VLAN Configuration
This section discusses the guidelines for configuring VLANs on the Catalyst switch. You will learn the steps to configure VLANs, how to enable VTP domains, how to define a trunk, how to create a VLAN, and how to verify proper VLAN operation.
You should remember several facts before you begin VLAN configuration:
The maximum number of VLANs that can operate on a switch is switch-dependent.
VLAN1 is one of the factory default VLANs.
Cisco Discovery Protocol (CDP) and VTP advertisements are sent on VLAN1.
The switch must be in VTP server mode or transparent mode to create, add, or delete VLANs.
VLAN Configuration Guidelines
The Catalyst switches have a factory default configuration in which various default VLANs are preconfigured. One of the default VLANs is VLAN1, which is used for CDP and VTP advertisements. The VLAN1 interface on a switch is also in the default VLAN1. As you'll recall, the switch requires an IP address for management purposes—for example, to allow Telnet connections into the switch, or to use the Visual Switch Manager (VSM) via an HTTP browser to configure the switch.
Before you can create a VLAN, the switch must be in VTP server mode or VTP transparent mode. If you want to propagate the VLAN to other switches in the domain, use server mode.
VLAN Configuration Steps
Before you create VLANs, you must decide whether to use VTP to maintain global VLAN configuration information for your network.
To allow VLANs to span multiple Catalyst switches on a single link, you must configure trunks to interconnect the switches.
By default, a switch is in VTP server mode so that VLANs can be added, changed, or deleted. If the switch is set to VTP client mode, VLANs cannot be added, changed, or deleted from that switch.
VLAN membership on the switch ports is assigned manually on a port-by-port basis. When you assign switch ports to VLANs using this method, it is known as port-based, or static, VLAN membership.
The following sections elaborate on the details of the steps to configure VLANs.
VTP Configuration Guidelines
The default VTP configuration parameters for the Catalyst switch are as follows:
VTP domain name: None
VTP mode: Server
VTP password: None
VTP pruning: Disabled
VTP trap: Disabled
The VTP domain name can be specified by the administrator or learned across a configured trunk line from a server with a domain name configured. By default, the domain name is not set.
By default, the switch is set to the VTP server mode.
A password can be set for the VTP management domain. The password entered must be the same for all switches in the domain. If you configure a VTP password, VTP does not function properly unless you assign the same password to each switch in the domain.
VTP pruning eligibility is one VLAN parameter advertised by the VTP protocol. Enabling or disabling VTP pruning on a VTP server propagates the change throughout the management domain. Enabling or disabling VTP pruning on a VTP server affects the entire management domain.
VTP trap is disabled by default. If you enable this feature, it causes an SNMP message to be generated every time a new VTP message is sent.
CAUTION
When adding a new switch to an existing domain, you should verify that the configuration revision number for the switch is 0 to prevent the new switch from propagating incorrect VLAN information. Example 3-1, in the "How VTP Works" section, demonstrated one method for resetting the VTP configuration revision number on the new switch.
Configuring VTP
Use the vtp global configuration command to specify the operating mode, domain name, password, generation of traps, and pruning capabilities of VTP. The syntax for this command is as follows:
switch(config)# vtp { [mode {server | transparent | client}] [domain domain-name]
[password password] [pruning {enable | disable}]}
To verify a recent configuration change, or to just view the VTP configuration information, use the show vtp status privileged EXEC command, as demonstrated in Example 3-2. Also displayed is the IP address of the device that last modified the configuration and a time stamp showing when the modification was made. VTP has two versions:
VTP version 1 only supports Ethernet.
VTP version 2 supports Ethernet and Token Ring.
Example 3-2 show vtp status Output
Switch#show vtp status VTP Version : 2 Configuration Revision : 5 Maximum VLANs supported locally : 250 Number of existing VLANs : 10 VTP Operating Mode : Server VTP Domain Name : switch_domain_1 VTP Pruning Mode : Disabled VTP V2 Mode : Disabled VTP Traps Generation : Disabled MD5 digest : 0x1E 0xED 0x19 0x49 0x0F 0x37 0x65 0x64 Configuration last modified by 192.168.255.21 at 3-1-93 00:02:39 Local updater ID is 192.168.255.21 on interface Vl1 (lowest numbered VLAN interface found)
Trunk Line Configuration
Use the command switchport mode trunk at the interface configuration mode to set a port to trunk. On the Catalyst 2950, this enables 802.1Q trunking. On other Cisco IOS Software-based switches, such as the 3550, 4500, or 6500, you will need to choose an encapsulation method before you can enable trunking. The command switchport trunk encapsulation [isl | dot1q] chooses an encapsulation mode.
