VLANs (Virtual Local Area Networks)

vlans

What is a VLAN?

In this lesson, we will talk about one of the key lessons in networking, especially in switching. We will explain VLAN definition and we will answer the question what is a VLAN?  In the following lessons, we will learn the details of Virtual LANs and we will practice with VLAN Configuration Examples . You can also check the definion on Wiki.

 


 

VLAN Definition

If we do a simple VLAN definition, Virtual Local Area Networks are the Logical Virtual Networks that groups network devices in it. In another definition, they are a layer 2 technology with which you can seperate big networks into smaller networks. This can be done for reducing broadcast traffic, network performance improvement, security purpose or to seperate different departments each other and for network flexibility.

vlan(virtual local area network)
Basically VLAN (Virtual Local Area Network)
 

In a company, different Virtual LANs can be used for different departments. Think about that these departments are IT, HR and Finance. In a single company LAN, with Virtual LANs, each of these department networks become separate networks.

 

Virtual LANs are Logical networks. In the first place they are defined on the switches and then the ports are assigned to them. By doing this, VLANs members ports appear.


vlans
 

VLAN (Virtual Local Area Network)
We will learn the key points of Virtual LANs, but firstly, let’s check two important domain terms used in networks.

1.Virtual LAN Example

2.Virtual LAN Example

 


 

Collision and Broadcast Domains

Generally fistly two terms are learned by new engineers about computer networks. These terms are Collision Domain and Broadcast Domain. It can be good to define these terms again. Because in the VLAN lesson, these terms are ciritically important.

 

Collision domain : A single physical line that a colision can occur. Example: Hubs have one collision domain and only one connected node can make a transfer at any time. Switches collision domain number is like their port number by default.

 

Broadcast domain : A logical division of networks that all nodes can reach eachother at data link layer(layer 2). Example: Switches are one broadcast domain. Because without any restriction, if one node sends something from one port, all other ports receive it. Routers’ each port is one broadcast domain.


 

VLANs What is it?

Let’s return our main lesson again. Virtual Local Area Networks help you to build new child broadcast domains in one switch or in one broadcast domain. After Configuring VLANs, each Virtual LAN become a single broadcast domain and without routing, there is no communication between Virtual LANs.

 

There are also Collision Domains in the Virtual LANs again. Each VLAN has Collision Domain as the number of their assigned ports.

 

Here, Virtual LANs can be thinked like small switches in the main switch.

 

On Cisco switches, all the ports are the member of VLAN 1 by default. So if no VLAN Configuration done, all the ports are in the same Virtual LAN, Virtual LAN 1. And they are in the same Broadcast Domain as mentioned above.

 


 

What is Native VLAN?

By default Native VLAN is 1. By default, all untagged frames are member of it. This Native VLAN can be changed by a trunk port. For example, think about that, one trunk’s NativeVLAN is 5. Here, all the untagged and 5 tagged frames are belong to that Virtual LAN 5. Here the important point is, each end of the connection must be configured with the same configuration.

 

ISL trunks does not support the NativeVLAN and untagged frames. But dot1.q trunks supports.

 

On the other hand, Native VLAN is a security risk. To avoid this risk, NativeVLAN can be assigned to an unused port or disabled port. You can also make the trunk ports to tag the Native Virtual LAN.

 

vlan(virtual local area network)
 

In this lesson, we have talked about Virtual LAN definition, what is a VLAN simply and Native VLAN. Beside, we have remembered Collision and Broadcast Domains. In the next lessons, we will learn more on Virtual LANs and we will learn How to Configure VLANs.

 


 

Cisco VLAN Example

After learning what is a VLAN meaning and some of its details, now, it is time to configure an example for vlanes. For our cisco vlan example, we will use the below topology. Here, we will use three switches which have three PCs connected to them. We will create VLAN 10 and 20.

 

vlanes-cisco-vlan-example-vlan-meaning-ipcisco

 

To configure vlanes, we will follow the below steps one by one:

  • Setting PC ip addresses
  • Creating vlanes
  • Setting Access ports
  • Assigning vlans to access ports
  • Setting Trunk ports
  • Allow VLANs on trunks
  • Verification

 

Setting IP Addresses

The first configuration step is about ip addressing. We will configure ip addresses of the PCs as it is given on the topology. We will set only ip address and subnet mask.

