Configuring VLans.pdf

Configuring VLANs

This chapter provides information about configuring virtual LANs (VLANs). It includes

command-line interface (CLI) procedures for using commands that have been specifically

created or changed for the Catalyst 2950 switches. For complete syntax and usage

information for the commands used in this chapter, refer to the Catalyst 2950 Desktop

Switch Command Reference .

This chapter does not repeat the concepts and CLI procedures provided in the standard

Cisco IOS Release 12.1 documentation. For information about the standard IOS Release

12.1 commands, refer to the IOS documentation set available from the Cisco.com home

page at Service and Support > Technical Documents . On the Cisco Product

Documentation home page, select Release 12.1 from the Cisco IOS Software drop-down

list.

For information about configuring these settings from Cluster Management Suite (CMS),

refer to the online help.

Note Some features can be implemented only by using the CLI.

Overview

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. Any switch port can

belong to a VLAN, and unicast, broadcast, and multicast packets are forwarded and

flooded only to 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 8-1. VLANs are identified with a number from 1 to

1001.

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 the Spanning Tree Protocol (STP).

Figure 8-1 VLANs as Logically Defined Networks

Table 8-1 lists the number of supported VLANs on the switches.

Table 8-1 Maximum Number of Supported VLANs

Switch

Number of Supported VLANs Trunking Supported?

Catalyst 2950 switches 250

Yes

The Catalyst 2950 switches support IEEE 802.1Q trunking methods for transmitting

VLAN traffic over 100BASE-T and Gigabit Ethernet ports.

The GigaStack GBIC also supports both trunking methods. When you are configuring a

cascaded stack of Catalyst 3500 XL switches that use the GigaStack GBIC and want to

include more than one VLAN in the stack, be sure to configure all of the GigaStack

GBIC interfaces as trunk ports by using the switchport mode trunk interface

configuration command. For more information on these commands, refer to the

Catalyst 2950 Desktop Switch Command Reference .

Note The Catalyst 2950 switches do not support ISL trunking.

Trunking is not supported on all switches. For the list of products that support trunking,

refer to the release notes.

Management VLANs

Communication with the switch management interfaces is through the switch IP address.

The IP address is associated with the management VLAN, which by default is VLAN 1.

The management VLAN has these characteristics:

• It is created from CMS or through the CLI on static-access and dynamic-access

and trunk ports. You cannot create or remove the management VLAN through

Simple Network Management Protocol (SNMP).

• Only one management VLAN can be administratively active at a time.

• With the exception of VLAN 1, the management VLAN can be deleted.

• When created, the management VLAN is administratively down.

Before changing the management VLAN on your switch network, make sure you follow

these guidelines:

• The new management VLAN should not have an Hot Standby Router Protocol

(HSRP) standby group configured on it.

• You must be able to move your network management station to a switch port

assigned to the same VLAN as the new management VLAN.

• Connectivity through the network must exist from the network management

station to all switches involved in the management VLAN change.

• Switches running a IOS software version that is earlier than Cisco IOS 12.0(5)XP

cannot have the management VLAN changed.

• Switches running Cisco IOS 12.0(5)XP should be upgraded to the current

software release as described in the release notes.

If you are using SNMP or CMS to manage the switch, ensure that the port through which

you are connected to a switch is in the management VLAN.

Changing the Management VLAN for a New Switch

If you add a new switch to an existing cluster and the cluster is using a management

VLAN other than the default VLAN 1, the command switch automatically senses that the

new switch has a different management VLAN and has not been configured. The

command switch issues commands to change the management VLAN on the new switch

to match the used by the cluster. This automatic change occurs for new, out-of-box

switches that do not have a config.text file and for which there have been no changes to

the running configuration.

Before a new switch can be added to a cluster, it must be connected to a port that belongs

to the cluster management VLAN. If the cluster is configured with a management VLAN

other than the default, the command switch changes the management VLAN for new

switches when they are connected to the cluster. In this way, the new switch can

exchange Cisco Discovery Protocol (CDP) messages with the command switch and be

proposed as a cluster candidate.

Note For the command switch to change the management VLAN on a new switch,

there must have been no changes to the new switch configuration, and there must be

no config.text file.

Because the switch is new and unconfigured, its management VLAN is changed to the

cluster management VLAN when it is first added to the cluster. All ports that have an

active link at the time of this change become members of the new management VLAN.

Changing the Management VLAN Through a Telnet Connection

Before you start, review the "Management VLANs" section. Beginning in privileged

EXEC mode on the command switch, follow these steps to configure the management

VLAN interface through a Telnet connection:

Command

Purpose

Step 1 configure

terminal

Enter global configuration mode.

Step 2

cluster

management-vlan

vlanid

Change the management VLAN for the cluster. This ends

your Telnet session. Move the port through which you are

connected to the switch to a port in the new management

VLAN.

Step 3 show running-

config

Verify the change.

Assigning 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 it can belong to.

Table 8-2 lists the membership modes and characteristics.

Table 8-2 Port Membership Modes

Membership

Mode

VLAN Membership Characteristics

Static-access

A static-access port can belong to one VLAN and is manually assigned.

By default, all ports are static-access ports assigned to VLAN 1.

IEEE 802.1Q

A trunk is a member of all VLANs in the VLAN database by default,

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.

VLAN Trunking Protocol (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 VLAN and is dynamically

assigned by a VLAN Membership Policy Server (VMPS). The VMPS

can be a Catalyst 5000 series switch but never a Catalyst 2950,

Catalyst 2900 XL, or Catalyst 3500 XL switch.

When a port belongs to a VLAN, the switch learns and manages the addresses associated

with the port on a per-VLAN basis.

VLAN Membership Combinations

You can configure your switch ports in the various VLAN membership combinations in

Table 8-3.

Table 8-3 VLAN Combinations

Port

Mode

VTP

Required?

Configuration

Procedure

Comments

Static-

access

ports

"Assigning Static-Access

Ports to a VLAN" section

If you do not want to use VTP

to globally propagate the

VLAN configuration

information, you can assign a

static-access port to a VLAN

and set the VTP mode to

transparent to disable VTP.

The switch automatically

transitions to VTP transparent

mode (VTP is disabled). No

VTP configuration is required.

"Configuring VTP Server

Mode" section

You can configure at least one

trunk port on the switch and

make sure that this trunk port is

connected to the trunk port of a

second switch.

Static-

access and

trunk ports

Adding, modifying, or

removing VLANs in the

database as described in

the "Configuring VLANs

in the VTP Database"

section

Recommended

Some restrictions apply to

trunk ports.

You can change the VTP

version on the switch and

"Assigning Static-Access

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