The Catalyst IOS switches also support Dynamic Trunking Protocol (DTP), which manages automatic trunk negotiation. The switchport mode command specifies a Layer 2 ports operation:
switch(config-if)# switchport mode [trunk | access | dynamic [desirable | auto | nonegotiate]]
The options for the switchport mode command are as follows:
trunk—Configures the port to permanent trunk mode and negotiates with the connected device on the other side to convert the link to trunk mode. If multiple trunk encapsulations are available, the encapsulation must be chosen before this command will work.
access—Disables port trunk mode and negotiates with the connected device to convert the link to nontrunk. This port will belong to only the configured access VLAN.
dynamic desirable—Triggers the port to negotiate the link from nontrunk to trunk mode. The port negotiates to a trunk port if the connected device is in the trunk, dynamic desirable, or dynamic auto state. Otherwise, the port becomes a nontrunk port. This is the default for IOS switch ports
dynamic auto—Enables the port to become a trunk only if the connected device has the state set to trunk or dynamic desirable.
nonnegotiate—Configures the port to permanent trunk mode. No negotiation takes place with the partner. The other side must be trunk or nonegotiate for the trunk to work. You must also specify the encapsulation before choosing this mode.
Verifying Trunk Line Configuration
To verify a trunk configuration, use the command show interface switchport or show interface trunk privileged EXEC command. The syntax for the show interface switchport and privileged EXEC command is as follows:
switch(config)# show interface [type module/port] switchport
The syntax for the show interface trunk and privileged EXEC command is as follows:
switch(config)# show interface [type module/port] trunk
These commands display the trunk parameters, as demonstrated in Example 3-3.
Example 3-3show interface trunk and show interface switchport Output
Switch#show interface trunk Port Mode Encapsulation Status Native vlan Fa0/1 on 802.1q trunking 1 Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Fa0/1 1-4094 Gi0/1 1-4094 Port Vlans allowed and active in management domain Fa0/1 1,101,202,303,404,505 Gi0/1 1,101,202,303,404,505 Port Vlans in spanning tree forwarding state and not pruned Fa0/1 1,101,202,303,404,505 Gi0/1 1,101,202,303,404,505 Switch#show interfaces fastEthernet 0/1 switchport Name: Fa0/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Administrative private-vlan host-association: none Administrative private-vlan mapping: none Operational private-vlan: none Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Protected: false Voice VLAN: none (Inactive) Appliance trust: none
Adding a VLAN
Use the vlan global configuration command to configure a VLAN. The syntax for the vlan global configuration command is as follows:
Switch(config)#vlan number Switch(config-vlan)#[name | mtu | shutdown | exit]
Each VLAN has a unique four-digit ID that can be a number from 0001 to 4096. To add a VLAN to the VLAN database, assign a number and name to the VLAN. After creating the VLAN, you will be in VLAN configuration mode. In this mode, use the name command to give the VLAN a name. VLAN1, VLAN1002, VLAN1003, VLAN1004, and VLAN1005 are the factory default VLANs. These VLANs exist on all Catalyst switches and are used as default VLANs for other topologies, such as Token Ring and FDDI. None of the default VLANs can be modified or deleted.
To add an Ethernet VLAN, you must specify at least a VLAN number. If no VLAN name is entered for the VLAN, the default is to append the VLAN number to the word VLAN. For example, VLAN0404 could be a default name for VLAN404 if no name is assigned.
Remember that to add, change, or delete VLANs, the switch must be in VTP server or transparent mode.
Verifying a VLAN/Modifying VLAN Parameters
When the VLAN is configured, the parameters for that VLAN should be confirmed to ensure validity. To verify the VLAN's parameters, use the show vlan id vlan# privileged EXEC command to display information about a particular VLAN. Use show vlan to show all configured VLANs.
The show vlan command output in Example 3-4 also shows which switch ports are assigned to the VLAN.