 

Here, we will use 10.10.10.0/24 prefix for VLAN 10 and 20.20.20.0/24 for VLAN 20. We will give the below ip addresses to the PCs.

 

PC1: 10.10.10.1

PC2: 10.10.10.2

PC3 10.10.10.3

PC4: 10.10.10.4

PC5: 10.10.10.5

PC6: 10.10.10.6

PC7: 10.10.10.7

PC8: 10.10.10.8

PC9: 10.10.10.9

 

We will use 255.255.255.0 subnet mask.

 


 

Creating VLANes

The second thing we will do is creating vlanes. Normally, we do not need to use this step, because during access port vlan assigning, vlanes are automatically created. But here, we are writing these steps to show you how to create them manually.

 

Switch A# configure terminal

Switch A (config)# vlan 10

Switch A (config-vlan)# vlan 20

 

Switch B# configure terminal

Switch B (config)# vlan 10

Switch B (config-vlan)# vlan 20

 

Switch C# configure terminal

Switch C (config)# vlan 10

Switch C (config-vlan)# vlan 20

 


 

Setting Access Ports and Assigning VLANs

In the third step, we will set access ports and assign created vlans to these access ports. To do this, we will use the below commands on each switch.

 

Switch A (config)# interface FastEthernet0/2

Switch A (config-if)# switchport mode access

Switch A (config-if)# switchport access vlan 10

Switch A (config)# interface FastEthernet0/3

Switch A (config-if)# switchport mode access

Switch A (config-if)# switchport access vlan 10

Switch A (config)# interface FastEthernet0/4

Switch A (config-if)# switchport mode access

Switch A (config-if)# switchport access vlan 20

 

Switch B (config)# interface FastEthernet0/2

Switch B (config-if)# switchport mode access

Switch B (config-if)# switchport access vlan 20

Switch B (config)# interface FastEthernet0/3

Switch B (config-if)# switchport mode access

Switch B (config-if)# switchport access vlan 10

Switch B (config)# interface FastEthernet0/4

Switch B (config-if)# switchport mode access

Switch B (config-if)# switchport access vlan 10

 

Switch C (config)# interface FastEthernet0/2

Switch c (config-if)# switchport mode access

Switch C (config-if)# switchport access vlan 20

Switch C (config)# interface FastEthernet0/3

Switch c (config-if)# switchport mode access

Switch C (config-if)# switchport access vlan 10

Switch C (config)# interface FastEthernet0/4

Switch c (config-if)# switchport mode access

Switch C (config-if)# switchport access vlan 10

 


 

Setting Trunk Ports and Allowing VLANs

After setting access ports, now it is time to configure trunk ports. To do this, we will use the below commands under trunk interfaces. On Switch C, there are two trunk interfaces, so we will configure two interface as trunks.

 

Switch A (config)# interface FastEthernet0/1

Switch A (config-if)# switchport mode trunk

Switch A (config-if)# switchport nonegotiate

Switch A (config-if)# switchport trunk allowed vlan 10,20

 

Switch B (config)# interface FastEthernet0/1

Switch B (config-if)# switchport mode trunk

Switch B (config-if)# switchport nonegotiate

Switch B (config-if)# switchport trunk allowed vlan 10,20

 

Switch C (config)# interface FastEthernet0/1

Switch C (config-if)# switchport mode trunk

Switch C (config-if)# switchport nonegotiate

Switch C (config-if)# switchport trunk allowed vlan 10,20

Switch C (config-if)# interface FastEthernet0/2

Switch C (config-if)# switchport mode trunk

Switch C (config-if)# switchport nonegotiate

Switch C (config-if)# switchport trunk allowed vlan 10,20

 


 

VLAN Verification

At last step, we will verify our configuration. To do this, we will use “show vlan brief” and “show interface trunk” commands for the verification.

