Module 3: VLANs
Switching, Routing, and Wireless Essentials v7.0 (SRWE)
Module Objectives
Module Title: Protocols and Models
Module Objective: Explain how network protocols enable devices to access local and remote network resources.
Topic Title
Topic Objective
Overview of VLANs
Explain the purpose of VLANs in a switched network.
VLANs in a Multi-Switched Environment
Explain how a switch forwards frames based on VLAN configuration in a multi-switch environment.
VLAN Configuration
Configure a switch port to be assigned to a VLAN based on requirements.
VLAN Trunks
Configure a trunk port on a LAN switch.
Dynamic Trunking Protocol
Configure Dynamic Trunking Protocol (DTP).
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 2
3.1 Overview of VLANs
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 3
Overview of VLANs VLAN Definitions
VLANs are logical connections with other similar devices.
Placing devices into various VLANs have the following characteristics:
• •
Provides segmentation of the various groups of devices on the same switches
Provide organization that is more manageable
• Broadcasts, multicasts and unicasts are isolated in the individual VLAN
• Each VLAN will have its own unique range of IP addressing
• Smaller broadcast domains
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 4
Overview of VLANs
Benefits of a VLAN Design
Benefits of using VLANs are as follows:
Benefits
Smaller Broadcast Domains
Improved Security
Improved IT Efficiency
Reduced Cost
Better Performance
Simpler Management
Description
Dividing the LAN reduces the number of broadcast domains
Only users in the same VLAN can communicate together
VLANs can group devices with similar requirements, e.g. faculty vs. students
One switch can support multiple groups or VLANs
Small broadcast domains reduce traffic, improving bandwidth
Similar groups will need similar applications and other network resources
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 5
Overview of VLANs Types of VLANs
Default VLAN
VLAN 1 is the following:
• The default VLAN
• The default Native VLAN
• The default Management VLAN
• Cannot be deleted or renamed
Note: While we cannot delete VLAN1 Cisco will recommend that we assign these default features to other VLANs
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 6
Overview of VLANs
Types of VLANs (Cont.)
Data VLAN
• Dedicated to user-generated traffic (email and web traffic).
• VLAN 1 is the default data VLAN because all interfaces are assigned to this VLAN. Native VLAN
• This is used for trunk links only.
• All frames are tagged on an 802.1Q trunk link except for those on the native VLAN. Management VLAN
• This is used for SSH/Telnet VTY traffic and should not be carried with end user traffic. • Typically, the VLAN that is the SVI for the Layer 2 switch.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 7
Overview of VLANs
Types of VLANs (Cont.)
Voice VLAN
•
A separate VLAN is required because Voice traffic requires:
• Assured bandwidth
• High QoS priority
• Ability to avoid congestion
• Delay less that 150 ms from source to destination
The entire network must be designed to support voice.
•
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 8
3.2 VLANs in a Multi-Switched Environment
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 9
VLANs in a Multi-Switched Environment Defining VLAN Trunks
A trunk is a point-to-point link between two network devices.
Cisco trunk functions:
• Allow more than one VLAN
• Extend the VLAN across the entire network
• By default, supports all VLANs • Supports 802.1Q trunking
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 10
VLANs in a Multi-Switched Environment Networks without VLANs
Without VLANs, all devices connected to the switches will receive all unicast, multicast, and broadcast traffic.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 11
VLANs in a Multi-Switched Environment Networks with VLANs
With VLANs, unicast, multicast, and broadcast traffic is confined to a VLAN. Without a Layer 3 device to connect the VLANs, devices in different VLANs cannot communicate.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 12
VLANs in a Multi-Switched Environment VLAN Identification with a Tag
• The IEEE 802.1Q header is 4 Bytes
• When the tag is created the FCS must be
recalculated.
• When sent to end devices, this tag must be removed and the FCS recalculated back to its original number.
802.1Q VLAN Tag Field
Function
Type
• 2-Byte field with hexadecimal 0x8100
• This is referred to as Tag Protocol ID (TPID)
User Priority
• 3-bit value that supports
Canonical Format Identifier (CFI)
• 1-bit value that can support token ring frames on Ethernet
VLAN ID (VID)
• 12-bit VLAN identifier that can support up to 4096 VLANs
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 13
VLANs in a Multi-Switched Environment
Native VLANs and 802.1Q Tagging
802.1Q trunk basics:
• Tagging is typically done on all VLANs.
• The use of a native VLAN was designed for legacy use, like the hub in the example.
• Unless changed, VLAN1 is the native VLAN.
• Both ends of a trunk link must be configured
with the same native VLAN.
• Each trunk is configured separately, so it is possible to have a different native VLANs on separate trunks.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 14
VLANs in a Multi-Switched Environment Voice VLAN Tagging
The VoIP phone is a three port switch:
• The switch will use CDP to inform the phone of the Voice VLAN.
• The phone will tag its own traffic (Voice) and can set Cost of Service (CoS). CoS is QoS for layer 2.
• The phone may or may not tag frames from the PC.
