Understanding Virtual LANs

Understanding Virtual LANs

• Logical Segmentation/Partitioning of a switch
• Traffic can only pass to end devices of the same vlan
• Totally no traffic can pass to other vlans
• Best practice is to use a unique subnet (network) for each vlan

 How does VLAN work?

•  When a frame enters a switch, it will be tagged with a vlan information.

Creating VLANs

1. Creating a VLAN From Privilege Mode
    Switch#vlan database
    Switch(vlan)#vlan 2
    Switch(vlan)#exit


2. Creating VLAN from Global Configuration
    Switch(config)#vlan 2

Assigning Ports to a VLAN

    Switch(config)#interface fastethernet 0/1
    Switch(config-if)#switchport mode access
    Switch(config-if)#switchport access vlan 2

Note: You can also create a vlan by assigning a port to a vlan that is not yet created.

Note: The vlan information is saved in vlan.dat located in flash memory.


VLAN Creation Guidelines

• The maximum number of VLANs is switch-dependent.
• Most Cisco Catalyst desktop switches support 128 separate spanning-tree instances, one per VLAN.
• VLAN 1 is the factory default Ethernet VLAN. 
• Cisco Discovery Protocol and VTP advertisements are sent on VLAN 1.
• The Cisco Catalyst switch IP address is in the management VLAN (VLAN 1 by default).

VLAN Membership Modes

Verifying the VLANs Created

witch#show vlan

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                Fa0/21, Fa0/22, Fa0/23, Fa0/24
2    VLAN0002                         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
2    enet  100002     1500  -      -      -        -    -        0      0
1002 enet  101002     1500  -      -      -        -    -        0      0
1003 enet  101003     1500  -      -      -        -    -        0      0
1004 enet  101004     1500  -      -      -        -    -        0      0
1005 enet  101005     1500  -      -      -        -    -        0      0

 Switch#show vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                Fa0/21, Fa0/22, Fa0/23, Fa0/24
2    VLAN0002                         active  
1002 fddi-default                     active  
1003 token-ring-default               active  
1004 fddinet-default                  active  
1005 trnet-default                    active