 

Switch A# show vlan brief

VLAN Name                             Status    Ports

—- ——————————– ——— ——————————-

1    default                          active    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

Gig0/1, Gig0/2

10   VLAN0010                         active    Fa0/2, Fa0/3

20   VLAN0020                         active    Fa0/4

1002 fddi-default                     active

1003 token-ring-default               active

1004 fddinet-default                  active

1005 trnet-default                    active

 

Switch A# show interfaces trunk

Port        Mode         Encapsulation  Status        Native vlan

Fa0/1       on           802.1q         trunking      1

Port        Vlans allowed on trunk

Fa0/1       10,20

Port        Vlans allowed and active in management domain

Fa0/1       10,20

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/1       10,20

 

 

Switch B# show vlan brief

VLAN Name                             Status    Ports

—- ——————————– ——— ——————————-

1    default                          active    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

Gig0/1, Gig0/2

10   VLAN0010                         active    Fa0/3, Fa0/4

20   VLAN0020                         active    Fa0/2

1002 fddi-default                     active

1003 token-ring-default               active

1004 fddinet-default                  active

1005 trnet-default                    active

 

Switch B# show interfaces trunk

Port        Mode         Encapsulation  Status        Native vlan

Fa0/1       on           802.1q         trunking      1

Port        Vlans allowed on trunk

Fa0/1       10,20

Port        Vlans allowed and active in management domain

Fa0/1       10,20

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/1       10,20

 

Switch C# show vlan brief

VLAN Name                             Status    Ports

—- ——————————– ——— ——————————-

1    default                          active    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, Gig0/1

Gig0/2

10   VLAN0010                         active    Fa0/3, Fa0/4

20   VLAN0020                         active    Fa0/5

1002 fddi-default                     active

1003 token-ring-default               active

1004 fddinet-default                  active

1005 trnet-default                    active

 

Switch C# show interfaces trunk

Port        Mode         Encapsulation  Status        Native vlan

Fa0/1       on           802.1q         trunking      1

Fa0/2       on           802.1q         trunking      1

Port        Vlans allowed on trunk

Fa0/1       10,20

Fa0/2       10,20

Port        Vlans allowed and active in management domain

Fa0/1       10,20

Fa0/2       10,20

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/1       10,20

Fa0/2       10,20

 

Beside we will ping PCs in the same VLAN but connected to different VLANs. For example, we will ping from PC1 to PC5 or PC3 to PC4.

 

Firstly, let’s ping from PC 1 to PC 5 and PC8. These PCs are in VLAN 10.

 

C:\> ping 10.10.10.5

Pinging 10.10.10.5 with 32 bytes of data:

Reply from 10.10.10.5: bytes=32 time<1ms TTL=128

Reply from 10.10.10.5: bytes=32 time=1ms TTL=128

Reply from 10.10.10.5: bytes=32 time<1ms TTL=128

Reply from 10.10.10.5: bytes=32 time<1ms TTL=128

Ping statistics for 10.10.10.5:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 1ms, Average = 0ms

 

 

C:\> ping 10.10.10.8

Pinging 10.10.10.8 with 32 bytes of data:

Reply from 10.10.10.8: bytes=32 time<1ms TTL=128

Reply from 10.10.10.8: bytes=32 time=1ms TTL=128

Reply from 10.10.10.8: bytes=32 time<1ms TTL=128

Reply from 10.10.10.8: bytes=32 time<1ms TTL=128

Ping statistics for 10.10.10.8:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 1ms, Average = 0ms

 

Then, let’s ping from PC3 to PC4 and PC9. These PCs are in VLAN 20.

 

C:\> ping 20.20.20.4

Pinging 20.20.20.4 with 32 bytes of data:

Reply from 20.20.20.4: bytes=32 time=1ms TTL=128

Reply from 20.20.20.4: bytes=32 time<1ms TTL=128

Reply from 20.20.20.4: bytes=32 time<1ms TTL=128

Reply from 20.20.20.4: bytes=32 time=1ms TTL=128

Ping statistics for 20.20.20.4:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 1ms, Average = 0ms

 

C:\> ping 20.20.20.9

Pinging 20.20.20.9 with 32 bytes of data:

Reply from 20.20.20.9: bytes=32 time<1ms TTL=128

Reply from 20.20.20.9: bytes=32 time<1ms TTL=128

Reply from 20.20.20.9: bytes=32 time=2ms TTL=128

Reply from 20.20.20.9: bytes=32 time=1ms TTL=128

Ping statistics for 20.20.20.9:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 2ms, Average = 0ms

 

Lesson tags: VLANs, switching
Back to: CCNA 200-301 > VLANs

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