the switch uses the VLAN database configuration.

Caution If the VLAN database configuration is used at startup and the startup configuration file contains extended-range VLAN configuration, this information is lost when the system boots up.

Default Ethernet VLAN Configuration

Table 11-2 shows the default configuration for Ethernet VLANs.

Note The switch supports Ethernet interfaces exclusively. Because FDDI and Token Ring VLANs are not locally supported, you only configure FDDI and Token Ring media-specific characteristics for VTP global advertisements to other switches.

Table 11-2 Ethernet VLAN Defaults and Ranges

Parameter

Default

Range

VLAN ID

1

1 to 4094.

Note Extended-range VLANs (VLAN IDs 1006 to 4094) are not saved in the VLAN database.

VLAN name

VLANxxxx, where xxxx represents four numeric digits (including leading zeros) equal to the VLAN ID number

No range

802.10 SAID

100001 (100000 plus the VLAN ID)

1-4294967294

MTU size

1500

1500-18190

Translational bridge 1

0

0-1005

Translational bridge 2

0

0-1005

VLAN state

active

active, suspend

Remote SPAN

disabled

enabled, disabled

Creating or Modifying an Ethernet VLAN

Each Ethernet VLAN in the VLAN database has a unique, 4-digit ID that can be a number from 1 to 1001. VLAN IDs 1002 to 1005 are reserved for Token Ring and FDDI VLANs. To create a normal-range VLAN to be added to the VLAN database, assign a number and name to the VLAN.

Note When the switch is in VTP transparent mode, you can assign VLAN IDs greater than 1006, but they are not added to the VLAN database. See the "Configuring Extended-Range VLANs" section.

For the list of default parameters that are assigned when you add a VLAN, see the "Configuring Normal-Range VLANs" section.

Beginning in privileged EXEC mode, follow these steps to use config-vlan mode to create or modify an Ethernet VLAN:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

vlan vlan-id

Enter a VLAN ID, and enter config-vlan mode. Enter a new VLAN ID to create a VLAN, or enter an existing VLAN ID to modify a VLAN.

Note The available VLAN ID range for this command is 1 to 4094. For information about adding VLAN IDs greater than 1005 (extended-range VLANs), see the "Configuring Extended-Range VLANs" section.

Step 3

name vlan-name

(Optional) Enter a name for the VLAN. If no name is entered for the VLAN, the default is to append the vlan-id with leading zeros to the word VLAN. For example, VLAN0004 is a default VLAN name for VLAN 4.

Step 4

mtu mtu-size

(Optional) Change the MTU size (or other VLAN characteristic).

Step 5

remote-span

(Optional) Configure the VLAN as the RSPAN VLAN for a remote SPAN session. For more information on remote SPAN, see "Configuring SPAN and RSPAN."

Step 6

end

Return to privileged EXEC mode.

Step 7

show vlan {name vlan-name | id vlan-id}

Verify your entries.

Step 8

copy running-config startup config

(Optional) If the switch is in VTP transparent mode, the VLAN configuration is saved in the running configuration file as well as in the VLAN database. This saves the configuration in the switch startup configuration file.

To return the VLAN name to the default settings, use the no vlan name, no vlan mtu, or no remote span config-vlan commands.

This example shows how to use config-vlan mode to create Ethernet VLAN 20, name it test20, and add it to the VLAN database:
Switch# configure terminal
Switch(config)# vlan 20
Switch(config-vlan)# name test20
Switch(config-vlan)# end
Beginning in privileged EXEC mode, follow these steps to use VLAN configuration mode to create or modify an Ethernet VLAN:

Command

Purpose

Step 1

vlan database

Enter VLAN database configuration mode.

Step 2

vlan vlan-id name vlan-name

Add an Ethernet VLAN by assigning a number to it. The range is 1 to 1001; do not enter leading zeros.

If no name is entered for the VLAN, the default is to append the vlan-id with leading zeros to the word VLAN. For example, VLAN0004 is a default VLAN name for VLAN 4.

Step 3

vlan vlan-id mtu mtu-size

(Optional) To modify a VLAN, identify the VLAN and change a characteristic, such as the MTU size.

Step 4

exit

Update the VLAN database, propagate it throughout the administrative domain, and return to privileged EXEC mode.

