Switch Working--2 : VLAN

In this section we will try to cover the following questions

• what is Vlan?
• what are the features of vlan?
• what is the range of normal vlan?
• what is the range of extended vlan?
• what are the restriction while configuring an extended vlan?
• How to remove a existing vlan?

VLAN:- (Virtual LANs) are logical grouping of devices in the same broadcast domain. Each VLAN being treated as its own subnet or broadcast domain. This means that frames broadcasted onto the network will be switched only between the ports within the same vlan, in other word we can say

Virtual LAN, i.e something virtual, which in not in real, i.e “VLAN is the method of breaking a single switch to act as a multiple switch”, for example if I create multiple vlan (vlan-1 and vlan-2) and assign some port on each vlan (let port no. 1-to-5 assign in vlan-1 and port no 6 to 10 are assign in vlan-2) then the user connected within vlan-1 could communicate to each other but not with users connected with vlan-2. "communication will happen within the usergroup connected with same vlan "

Features of Vlan

• Virtual vlan is independent vlan
• Vlan can be named on the basis of user group
• It is the method of reducing Broadcast traffic at layer-2
• Grouping of special users, enhance the security of network, how? because of newly connected user could be the part of another vlan.

Depending on the range and requirement, vlan are as:-

• Normal VLAN
• Extended VLAN
• Voice VLAN
• Native VLAN

Normal VLAN

• VLAN 1 is the default vlan and can’t be renamed
• by default all ports are the part of vlan-1
• VLAN 2-1001 can be created and basically used for Ethernet network
• VLAN 1002-1005 are reserved for FDDI/ token ring etc  networks
• Default range of VLANs is 1024 (Cisco Catalyst switches supported only upto 1024 VLANs)
• ISL used 10-bit VLAN id  (upto 1024 Vlan)

Extended VLAN

• 802.1Q include a 12-bit Vlan ID field (upto 4096 Vlans)
• Cisco refers to the VLANs between 1025 and 4096 as extended –range VLANs

i.e Basically Vlan range from 1025-4096 is called the extended vlan,These extended vlan range is supported on some of the specific platforms and we can use these extended vlan range for adding more vlan into our networks, most of the catalyst switches use these Extended vlans under some restrictions and these are as:

Restrictions:-1

• ·   VTP cannot be used for VLAN management (vtp must be configured as transparent or off)

·   i.e Extended vlan range will not work if VTP mode is other then transparent mode or off mode, If we tried to create a extended vlan in server mode , we get the message as shown below

Sw1(config)# vtp mode server

Sw1(config)#vlan 3500

Sw1(config)#exit

% fail to create vlan 3500

Extended vlan(s) not allowed in current VTP mode

%failed to commit extended VLANs changes.

so if we are using the extended vlan , we have to use VTP  either transparent mode or off mode, so if we create a extended vlan in transparent mode we would be able to create the vlan without any problem.

Sw1(config)# vtp mode transparent

Sw1(config)#vlan 3500

Sw1(config)#management

Sw1(config)#exit

Result can be seen by as :-

Sw1(config)#show vlan

Restrictions:-2

• STP – extended system ID feature has to be enabled, by default it is enabled and can’t be disabled.
• Extended system id is the combination of priority + vlan information, i.e  when stp going to select the root bridge it select the priority value and add the vlan no. which is called as extended system id.

It can be verified by

SW1# show spanning-tree summary

           Switch#sh spanning-tree sum

           Switch is in pvst mode

           Root bridge for:

           Extended system ID is ----------------- enabled

Portfast Default is -------------------------- disabled

PortFast BPDU Guard Default is ------- disabled

Portfast BPDU Filter Default is ----------disabled

Loopguard Default is -----------------------disabled

EtherChannel misconfig guard is -------disabled

UplinkFast is ----------------------------------disabled

BackboneFast is -----------------------------disabled

Configured Pathcost method used is short

It can’t be removed by command

SW1# no spanning-tree extend system-id--------can’t be removed error message shows

So overall conclusion is for extended vlan is to have a

• vtp in either on transparent mode or off mode
• the system must have extended system id support

Vlan Configuration :-
older method:- vlan could be created in privileged mode

           Switch#vlan database
           Switch(Vlan)#vlan 2 name Management
           now the new vlan with name Management is created, to see name and No. of vlan 
           Switch(vlan)#exit
           Switch#show vlan-----------------vlan trouble shooting command
Note :- this is the older method of creating vlan, may be could't available for new devices a

Newer Method:- vlan created in global configuration mode

Switch>en

Switch#config t

           Enter configuration commands, one per line. End with CNTL/Z

   Creating a vlan

           Switch(config)#vlan 10----------------------------new vlan with vlan ID 10

Switch(config-vlan)#name swan-----------------swan is the name of created vlan-10

Switch(config-vlan)#vlan 20

Switch(config-vlan)#name Nic---------------- Nic is the name of vlan 20

Assigning port to vlan,

Switch(config-vlan)#in fa 0/23

Switch(config-if)#switchport mode access-----By default all ports are in dynamic mode

Switch(config-if)#switchport access vlan 10----Assigning a port no.fa 0/23 to vlan 10

similarly 

Switch(config-if)#interface fa 0/24

Switch(config-if)#switchport mode access

Switch(config-if)#switchport voice vlan 20
Switch(config-if)#show vlan -------------------------vlan troubleshooting 

To Remove Vlan:-  if by mistake we have created a wrong vlan then following command is used to remove existing vlan,

Switch>en

Switch#configure terminal

           Switch(config)#No vlan 10----------------------------to delete vlan10

Switch(config-vlan)# do wr me 

Note:- if we delete the existing vlan which have some ports , then after deleting vlan all port will be the part of default vlan i.e vlan 1.

In Next section we will cover the voice vlan

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