Example 3-4 show vlan Output
Switch#show vlan
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/2, Fa0/3, Fa0/4, Fa0/6
Fa0/7, Fa0/8, Fa0/9, Fa0/10
Fa0/11, Fa0/12, Gi0/2
101 VLAN0101 active
202 VLAN0202 active
303 VLAN0303 active
404 VLAN0404 active
505 VLAN0505 active
986 VLAN0986 active
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active
VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 0 0
101 enet 100101 1500 - - - - - 0 0
202 enet 100202 1500 - - - - - 0 0
303 enet 100303 1500 - - - - - 0 0
VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
404 enet 100404 1500 - - - - - 0 0
505 enet 100505 1500 - - - - - 0 0
986 enet 100986 1500 - - - - - 0 0
1002 fddi 101002 1500 - - - - - 0 0
1003 tr 101003 1500 - - - - - 0 0
1004 fdnet 101004 1500 - - - ieee - 0 0
1005 trnet 101005 1500 - - - ibm - 0 0
Remote SPAN VLANs
------------------------------------------------------------------------------
Primary Secondary Type Ports
------- --------- ----------------- ------------------------------------------
Other VLAN parameters shown in Example 3-4 include the following:
Type (default is Ethernet)
Security Association ID (SAID), which is used for the FDDI trunk
Maximum transmission unit (MTU, where the default is 1500 for Ethernet VLAN)
Other parameters used for Token Ring or FDDI VLANs
To modify an existing VLAN parameter (such as the VLAN name), use the same command syntax used to add a VLAN.
In Example 3-5, the VLAN name for VLAN986 is changed to CSR_VLAN.
Example 3-5 Change VLAN Name
Switch# config t Enter configuration commands, one per line. End with CNTL/Z Switch(config)#vlan 986 Switch(config-vlan)#name CSR_VLAN
Use the show vlan id 986 command, as demonstrated in Example 3-6, to verify the change.
Example 3-6 Verify VLAN Change
Switch# show vlan id 986 VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 986 CSR_VLAN active Fa0/1, Gi0/1 VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2 ---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------ 986 enet 100986 1500 - - - - - 0 0 Remote SPAN VLAN ---------------- Disabled Primary Secondary Type Ports ------- --------- ----------------- ------------------------------------------
Assigning Ports to a VLAN
After creating a VLAN, you can statically assign a port or a number of ports to that VLAN. A port can belong to only one VLAN at a time.
Configure the VLAN port assignment from the interface configuration mode using the interface command switchport access vlan number, as shown in the following syntax:
Switch(config-if)#switchport access vlan [1-4096 | dynamic]
dynamic means that the Catalyst switch queries a VMPS for VLAN information based on a MAC address. A number in the range of 1 to 4096 would represent the VLAN assignment for the port.
By default, all ports are members of the default VLAN—VLAN1.
Use the show vlan brief privileged EXEC command to display the VLAN assignment for all switch ports, as demonstrated in Example 3-7.
Example 3-7 Displaying VLAN Assignments and Membership for All Switch Ports
Switch#show vlan brief
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/2, Fa0/3, Fa0/4, Fa0/6
Fa0/7, Fa0/8, Fa0/9, Fa0/10
Fa0/11, Fa0/12, Gi0/2
101 VLAN0101 active
202 VLAN0202 active
303 VLAN0303 active
404 VLAN0404 active
505 VLAN0505 active
986 CSR_VLAN active
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active
Displaying Spanning Tree Protocol Configuration Status
Use the show spanning-tree privileged EXEC command to display the switch's Spanning Tree Protocol configuration status, as demonstrated in Example 3-8. The basic syntax for the show spanning-tree privileged EXEC command is as follows:
Switch# show spanning-tree [vlan number]
Example 3-8 show spanning-tree Output
Switch# show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 0
Address 0005.00a9.2401
Cost 8
Port 13 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000b.5f2a.5a40
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Port ID Designated Port ID
Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr
---------------- -------- --------- --- --------- -------------------- --------
Fa0/1 128.1 100 FWD 8 32769 000b.5f2a.5a40 128.1
Gi0/1 128.13 4 FWD 4 32768 0005.3104.c000 32.65
Example 3-8 displays various spanning tree information for VLAN1, including the following:
Port Fa0/1 and G0/1 are in the forwarding state for VLAN1.
The root bridge for VLAN1 has a bridge priority of 0 with a MAC address of 0005.00a9.2401.
The switch is running the IEEE 802.1D Spanning Tree Protocol.
Recall that a Catalyst switch can support a separate Spanning Tree instance per VLAN. This allows for load balancing between switches. For example, one switch can be the root for VLAN1, and another switch can be the root for VLAN2.