What Happens When Vlans Are Configured On A Switch

When VLANs are configured on a switch, the device transforms from a simple hub-like environment to a sophisticated network segmentation tool. This fundamental shift affects how data is forwarded, security is enforced, and overall network management is simplified. VLANs, or Virtual LANs, logically divide a physical network into multiple broadcast domains.

Introduction to VLANs and Switch Configuration

A VLAN is a logical grouping of network devices that allows them to communicate as if they were on the same physical network, regardless of their actual physical location. Think of it as drawing virtual lines to separate departments within a company, even though everyone is connected to the same switch. Without VLANs, all devices connected to a switch belong to the same broadcast domain, meaning any broadcast traffic (like ARP requests) is sent to every device. This can lead to network congestion and security vulnerabilities. Configuring VLANs on a switch mitigates these issues by isolating traffic and creating more manageable network segments.

Benefits of Using VLANs

• Enhanced Security: VLANs isolate traffic, preventing unauthorized access to sensitive data. A user on one VLAN cannot "see" or interact with traffic on another VLAN unless explicitly permitted.
• Improved Performance: By reducing the size of broadcast domains, VLANs minimize unnecessary traffic and improve network performance. Devices only receive broadcasts relevant to their VLAN.
• Simplified Network Management: VLANs make it easier to manage large networks by grouping users and devices logically. Changes can be applied to a VLAN instead of individual devices.
• Flexibility and Scalability: VLANs provide flexibility in network design and allow for easy expansion. New devices can be added to a VLAN without requiring physical rewiring.
• Cost Reduction: By logically segmenting the network, VLANs reduce the need for physical routers and switches, leading to cost savings.

The Inner Workings: What Happens When VLANs are Configured

When you configure VLANs on a switch, several key processes and changes occur that affect how the switch operates:

1. VLAN Database Creation

The first step is the creation of a VLAN database within the switch's memory. This database stores information about each VLAN, including:

• VLAN ID (VID): A unique number (typically between 1 and 4094) that identifies the VLAN. VLAN 1 is usually the default VLAN and should not be used for user traffic.
• VLAN Name: A descriptive name for the VLAN (e.g., "Marketing," "Finance," "Guest").
• Status: Indicates whether the VLAN is active or inactive.
• Associated Ports: Lists the switch ports that belong to the VLAN.

This database is critical for the switch to determine which VLAN a packet belongs to and how to forward it accordingly.

2. Port Assignment to VLANs

Each port on the switch is assigned to one or more VLANs. This is the core of VLAN configuration. There are two primary types of port assignments:

• Access Ports: These ports are assigned to a single VLAN and are typically used to connect end-user devices like computers, printers, and IP phones. Traffic entering an access port is assumed to belong to the VLAN assigned to that port.
• Trunk Ports: These ports carry traffic for multiple VLANs. They are used to connect switches to each other or to routers that perform inter-VLAN routing. Trunk ports use tagging protocols to identify which VLAN each frame belongs to.

When a frame enters an access port, the switch adds a VLAN tag to the frame's header, indicating the VLAN to which the port belongs. This tag is used by other switches to forward the frame correctly. When a frame exits an access port, the VLAN tag is removed.

3. VLAN Tagging (802.1Q)

Trunk ports use a tagging protocol, most commonly IEEE 802.1Q, to identify the VLAN to which each frame belongs. The 802.1Q standard adds a 4-byte tag to the Ethernet frame header. This tag contains:

• TPID (Tag Protocol Identifier): A 2-byte field that identifies the frame as an 802.1Q tagged frame (typically 0x8100).
• Priority Code Point (PCP): A 3-bit field used for Quality of Service (QoS) prioritization.
• Drop Eligible Indicator (DEI): A 1-bit field used to indicate whether the frame is eligible to be dropped during congestion.
• VLAN ID (VID): A 12-bit field that specifies the VLAN to which the frame belongs. This field can represent up to 4096 VLANs (0-4095), although VLANs 0 and 4095 are reserved.

When a frame enters a trunk port, the switch examines the frame to see if it is already tagged. If it is not, the switch adds an 802.1Q tag with the appropriate VLAN ID. If the frame is already tagged, the switch forwards it as is. When a frame exits a trunk port, the VLAN tag is either kept (if the destination switch needs it) or removed (if the destination is an access port).