Step 5

show vlan {name vlan-name | id vlan-id}

Verify your entries.

Step 6

copy running-config startup config

(Optional) If the switch is in VTP transparent mode, the VLAN configuration is saved in the running configuration file as well as in the VLAN database. This saves the configuration in the switch startup configuration file.

Note You cannot configure an RSPAN VLAN in VLAN database configuration mode.

To return the VLAN name to the default settings, use the no vlan vlan-id name or no vlan vlan-id mtu VLAN configuration command.

This example shows how to use VLAN database configuration mode to create Ethernet VLAN 20, name it test20, and add it to the VLAN database:
Switch# vlan database
Switch(vlan)# vlan 20 name test20
Switch(vlan)# exit
APPLY completed.
Exiting....
Switch# 
Deleting a VLAN

When you delete a VLAN from a switch that is in VTP server mode, the VLAN is removed from the VLAN database for all switches in the VTP domain. When you delete a VLAN from a switch that is in VTP transparent mode, the VLAN is deleted only on that specific switch.

You cannot delete the default VLANs for the different media types: Ethernet VLAN 1 and FDDI or Token Ring VLANs 1002 to 1005.

Caution When you delete a VLAN, any ports assigned to that VLAN become inactive. They remain associated with the VLAN (and thus inactive) until you assign them to a new VLAN.

Beginning in privileged EXEC mode, follow these steps to delete a VLAN on the switch by using global configuration mode:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

no vlan vlan-id

Remove the VLAN by entering the VLAN ID.

Step 3

end

Return to privileged EXEC mode.

Step 4

show vlan brief

Verify the VLAN removal.

Step 5

copy running-config startup config

(Optional) If the switch is in VTP transparent mode, the VLAN configuration is saved in the running configuration file as well as in the VLAN database. This saves the configuration in the switch startup configuration file.

To delete a VLAN in VLAN database configuration mode, use the vlan database privileged EXEC command to enter VLAN database configuration mode and the no vlan vlan-id VLAN configuration command.

Assigning Static-Access Ports to a VLAN

You can assign a static-access port to a VLAN without having VTP globally propagate VLAN configuration information by disabling VTP (VTP transparent mode). If you are assigning a port on a cluster member switch to a VLAN, first use the rcommand privileged EXEC command to log in to the member switch.

Note If you assign an interface to a VLAN that does not exist, the new VLAN is created. (See the "Creating or Modifying an Ethernet VLAN" section.)

Beginning in privileged EXEC mode, follow these steps to assign a port to a VLAN in the VLAN database:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode

Step 2

interface interface-id

Enter the interface to be added to the VLAN.

Step 3

switchport mode access

Define the VLAN membership mode for the port (Layer 2 access port).

Step 4

switchport access vlan vlan-id

Assign the port to a VLAN. Valid VLAN IDs are 1 to 4094.

Step 5

end

Return to privileged EXEC mode.

Step 6

show running-config interface interface-id

Verify the VLAN membership mode of the interface.

Step 7

show interfaces interface-id switchport

Verify your entries in the Administrative Mode and the Access Mode VLAN fields of the display.

Step 8

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return an interface to its default configuration, use the default interface interface-id interface configuration command.

This example shows how to configure Fast Ethernet interface 0/1 as an access port in VLAN 2:
Switch# configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)# interface fastethernet0/1 
Switch(config-if)# switchport mode access 
Switch(config-if)# switchport access vlan 2
Switch(config-if)# end 
Switch#
Configuring Extended-Range VLANs

When the switch is in VTP transparent mode (VTP disabled), you can create extended-range VLANs (in the range 1006 to 4094). Extended-range VLANs enable service providers to extend their infrastructure to a greater number of customers. The extended-range VLAN IDs are allowed for any switchport commands that allow VLAN IDs. You always use config-vlan mode (accessed by entering the vlan vlan-id global configuration command) to configure extended-range VLANs. The extended range is not supported in VLAN database configuration mode (accessed by entering the vlan database privileged EXEC command).

Extended-range VLAN configurations are not stored in the VLAN database, but because VTP mode is transparent, they are stored in the switch running configuration file, and you can save the configuration in the startup configuration file by using the copy running-config startup-config privileged EXEC command.

Note Although the switch supports 4094 VLAN IDs, see the "Supported VLANs" section for the actual number of VLANs supported.

