Introduction
A VLAN (Virtual Local Area Network) is a way to break a large network into smaller networks. This is helpful because if all devices are in one big network, it can become slow and unsafe. With VLANs, devices can be grouped, like putting IT in one VLAN, HR in another, and Guests in a separate one.
Even though they share the same switches and cables, VLANs keep their traffic apart. This makes the network faster, safer, and easier to manage. Setting up VLANs on switches helps keep the network organized and ready to support more devices as the network grows.
What is a VLAN?
A VLAN (Virtual Local Area Network) is a way to break a big network into smaller networks. Devices in the same VLAN can talk to each other as if they are connected to the same cable, even if they are far away or on different switches. VLANs are not about where the devices are physically connected, but how the network is set up. This lets groups like IT, HR, and Sales stay in their own networks while still being part of the main network.
Key Benefits of VLANs
Each VLAN in a switched network is linked to an IP network. This means when designing VLANs, you need to plan how IP addresses will be used across the whole network. This is called hierarchical network addressing. It means you give network numbers to VLANs or segments in an organized way, so the entire network works smoothly. For example, a block of IP addresses can be set aside for devices in one part of the network and another block for a different part.
Benefits of Designing a Network with VLANs.
Smaller Broadcast Domains
Dividing the network into groups like IT, HR, Sales, and Guest makes sure that only the devices in each group see each other's broadcasts. For example, IT computers only receive IT traffic, HR only sees HR traffic, and so on. This reduces extra noise on the network.
Improved Security
When users are in groups like IT, HR, Sales, or Guest, they can only talk directly to others in the same group. If IT needs to send data to HR, it must pass through a router that can block or allow the traffic. This separation makes the network safer.
Improved IT Efficiency
Organizing devices into groups like IT, HR, Sales, and Guest makes the network easier to manage. People with similar needs are in the same group, and each group can be clearly named, making it simple for administrators to identify and control them.
Reduced Cost
Using groups such as IT, HR, Sales, and Guest saves money because it reduces the need for expensive new hardware. The existing network works more efficiently, so upgrades are not always necessary.
Better Performance
When the network is divided into groups, less unnecessary traffic is shared. For example, IT does not have to process HR traffic, and Sales does not have to process Guest traffic. This keeps the network faster and more reliable.
Understanding VLAN Types
Default VLAN
On a Cisco switch, the default VLAN is VLAN 1. This means that all switch ports belong to VLAN 1 unless you change them to another VLAN. By default, all Layer 2 control traffic uses VLAN 1.
Key points about VLAN 1:
All ports are in VLAN 1 by default.
The native VLAN is VLAN 1 by default.
The management VLAN is VLAN 1 by default.
VLAN 1 cannot be renamed or deleted.
For example, if you check the switch using show vlan brief, you will see that all ports are on VLAN 1. If no other VLANs are set and the native VLAN is the same as the management VLAN, it can create a security risk.
show vlan brief
Data VLAN
A Data VLAN is used to keep user traffic separate on a network. It is also called a user VLAN because it groups users or devices into their own sections. In most modern networks, there are many Data VLANs depending on what the organization needs. Data VLANs should only carry user traffic—voice traffic and network management traffic should not be sent on them.
Native VLAN
When traffic moves between switches, it is usually tagged with a VLAN ID so the switch knows which VLAN it belongs to. This is done on trunk ports. An 802.1Q trunk port adds a 4-byte tag to the Ethernet frame to show the VLAN number.
Sometimes, a switch needs to send traffic without a tag. This untagged traffic can be made by the switch itself or come from older devices. On an 802.1Q trunk port, untagged traffic is placed into the native VLAN. By default, Cisco switches use VLAN 1 as the native VLAN.
For better security, it is recommended not to use VLAN 1 as the native VLAN. Instead, choose another unused VLAN and set it as the native VLAN on all trunk ports. Many networks use one dedicated VLAN just for this purpose.
Management VLAN
A Management VLAN is a special VLAN used only for network management traffic. This includes things like SSH, Telnet, HTTPS, HTTP, and SNMP, which are used by administrators to control and monitor the network. By default, on a Layer 2 switch, VLAN 1 is set as the management VLAN.
What are VLAN Trunks?
VLANs would not work well without trunks. VLAN trunks let traffic from all VLANs travel between switches. This allows devices on different switches but in the same VLAN to talk to each other without needing a router.
A trunk is a direct link between two network devices that can carry traffic from many VLANs. It helps extend VLANs across the whole network. Cisco uses the IEEE 802.1Q standard for trunks on Fast Ethernet, Gigabit Ethernet, and 10-Gigabit Ethernet links.
A trunk does not belong to just one VLAN. Instead, it carries traffic for multiple VLANs between switches, routers, or even servers with special 802.1Q network cards. On Cisco Catalyst switches, a trunk port supports all VLANs by default.
For example, in the figure, the trunk links between switches S1–S2 and S1–S3 carry traffic for VLANs 10, 20, 30, and VLAN 99 (native VLAN). Without VLAN trunks, this network would not work.
A Network Without VLANs
If a switch gets a broadcast message on one of its ports, it sends that message out to all other ports except the one it came from.
In the example, the whole network is in the same subnet (172.16.10.0/24) and no VLANs are set up. So, when the IT computer (PC1) sends a broadcast message, switch S2 sends it to all its ports. Eventually, every device in the network receives the broadcast because the entire network is one big broadcast domain.
A Network with VLANs
VLANs are set up on individual switch ports. The devices connected to those ports don’t know about VLANs—they just use IP addresses and belong to a specific IP network. A VLAN is like an IP network or subnet. VLANs are configured on the switch, while IP addresses are set on the devices.
In the example, the network is split into two VLANs: IT and HR. Devices for the IT department are in VLAN IT, and devices for HR are in VLAN HR. When the IT computer sends a broadcast message, the switch only sends it to ports in VLAN IT. Devices in VLAN HR do not receive the message.
VLAN Traffic Forwarding
The links between switches S2–S1 (ports F0/1) and S1–S3 (ports F0/3) are trunks. They are set up to carry traffic for all VLANs in the network.
When switch S1 gets a broadcast message on port F0/1, it sends that message out only through port F0/3, which also supports VLAN 10. Then, when switch S3 receives the message on port F0/3, it forwards it only to port F0/11, which is also in VLAN 10. Finally, the broadcast reaches PC4, the only other computer in VLAN 10.
With VLANs, any traffic (unicast, multicast, or broadcast) sent by a device in one VLAN stays inside that VLAN and is not shared with devices in other VLANs.
Conclusion
VLANs are very useful in modern networks. They make the network more secure, faster, and easier to manage. By splitting one big network into smaller groups, VLANs reduce extra traffic and keep information safe.
Without VLANs, the whole network is one big group, so every device gets all the traffic, which causes problems. With VLANs, each group, like IT, HR, or Sales, has its own network, even though they share the same switches. Trunks help these VLANs work across many switches.
In short, VLANs are important for creating a safe, organized, and efficient network.
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