4. Broadcast Domain Segmentation

One of the primary effects of configuring VLANs is the segmentation of the broadcast domain. Without VLANs, any broadcast traffic sent by a device is received by all other devices on the network. With VLANs, broadcast traffic is confined to the VLAN to which the sending device belongs. This significantly reduces the amount of unnecessary traffic on the network and improves performance.

For example, if a device on VLAN 10 sends a broadcast, only devices on VLAN 10 will receive it. Devices on VLAN 20 or VLAN 30 will not be affected.

5. MAC Address Table Modifications

The switch maintains a MAC address table that maps MAC addresses to switch ports and VLANs. When VLANs are configured, the MAC address table is modified to include VLAN information. This allows the switch to forward frames correctly based on both the destination MAC address and the VLAN.

Each entry in the MAC address table now includes the VLAN ID. When a frame arrives, the switch looks up the destination MAC address in the table and checks if the VLAN ID matches the VLAN of the incoming port. If they match, the switch forwards the frame to the appropriate port. If they do not match, the switch discards the frame.

6. Inter-VLAN Routing

By default, devices on different VLANs cannot communicate with each other. To enable communication between VLANs, you need to implement inter-VLAN routing. This is typically done using a router or a Layer 3 switch.

• Router-on-a-Stick: This method involves connecting a router to the switch via a trunk port. The router has subinterfaces configured for each VLAN, each with its own IP address. Traffic between VLANs is routed through the router.
• Layer 3 Switch: A Layer 3 switch can perform routing functions internally. It has virtual interfaces (Switch Virtual Interfaces or SVIs) configured for each VLAN, each with its own IP address. Traffic between VLANs is routed directly by the switch.

When a device on one VLAN needs to communicate with a device on another VLAN, it sends the traffic to its default gateway (the IP address of the SVI or router interface for its VLAN). The gateway then routes the traffic to the destination VLAN.

7. Security Policy Enforcement

VLANs provide a framework for implementing security policies. Access control lists (ACLs) can be configured on routers or Layer 3 switches to control traffic flow between VLANs. This allows you to restrict communication between sensitive VLANs and less secure VLANs.

For example, you might want to allow traffic from the "Marketing" VLAN to the "Internet" VLAN, but block traffic from the "Internet" VLAN to the "Finance" VLAN. ACLs can be configured to enforce these types of policies.

8. Spanning Tree Protocol (STP) Considerations

When using VLANs, it's important to configure Spanning Tree Protocol (STP) correctly. STP prevents loops in the network by blocking redundant paths. When VLANs are configured, you can run multiple instances of STP, one for each VLAN, or use more advanced protocols like Rapid Spanning Tree Protocol (RSTP) or Multiple Spanning Tree Protocol (MSTP).

MSTP allows you to map multiple VLANs to a single spanning tree instance, which can reduce the overhead of running separate STP instances for each VLAN.

Practical Examples and Configuration Commands

Let's look at some practical examples and configuration commands for configuring VLANs on a Cisco switch.

Basic VLAN Configuration

Switch> enable
Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name Marketing
Switch(config-vlan)# exit
Switch(config)# vlan 20
Switch(config-vlan)# name Finance
Switch(config-vlan)# exit
Switch(config)# interface FastEthernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# exit
Switch(config)# interface FastEthernet 0/2
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20
Switch(config-if)# exit
Switch(config)# interface FastEthernet 0/24
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport trunk allowed vlan 10,20
Switch(config-if)# end
Switch# show vlan brief

Explanation:

• enable: Enters privileged EXEC mode.
• configure terminal: Enters global configuration mode.
• vlan 10: Creates VLAN 10.
• name Marketing: Assigns the name "Marketing" to VLAN 10.
• interface FastEthernet 0/1: Enters interface configuration mode for interface FastEthernet 0/1.
• switchport mode access: Configures the interface as an access port.
• switchport access vlan 10: Assigns the interface to VLAN 10.
• interface FastEthernet 0/24: Enters interface configuration mode for interface FastEthernet 0/24.
• switchport mode trunk: Configures the interface as a trunk port.
• switchport trunk encapsulation dot1q: Specifies 802.1Q encapsulation for the trunk port.
• switchport trunk allowed vlan 10,20: Allows VLANs 10 and 20 on the trunk port.
• show vlan brief: Displays a summary of VLAN configuration.

