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CCNA – Semester 3

Chapter 3: VLANs

CCNA Exploration 4.0

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Objectives

• Explain the role of VLANs in a network

• Explain the role of trunking VLANs in a network

• Configure VLANs on the switches in a network

topology

• Troubleshoot the common software or hardware

configuration problems associated with VLANs on

switches in a network topology

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Introducing VLANs

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Introducing VLANs

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What is a VLAN?

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What is a VLAN?

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Benefits of a VLAN

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Introducing VLANs

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Types of VLANs

• Today there is essentially one way of implementing VLANs -

port-based VLANs. A port-based VLAN is associated with a

port called an access VLAN.

• However in the network there are a number of terms for

VLANs. Some terms define the type of network traffic they

carry and others define a specific function a VLAN performs.

VLAN Types:

• Data VLAN:

– Is a VLAN that is configured to carry only user-generated

traffic

– A VLAN could carry voice-based traffic or traffic used to

manage the switch, but this traffic would not be part of a

data VLAN.

– Sometimes referred to as a user VLAN

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Types of VLANs

• Default VLAN:

– All switch ports become a member of the default VLAN

after the initial boot up of the switch.

– The default VLAN for Cisco switches is VLAN 1, you

cannot rename it and you can not delete it.

– Layer 2 control traffic, such as CDP and spanning tree

protocol traffic, will always be associated with VLAN 1 -

this cannot be changed.

– It is a security best practice to change the default VLAN to

a VLAN other than VLAN 1

• Note: Some network administrators use the term "default VLAN" to mean

a VLAN other than VLAN 1 defined by the network administrator as the

VLAN that all ports are assigned to when they are not in use. In this

case, the only role that VLAN 1 plays is that of handling Layer 2 control

traffic for the network.

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Types of VLANs

• Native VLAN:

– A native VLAN is assigned to an 802.1Q trunk port. An

802.1Q trunk port supports traffic coming from many

VLANs (tagged traffic) as well as traffic that does not

come from a VLAN (untagged traffic). The 802.1Q trunk

port places untagged traffic on the native VLAN.

– Native VLANs are set out in the IEEE 802.1Q

specification to maintain backward compatibility with

untagged traffic common to legacy LAN scenarios.

– It is a best practice to use a VLAN other than VLAN 1 as

the native VLAN.

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Types of VLANs

• Management VLAN

– A management VLAN is any VLAN you configure to

access the management capabilities of a switch.

– VLAN 1 would serve as the management VLAN if you did

not proactively define a unique VLAN to serve as the

management VLAN.

– You assign the management VLAN an IP address and

subnet mask. A switch can be managed via HTTP,

Telnet, SSH, or SNMP.

Since the out-of-the-box configuration of a Cisco switch has

VLAN 1 as the default VLAN, you see that VLAN 1 would be

a bad choice as the management VLAN.

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Types of VLANs

• Voice VLANs

VoIP traffic requires:

– Assured bandwidth to ensure voice quality

– Transmission priority over other types of network traffic

– Ability to be routed around congested areas on the network

– Delay of less than 150 milliseconds (ms) across the network

A Cisco Phone is a Switch: contains an integrated three-port 10/100 switch:

– Port 1 connects to the switch or other voice-over-IP (VoIP) device.

– Port 2 is an internal 10/100 interface that carries the IP phone traffic.

– Port 3 (access port) connects to a PC or other device.

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Types of VLANs

• Voice VLAN

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Types of VLANs

• Cisco IP Phone

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Network Traffic Types

• Network Management and Control Traffic

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Network Traffic Types

• IP Telephony

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Network Traffic Types

• IP Multicast

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Network Traffic Types

• Normal Data

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Switch Port Membership Modes

• Switch ports are Layer 2-only interfaces associated with a physical port.

A port can be configured to support these VLAN types:

• Static VLAN - Ports on a switch are manually assigned to a VLAN.

• Dynamic VLAN - This mode is not widely used in production networks and is not explored in this course. However, it is useful to know what a dynamic VLAN is. A dynamic port VLAN membership is configured using a special server called a VLAN Membership Policy Server (VMPS), based on the source MAC address of the device connected to the port.

• Voice VLAN - A port is configured to be in voice mode so that it can support an IP phone attached to it. Before you configure a voice VLAN on the port, you need to first configure a VLAN for voice and a VLAN for data.

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Switch Port Membership Modes

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Switch Port Membership Modes

• Voice Mode configuration

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Controlling Broadcast Domains with VLANs

• Without VLANs: In normal operation, when a switch

receives a broadcast frame on one of its ports, it forwards

the frame out all other ports on the switch.

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Controlling Broadcast Domains with VLANs

• With VLANs: the broadcast frame arrives at the only other

computer in the network configured on the same VLAN

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Controlling Broadcast Domains with

Switches and Routers

Intra-VLAN Communication

• PC1 want to communicate with PC4.

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Controlling Broadcast Domains with

Switches and Routers

Inter-VLAN Communication

• PC1 want to communicate with PC5.

