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.