Inter-VLAN Routing Configuration (Router-on-a-Stick)

Router> enable
Router# configure terminal
Router(config)# interface GigabitEthernet 0/0
Router(config-if)# no shutdown
Router(config-if)# exit
Router(config)# interface GigabitEthernet 0/0.10
Router(config-subif)# encapsulation dot1Q 10
Router(config-subif)# ip address 192.168.10.1 255.255.255.0
Router(config-subif)# exit
Router(config)# interface GigabitEthernet 0/0.20
Router(config-subif)# encapsulation dot1Q 20
Router(config-subif)# ip address 192.168.20.1 255.255.255.0
Router(config-subif)# exit
Router(config)# ip routing
Router(config)# end
Router# show ip route

Explanation:

• interface GigabitEthernet 0/0: Enters interface configuration mode for interface GigabitEthernet 0/0.
• no shutdown: Enables the interface.
• interface GigabitEthernet 0/0.10: Creates a subinterface for VLAN 10.
• encapsulation dot1Q 10: Specifies 802.1Q encapsulation for VLAN 10.
• ip address 192.168.10.1 255.255.255.0: Assigns an IP address to the subinterface.
• ip routing: Enables IP routing on the router.
• show ip route: Displays the IP routing table.

Inter-VLAN Routing Configuration (Layer 3 Switch)

Switch> enable
Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name Marketing
Switch(config-vlan)# exit
Switch(config)# vlan 20
Switch(config-vlan)# name Finance
Switch(config-vlan)# exit
Switch(config)# interface vlan 10
Switch(config-if)# ip address 192.168.10.1 255.255.255.0
Switch(config-if)# no shutdown
Switch(config-if)# exit
Switch(config)# interface vlan 20
Switch(config-if)# ip address 192.168.20.1 255.255.255.0
Switch(config-if)# no shutdown
Switch(config-if)# exit
Switch(config)# ip routing
Switch(config)# interface FastEthernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# no shutdown
Switch(config-if)# exit
Switch(config)# interface FastEthernet 0/2
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20
Switch(config-if)# no shutdown
Switch(config-if)# exit
Switch(config)# end
Switch# show ip route

Explanation:

• interface vlan 10: Creates a Switch Virtual Interface (SVI) for VLAN 10.
• ip address 192.168.10.1 255.255.255.0: Assigns an IP address to the SVI.
• no shutdown: Enables the SVI.
• ip routing: Enables IP routing on the switch.

Advanced VLAN Concepts

Beyond the basics, several advanced VLAN concepts can further enhance network functionality:

• Voice VLANs: These VLANs are specifically designed for voice traffic, prioritizing voice packets to ensure high-quality VoIP communication.
• Private VLANs (PVLANs): PVLANs provide isolation within a VLAN, allowing devices to communicate only with a gateway device, enhancing security in shared environments.
• Dynamic VLANs: These VLANs automatically assign devices to a VLAN based on their MAC address or other criteria, simplifying network management.
• VLAN Trunking Protocol (VTP): VTP is a Cisco proprietary protocol that allows VLAN information to be propagated across multiple switches, simplifying VLAN management in large networks. (Note: VTP has security considerations and should be used with caution.)

Troubleshooting VLAN Issues

When troubleshooting VLAN issues, consider the following:

• Connectivity Problems: Ensure that devices are assigned to the correct VLAN and that trunk ports are configured correctly.
• Inter-VLAN Routing Issues: Verify that the router or Layer 3 switch is configured correctly and that routing protocols are functioning properly.
• VLAN Mismatch Errors: Check for VLAN mismatches between switches, which can cause connectivity problems.
• STP Issues: Ensure that STP is configured correctly to prevent loops in the network.

Conclusion

Configuring VLANs on a switch is a fundamental aspect of modern network design. By logically segmenting the network, VLANs enhance security, improve performance, and simplify network management. Understanding the inner workings of VLANs, including VLAN tagging, broadcast domain segmentation, MAC address table modifications, and inter-VLAN routing, is essential for any network administrator. By implementing VLANs correctly, you can create a more efficient, secure, and manageable network infrastructure. The examples provided offer a starting point for configuring VLANs on Cisco switches, but always tailor the configuration to your specific network requirements. Remember to test thoroughly after making any changes to ensure proper functionality.