This section includes this information about extended-range VLANs:

•Default VLAN Configuration

•Extended-Range VLAN Configuration Guidelines

•Creating an Extended-Range VLAN

•Creating an Extended-Range VLAN with an Internal VLAN ID

Default VLAN Configuration

See Table 11-2 for the default configuration for Ethernet VLANs. You can change only the MTU size on extended-range VLANs; all other characteristics must remain at the default state.

Extended-Range VLAN Configuration Guidelines

Follow these guidelines when creating extended-range VLANs:

•To add an extended-range VLAN, you must use the vlan vlan-id global configuration command and access config-vlan mode. You cannot add extended-range VLANs in VLAN database configuration mode (accessed by entering the vlan database privileged EXEC command).

•VLAN IDs in the extended range are not saved in the VLAN database and are not recognized by VTP.

•You cannot include extended-range VLANs in the pruning eligible range.

•The switch must be in VTP transparent mode when you create extended-range VLANs. If VTP mode is server or client, an error message is generated, and the extended-range VLAN is rejected.

•You can set the VTP mode to transparent in global configuration mode or in VLAN database configuration mode. See the "Disabling VTP (VTP Transparent Mode)" section. You should save this configuration to the startup configuration so that the switch will boot up in VTP transparent mode. Otherwise, you will lose extended-range VLAN configuration if the switch resets.

•VLANs in the extended range are not supported by VQP. They cannot be configured by VMPS.

•STP is enabled by default on extended-range VLANs, but you can disable it by using the no spanning-tree vlan vlan-id global configuration command. When the maximum number of spanning-tree instances(128) are on the switch, spanning tree is disabled on any newly created VLANs. If the number of VLANs on the switch exceeds the maximum number of spanning tree instances, we recommend that you configure the IEEE 802.1S Multiple STP (MSTP) on your switch to map multiple VLANs to a single STP instance. For more information about MSTP, see "Configuring RSTP and MSTP."

•Each routed port on a Catalyst 3550 switch creates an internal VLAN for its use. These internal VLANs use extended-range VLAN numbers, and the internal VLAN ID cannot be used for an extended-range VLAN. If you try to create an extended-range VLAN with a VLAN ID that is already allocated as an internal VLAN, an error message is generated, and the command is rejected.

–Because internal VLAN IDs are in the lower part of the extended range, we recommend that you create extended-range VLANs beginning from the highest number (4094) and moving to the lowest (1006) to reduce the possibility of using an internal VLAN ID.

–Before configuring extended-range VLANs, enter the show vlan internal usage privileged EXEC command to see which VLANs have been allocated as internal VLANs.

–If necessary, you can shut down the routed port assigned to the internal VLAN, which frees up the internal VLAN, and then create the extended-range VLAN and re-enable the port, which then uses another VLAN as its internal VLAN. See the "Creating an Extended-Range VLAN with an Internal VLAN ID" section.

Creating an Extended-Range VLAN

You create an extended-range VLAN in global configuration mode by entering the vlan global configuration command with a VLAN ID from 1006 to 4094. This command accesses the config-vlan mode. The extended-range VLAN has the default Ethernet VLAN characteristics (see Table 11-2) and the MTU size is the only parameter you can change. Refer to the description of the vlan global configuration command in the command reference for defaults of all parameters. If you enter an extended-range VLAN ID when the switch is not in VTP transparent mode, an error message is generated when you exit from config-vlan mode, and the extended-range VLAN is not created.

Extended-range VLANs are not saved in the VLAN database; they are saved in the switch running configuration file. You can save the extended-range VLAN configuration in the switch startup configuration file by using the copy running-config startup-config privileged EXEC command.

Note Before you create an extended-range VLAN, you can verify that the VLAN ID is not used internally by entering the show vlan internal usage privileged EXEC command. If the VLAN ID is used internally and you want to free it up, go to the"Creating an Extended-Range VLAN with an Internal VLAN ID" section before creating the extended-range VLAN.

Beginning in privileged EXEC mode, follow these steps to create an extended-range VLAN:

Command

Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

vtp mode transparent

Configure the switch for VTP transparent mode, disabling VTP.

Step 3

vlan vlan-id

Enter an extended-range VLAN ID and enter config-vlan mode. The range is 1006 to 4094.