Traffic
Tagging Function
Voice VLAN
tagged with an appropriate Layer 2 class of service (CoS) priority value
Access VLAN
can also be tagged with a Layer 2 CoS priority value
Access VLAN
is not tagged (no Layer 2 CoS priority value)
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 15
VLANs in a Multi-Switched Environment
Voice VLAN Verification Example
The show interfaces fa0/18 switchport command can show us both data and voice VLANs assigned to the interface.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 16
3.3 VLAN Configuration
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 17
VLAN Configuration
VLAN Ranges on Catalyst Switches
Catalyst switches 2960 and 3650 support over 4000 VLANs.
Normal Range VLAN 1 – 1005
Extended Range VLAN 1006 – 4095
Used in Small to Medium sized businesses
Used by Service Providers
1002 – 1005 are reserved for legacy VLANs
Are in Running-Config
1, 1002 – 1005 are auto created and cannot be deleted
Supports fewer VLAN features
Stored in the vlan.dat file in flash
Requires VTP configurations
VTP can synchronize between switches
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 18
VLAN Configuration
VLAN Creation Commands
VLAN details are stored in the vlan.dat file. You create VLANs in the global configuration mode.
Task
IOS Command
Enter global configuration mode.
Switch# configure terminal
Create a VLAN with a valid ID number.
Switch(config)# vlan vlan-id
Specify a unique name to identify the VLAN.
Switch(config-vlan)# name vlan-name
Return to the privileged EXEC mode.
Switch(config-vlan)# end
Enter global configuration mode.
Switch# configure terminal
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 19
VLAN Configuration
VLAN Creation Example
• If the Student PC is going to be in VLAN 20, we will create the VLAN first and then name it.
• If you do not name it, the Cisco IOS will give it a default name of vlan and the four digit number of the VLAN. E.g. vlan0020 for VLAN 20.
Prompt
Command
S1#
Configure terminal
S1(config)#
vlan 20
S1(config-vlan)#
name student
S1(config-vlan)#
end
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 20
VLAN Configuration
VLAN Port Assignment Commands
Once the VLAN is created, we can then assign it to the correct interfaces.
Task
Command
Enter global configuration mode.
Switch# configure terminal
Enter interface configuration mode.
Switch(config)# interface interface-id
Set the port to access mode.
Switch(config-if)# switchport mode access
Assign the port to a VLAN.
Switch(config-if)# switchport access vlan vlan-id
Return to the privileged EXEC mode.
Switch(config-if)# end
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 21
VLAN Configuration
VLAN Port Assignment Example
We can assign the VLAN to the port interface.
• Once the device is assigned the VLAN, then the end device will need the IP address information for that VLAN
• Here, Student PC receives 172.17.20.22
Prompt
Command
S1#
Configure terminal
S1(config)#
Interface fa0/18
S1(config-if)#
Switchport mode access
S1(config-if)#
Switchport access vlan 20
S1(config-if)#
end
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 22
VLAN Configuration
Data and Voice VLANs
An access port may only be assigned to one data VLAN. However it may also be assigned to one Voice VLAN for when a phone and an end device are off of the same switchport.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 23
VLAN Configuration
Data and Voice VLAN Example
• We will want to create and name both Voice and Data VLANs.
• In addition to assigning the data VLAN, we will also assign the Voice VLAN and turn on QoS for the voice traffic to the interface.
• The newer catalyst switch will automatically create the VLAN, if it does not already exist, when it is assigned to an interface.
Note: QoS is beyond the scope of this course. Here we do show the use of the mls qos trust [cos | device cisco-phone | dscp | ip- precedence] command.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 24
VLAN Configuration
Verify VLAN Information
Use the show vlan command. The complete syntax is:
show vlan [brief | id vlan-id | name vlan-name | summary]
Task
Command Option
Display VLAN name, status, and its ports one VLAN per line.
brief
Display information about the identified VLAN ID number.
id vlan-id
Display information about the identified VLAN name. The vlan-name is an ASCII string from 1 to 32 characters.
name vlan-name
Display VLAN summary information.
summary
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 25
VLAN Configuration
Change VLAN Port Membership
There are a number of ways to change VLAN membership:
• re-enter switchport access vlan vlan-id command
• use the no switchport access vlan to place interface back in VLAN 1
Use the show vlan brief or the show interface fa0/18 switchport commands to verify the correct VLAN association.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 26
VLAN Configuration Delete VLANs
Delete VLANs with the no vlan vlan-id command.
Caution: Before deleting a VLAN, reassign all member ports to a different VLAN. • Delete all VLANs with the delete flash:vlan.dat or delete vlan.dat commands. • Reload the switch when deleting all VLANs.
Note: To restore to factory default – unplug all data cables, erase the startup-configuration and delete the vlan.dat file, then reload the device.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 27
3.4 VLAN Trunks
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 28
VLAN Trunks
Trunk Configuration Commands
Configure and verify VLAN trunks. Trunks are layer 2 and carry traffic for all VLANs.
Task
IOS Command
Enter global configuration mode.