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Controlling Broadcast Domains with VLANs

and Layer 3 Forwarding

SVI: switch virtual interface

• SVI is a logical interface configured for a specific VLAN. You

need to configure an SVI for a VLAN if you want to route

between VLANs or to provide IP host connectivity to the

switch. By default, an SVI is created for the default VLAN

(VLAN 1) to permit remote switch administration.

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Controlling Broadcast Domains with VLANs

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VLAN Trunking

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VLAN Trunks

• It is hard to describe VLANs without mentioning VLAN

trunks.

• A trunk is a point-to-point link between one or more Ethernet

switch interfaces and another networking device, such as a

router or a switch.

• Ethernet trunks carry the traffic of multiple VLANs over a

single link.

• A VLAN trunk allows you to extend the VLANs across an

entire network. Cisco supports IEEE 802.1Q for coordinating

trunks on Fast Ethernet and Gigabit Ethernet interfaces.

• A VLAN trunk does not belong to a specific VLAN, rather it is

a conduit for VLANs between switches and routers.

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VLAN Trunks

• Without VLAN Trunks

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VLAN Trunks

• With VLAN Trunks

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VLAN Trunks

802.1Q Frame Tagging

• Switches are layer 2 devices, use the Ethernet frame header

information to forward packets. The frame header does not

contain information about which VLAN the frame should

belong to.

• When Ethernet frames are placed on a trunk they need

additional information about the VLANs they belong to. This

is accomplished by using the 802.1q encapsulation header.

This header adds a tag to the original Ethernet frame

specifying the VLAN for which the frame belongs to.

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VLAN Trunks

• VLAN Frame Tagging

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Native VLANs and 802.1Q Trunking

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Native VLANs and 802.1Q Trunking

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Trunking Operation

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Trunking Modes

• Although a Cisco switch can be configured to support two types of

trunk ports, IEEE 802.1Q and ISL, today only 802.1Q is used.

• 802.1Q: An IEEE 802.1Q trunk port supports simultaneous tagged

and untagged traffic. An 802.1Q trunk port is assigned a default

PVID, and all untagged traffic travels on the port default PVID. All

untagged traffic and tagged traffic with a null VLAN ID are

assumed to belong to the port default PVID. A packet with a VLAN

ID equal to the outgoing port default PVID is sent untagged. All

other traffic is sent with a VLAN tag.

• ISL (Inter-Switch Link): In an ISL trunk port, all received packets

are expected to be encapsulated with an ISL header, and all

transmitted packets are sent with an ISL header. Native (non-

tagged) frames received from an ISL trunk port are dropped.

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Trunking Modes

DTP (Dynamic Trunking Protocol )

• A Cisco proprietary protocol

• DTP manages trunk negotiation only if the port on the other

switch is configured in a trunk mode that supports DTP. DTP

supports both ISL and 802.1Q trunks.

• Some Cisco switches and routers do not support DTP

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Configure VLANs and Trunks

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Configuring VLANs &Trunks Overview

Use the following steps to configure and verify VLANs and

trunks on a switch network:

1. Create the VLANs

2. Assign switch ports to VLANs statically

3. Verify VLAN configuration

4. Enable trunking on the inter-switch connections

5. Verify trunk configuration

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Configure a VLAN

• Add a VLAN

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Configure a VLAN

• Assign a Switch Port

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Configure a VLAN

• Verification

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Managing VLANs

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Managing VLANs

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Managing VLANs

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Managing VLANs

• Reassign a Port to VLAN 1

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Managing VLANs

• Delete VLANs:

– No vlan vlan-id

• You can delete vlan database:

– Delete flash:vlan.dat

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Configure a Trunk

• Note: (config-if)# switchport trunk encapsultation

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Configure a Trunk

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Configure a Trunk

• Verify

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Configure a Trunk

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Configure a Trunk

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Troubleshooting VLANs and Trunks

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Common Problems witch Trunks

• Use: show interfaces trunk command

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Common Problem with VLAN configurations

VLAN and IP Subnets

• Each VLAN must correspond to a unique IP subnet. If two devices in the

same VLAN have different subnet addresses, they cannot communicate.

This type of incorrect configuration is a common problem, and it is easy

to solve by identifying the offending device and changing the subnet

address to the correct one.

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Troubleshooting

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Summary

• VLANs separate broadcast domains on switches.

• VLANs improve network performance, management, and security.

• VLAN can be used for data, voice, network protocol and network

management traffic.

• There are 3 different membership modes: Static, Dynamic, and Voice

VLAN mode.

• Routers or Layer 3 switches are required for inter-VLAN communication.

• Trunks allow multiple VLANs to traverse a single link to simplify intra-

VLAN communication across multiple switches.

• IEEE 802.1Q is the standard trunking protocol

• 802.1Q uses a process of frame tagging to keep VLAN traffic separate as

it traverse the trunk link.

• 802.1Q does not tag native VLAN traffic, which can result in problems

when trunking is misconfigured.