Step 4

mtu mtu-size

(Optional) Modify the VLAN by changing the MTU size.

Note Although all commands appear in the CLI help in config-vlan mode, only the mtu mtu-size command is supported for extended-range VLANs.

Step 5

end

Return to privileged EXEC mode.

Step 6

show vlan id vlan-id

Verify that the VLAN has been created.

Step 7

copy running-config startup config

Save your entries in the switch startup configuration file. To save extended-range VLAN configurations, you need to save the VTP transparent mode configuration and the extended-range VLAN configuration in the switch startup configuration file. Otherwise, if the switch resets, it will default to VTP server mode, and the extended-range VLAN IDs will not be saved.

To delete an extended-range VLAN, use the no vlan vlan-id global configuration command.

The procedure for assigning static-access ports to an extended-range VLAN is the same as for normal-range VLANs. See the "Assigning Static-Access Ports to a VLAN" section.

This example shows how to create a new extended-range VLAN with all default characteristics, enter config-vlan mode, and save the new VLAN in the switch startup configuration file:
Switch(config)# vtp mode transparent
Switch(config)# vlan 2000
Switch(config-vlan)# end
Switch# copy running-config startup config
Creating an Extended-Range VLAN with an Internal VLAN ID

If you enter an extended-range VLAN ID that is already assigned to an internal VLAN, an error message is generated, and the extended-range VLAN is rejected. To manually free an internal VLAN ID, you must temporarily shut down the routed port that is using the internal VLAN ID.

Beginning in privileged EXEC mode, follow these steps to release a VLAN ID that is assigned to an internal VLAN and to create an extended-range VLAN with that ID:

Command

Purpose

Step 1

show vlan internal usage

Display the VLAN IDs being used internally by the switch. If the VLAN ID that you want to use is an internal VLAN, the display shows the routed port that is using the VLAN ID. Enter that port number in Step 3.

Step 2

configure terminal

Enter global configuration mode.

Step 3

interface interface-id

Enter the interface ID for the routed port that is using the VLAN ID.

Step 4

shutdown

Shut down the port to free the internal VLAN ID.

Step 5

exit

Return to global configuration mode.

Step 6

vtp mode transparent

Set the VTP mode to transparent for creating extended-range VLANs.

Step 7

vlan vlan-id

Enter the new extended-range VLAN ID, and enter config-vlan mode.

Step 8

exit

Exit from config-vlan mode, and return to global configuration mode.

Step 9

interface interface-id

Enter the interface ID for the routed port that you shut down in Step 4.

Step 10

no shutdown

Re-enable the routed port. It will be assigned a new internal VLAN ID.

Step 11

end

Return to privileged EXEC mode.

Step 12

copy running-config startup config

Save your entries in the switch startup configuration file. To save an extended-range VLAN configuration, you need to save the VTP transparent mode configuration and the extended-range VLAN configuration in the switch startup configuration file. Otherwise, if the switch resets, it will default to VTP server mode, and the extended-range VLAN IDs will not be saved.

Displaying VLANs

Use the show vlan privileged EXEC command to display a list of all VLANs on the switch, including extended-range VLANs. The display includes VLAN status, ports, and configuration information. To view normal-range VLANs in the VLAN database (1 to 1005,) use the show VLAN configuration command (accessed by entering the vlan database privileged EXEC command). For a list of the VLAN IDs on the switch, use the show running-config vlan privileged EXEC command, optionally entering a VLAN ID range.

Table 11-3 lists the commands for monitoring VLANs.

Table 11-3 VLAN Monitoring Commands

Command

Command Mode

Purpose

show

VLAN configuration

Display status of VLANs in the VLAN database.

show current [vlan-id]

VLAN configuration

Display status of all or the specified VLAN in the VLAN database.

show interfaces [vlan vlan-id]

Privileged EXEC

Display characteristics for all interfaces or for the specified VLAN configured on the switch.

show running-config vlan

Privileged EXEC

Display all or a range of VLANs on the switch.

show vlan [id vlan-id]

Privileged EXEC

Display parameters for all VLANs or the specified VLAN on the switch.

For more details about the show command options and explanations of output fields, refer to the command reference for this release.