Switch# configure terminal
Enter interface configuration mode.
Switch(config)# interface interface-id
Set the port to permanent trunking mode.
Switch(config-if)# switchport mode trunk
Sets the native VLAN to something other than VLAN 1.
Switch(config-if)# switchport trunk native vlan vlan-id
Specify the list of VLANs to be allowed on the trunk link.
Switch(config-if)# switchport trunk allowed vlan vlan-list
Return to the privileged EXEC mode.
Switch(config-if)# end
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 29
VLAN Trunks
Trunk Configuration Example
The subnets associated with each VLAN are:
• VLAN 10 – Faculty/Staff – 172.17.10.0/24
• VLAN 20 – Students – 172.17.20.0/24
• VLAN 30 – Guests – 172.17.30.0/24
• VLAN 99 – Native – 172.17.99.0/24
F0/1 port on S1 is configured as a trunk port.
Note: This assumes a 2960 switch using 802.1q tagging. Layer 3 switches require the encapsulation to be configured before the trunk mode.
Prompt
Command
S1(config)#
Interface fa0/1
S1(config-if)#
Switchport mode trunk
S1(config-if)#
Switchport trunk native vlan 99
S1(config-if)#
Switchport trunk allowed vlan 10,20,30,99
S1(config-if)#
end
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 30
VLAN Trunks
Verify Trunk Configuration
Set the trunk mode and native vlan. Notice sh int fa0/1 switchport command:
• Is set to trunk administratively
• Is set as trunk operationally (functioning)
• Encapsulation is dot1q
• Native VLAN set to VLAN 99
• All VLANs created on the switch will pass traffic on this trunk
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 31
VLAN Trunks
Reset the Trunk to the Default State
• Reset the default trunk settings with the no command.
• AllVLANsallowedtopasstraffic • Native VLAN = VLAN 1
• Verify the default settings with a sh int fa0/1 switchport command.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 32
VLAN Trunks
Reset the Trunk to the Default State (Cont.)
Reset the trunk to an access mode with the switchport mode access command:
• Is set to an access interface administratively
• Is set as an access interface operationally (functioning)
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 33
3.5 Dynamic Trunking Protocol
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 34
Dynamic Trunking Protocol Introduction to DTP
Dynamic Trunking Protocol (DTP) is a proprietary Cisco protocol. DTP characteristics are as follows:
• On by default on Catalyst 2960 and 2950 switches
• Dynamic-auto is default on the 2960 and 2950 switches
• May be turned off with the nonegotiate command
• May be turned back on by setting the interface to dynamic-auto
• Setting a switch to a static trunk or static access will avoid negotiation issues with the switchport mode trunk or the switchport mode access commands.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 35
Dynamic Trunking Protocol Negotiated Interface Modes
The switchport mode command has additional options.
Use the switchport nonegotiate interface configuration command to stop DTP negotiation.
Option
Description
access
Permanent access mode and negotiates to convert the neighboring link into an access link
dynamic auto
Will becomes a trunk interface if the neighboring interface is set to trunk or desirable mode
dynamic desirable
Actively seeks to become a trunk by negotiating with other auto or desirable interfaces
trunk
Permanent trunking mode and negotiates to convert the neighboring link into a trunk link
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 36
Dynamic Trunking Protocol
Results of a DTP Configuration
DTP configuration options are as follows:
Dynamic Auto
Dynamic Desirable
Trunk
Access
Dynamic Auto
Access
Trunk
Trunk
Access
Dynamic Desirable
Trunk
Trunk
Trunk
Access
Trunk
Trunk
Trunk
Trunk
Limited connectivity
Access
Access
Access
Limited connectivity
Access
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 37
Dynamic Trunking Protocol Verify DTP Mode
The default DTP configuration is dependent on the Cisco IOS version and platform.
▪ Use the show dtp interface command to determine the current DTP mode.
▪ Best practice recommends that the interfaces be set to access or trunk and to turnoff DTP
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 38
3.6 Module Practice and Quiz
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 39
Module Practice and Quiz
What did I learn in this module?
• VLANs are based on logical instead of physical connections.
• VLANs can segment networks based on function, team, or application.
• Each VLAN is considered a separate logical network.
• A trunk is a point-to-point link that carries more than one VLAN.
• VLAN tag fields include the type, user priority, CFI and VID.
• A separate voice VLAN is required to support VoIP.
• Normal range VLAN configurations are stored in the vlan.dat file in flash.
• An access port can belong to one data VLAN at a time, but may also have a Voice VLAN.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 40
Module Practice and Quiz
What did I learn in this module? (Cont.)
• A trunk is a Layer 2 link between two switches that carries traffic for all VLANs.
• Trunks will need tagging for the various VLANs, typically 802.1q .
• IEEE 802.1q tagging makes provision for one native VLAN that will remain untagged.
• An interface can be set to trunking or nontrunking.
• Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP).
• DTP is a Cisco proprietary protocol that manages trunk negotiations.
© 2019, 2021 Cisco and/or its affiliates. All rights reserved. Cisco Confidential 41