Configuring VLAN Trunks

These sections describe how VLAN trunks function on the switch:

•Trunking Overview

•Encapsulation Types

•Default Layer 2 Ethernet Interface VLAN Configuration

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. Fast Ethernet and Gigabit Ethernet trunks carry the traffic of multiple VLANs over a single link, and you can extend the VLANs across an entire network.

Two trunking encapsulations are available on all Ethernet interfaces:

•Inter-Switch Link (ISL)—ISL is Cisco-proprietary trunking encapsulation.

•802.1Q—802.1Q is industry-standard trunking encapsulation.

Figure 11-2 shows a network of switches that are connected by ISL trunks.

Figure 11-2 Switches in an ISL Trunking Environment

You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more information about EtherChannel, see "Configuring EtherChannels."

Ethernet trunk interfaces support different trunking modes (see Table 11-4). 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. However, some internetworking devices might forward DTP frames improperly, which could cause misconfigurations.

To avoid this, you should 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.

Note On GigaStack GBICs, dynamic trunking is only supported when only one port of a GigaStack GBIC is being used. If trunking is required on a GigaStack GBIC where both ports are in use, use the switchport mode trunk and switchport nonegotiate interface configuration commands on both GBIC interfaces to cause the interfaces to become trunks.

You can also specify whether the trunk uses ISL or 802.1Q encapsulation or if the encapsulation type is autonegotiated. The DTP supports autonegotiation of both ISL and 802.1Q trunks.

Note Tunnel ports do not support DTP. See "Configuring 802.1Q and Layer 2 Protocol Tunneling," for more information on tunnel ports.

Table 11-4 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 even if the neighboring interface is not a trunk interface.

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. The default switch-port mode for all Ethernet interfaces is dynamic desirable.

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.

switchport mode trunk

Puts the interface into permanent trunking mode and negotiates to convert the 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 802.1Q trunk port. 802.1Q tunneling is used to maintain customer VLAN integrity across a service provider network. See "Configuring 802.1Q and Layer 2 Protocol Tunneling," for more information on tunnel ports.

Encapsulation Types

Table 11-5 lists the Ethernet trunk encapsulation types and keywords.

Table 11-5 Ethernet Trunk Encapsulation Types

Encapsulation

Function

switchport trunk encapsulation isl

Specifies ISL encapsulation on the trunk link.

switchport trunk encapsulation dot1q

Specifies 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 802.1Q trunk, depending on the configuration and capabilities of the neighboring interface.

Note 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.

The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected interfaces determine whether a link becomes an ISL or 802.1Q trunk.

802.1Q Configuration Considerations

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

•In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree 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 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 802.1Q switch. However, spanning-tree information for each VLAN is maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches.

•Make sure the native VLAN for an 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 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 802.1Q trunk or disable spanning tree on every VLAN in the network. Make sure your network is loop-free before disabling spanning tree.

Default Layer 2 Ethernet Interface VLAN Configuration

Table 11-6 shows the default Layer 2 Ethernet interface VLAN configuration.

Table 11-6 Default Layer 2 Ethernet Interface VLAN Configuration

Feature

Default Setting

Interface mode

switchport mode dynamic desirable

Trunk encapsulation

switchport trunk encapsulation negotiate

Allowed VLAN range

VLANs 1 to 4094

VLAN range eligible for pruning

VLANs 2 to 1001

Default VLAN (for access ports)

VLAN 1

Native VLAN (for 802.1Q trunks)

VLAN 1

Configuring an Ethernet Interface as a Trunk Port

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.

This section includes these procedures for configuring an Ethernet interface as a trunk port on the switch:

•Interaction with Other Features

•Defining the Allowed VLANs on a Trunk

•Changing the Pruning-Eligible List

•Configuring the Native VLAN for Untagged Traffic

Note By default, an interface is in Layer 2 mode. The default mode for Layer 2 interfaces is switchport mode dynamic desirable. 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. By default, trunks negotiate encapsulation. If the neighboring interface supports ISL and 802.1Q encapsulation and both interfaces are set to negotiate the encapsulation type, the trunk uses ISL encapsulation.

Interaction with Other Features

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 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 PVST mode and no more than 40 trunk ports in MST mode.

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

Configuring a Trunk Port

Beginning in privileged EXEC mode, follow these steps to configure a port as an ISL or 802.1Q trunk port: