2. Objectives
• VLAN Segmentation
• Explain the purpose of VLANs in a switched network.
• Explain how a switch forwards frames based on VLAN configuration in
a multi-switch environment.
• VLAN Implementations
• Configure a switch port to be assigned to a VLAN based on
requirements.
• Configure a trunk port on a LAN switch.
• Troubleshoot VLAN and trunk configurations in a switched network.
• Inter-VLAN Routing Using Routers
• Describe the two options for configuring Inter-VLAN routing.
• Configure Legacy Inter-VLAN Routing.
• Configure Router-on-a-Stick Inter-VLAN Routing
4. VLAN Definitions
• VLANs allow an administrator to segment networks based on factors such as
function, project team, or application, without regard for the physical location
of the user or device.
• VLANs enable the implementation of access and security policies according to
specific groupings of users.
• A VLAN is a logical partition of a Layer 2 network.
• Multiple partitions can be created, allowing for multiple VLANs to co-exist.
• Each VLAN is a broadcast domain, usually with its own IP network.
• VLANs are mutually isolated, and packets can only pass between them via a
router.
• The partitioning of the Layer 2 network takes place inside a Layer 2 device,
usually via a switch.
• The hosts grouped within a VLAN are unaware of the VLAN’s existence.
6. Types of VLANs
• Data VLAN – user generated traffic
• Default VLAN – all switch ports become part of this VLAN until switch is
configured, show vlan brief
• Native VLAN – used for untagged traffic
• Management VLAN – used to access management capabilities
7. Voice VLANs
• VoIP traffic is time-sensitive and
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 ms across
the network.
• The voice VLAN feature enables
access ports to carry IP voice traffic
from an IP phone.
9. VLAN Trunks
• The links between switches S1 and S2, and S1 and S3 are configured
to transmit traffic coming from VLANs 10, 20, 30, and 99 across the
network. This network could not function without VLAN trunks.
• A VLAN trunk is a point-to-point link that carries more than one
VLAN.
• A VLAN trunk is usually established between switches so same-
VLAN devices can communicate, even if physically connected to
different switches.
• A VLAN trunk is not associated to any VLANs; neither is the trunk
ports used to establish the trunk link.
• Cisco IOS supports IEEE802.1q, a popular VLAN trunk protocol.
12. Controlling Broadcast Domains
with VLANs
• VLANs can be used to limit the reach of broadcast frames.
• A VLAN is a broadcast domain of its own.
• A broadcast frame sent by a device in a specific VLAN is forwarded within that
VLAN only.
• VLANs help control the reach of broadcast frames and their impact in the
network.
• Unicast and multicast frames are forwarded within the originating VLAN.
13. Tagging Ethernet Frames for VLAN
Identification
• Frame tagging is the process of adding a VLAN identification header to the
frame.
• It is used to properly transmit multiple VLAN frames through a trunk link.
• Switches tag frames to identify the VLAN to which they belong.
• Different tagging protocols exist; IEEE 802.1Q is a vey popular example.
• The protocol defines the structure of the tagging header added to the frame.
• Switches add VLAN tags to the frames before placing them into trunk links
and remove the tags before forwarding frames through non-trunk ports.
• When properly tagged, the frames can transverse any number of switches via
trunk links and still be forwarded within the correct VLAN at the destination.
14. Native VLANs and 802.1Q Tagging
• Control traffic sent on the native VLAN should not be tagged.
• Frames received untagged, remain untagged and are placed in the native
VLAN when forwarded.
• If there are no ports associated to the native VLAN and no other trunk links,
an untagged frame is dropped.
• When configuring a switch port on a Cisco switch, configure devices so that
they do not send tagged frames on the native VLAN.
• In Cisco switches, the native VLAN is VLAN 1, by default.
16. Voice VLAN Tagging
• Scenario 1: PC 1
sends a
broadcast.
• Scenario 2: PC 2
sends a
broadcast.
• Scenario 3: PC 3
sends a broadcast
17. Creating a VLAN
Cisco Switch IOS Commands
Enter global
configuration
S1#configure terminal
Create a
VLAN with a
valid id
number
S1(config)#vlan vlan-id
Specify a
unique name
to identify the
VLAN
S1(config-vlan)#name
vlan-name
Return to the
privileged
EXEC mode
S1(config-vlan)#end
18. Assigning Ports to VLANs
Cisco Switch IOS Commands
Enter global
configuration
S1#configure
terminal
Enter interface
configuration mode
S1(config)#interface
interface_id
Set the port to access
mode
S1(config-
if)#switchport mode
access
Assign the port to a
VLAN
S1(config-
if)#switchport access
vlan vlan_id
Return to the
privileged EXEC
mode
S1(config-if)#end
19. Changing VLAN Port Membership
• Remove VLAN Assignment
• Interface F0/18 was previously assigned to VLAN 20 which is still active, F0/18
reset to VLAN1
Cisco Switch IOS Command
Enter global configuration mode S1#config t
Enter interface configuration
mode
S1(config)#interface interface_id
Remove the VLAN assignment
from the port
S1(config-if)#no switchport
access vlan
Return to the privileged EXEC
mode
S1(config-if)#end
22. Deleting VLANs
• The entire vlan.dat file can be deleted using the delete flash:vlan.dat privileged
EXEC mode command
• Abbreviated command version (delete vlan.dat) can be used if the vlan.dat file
has not been moved from its default location
23. Verifying VLAN Information
Show vlan command
Show vlan |brief | id vlan-id | name vlan-name | summary|
Display one line for each vlan with the VLAN name, status and its ports brief
Display information about a single VLAN identified by VLAN ID
number, for vlan-id the range is 1 to 4094
id vlan-id
Display information about single VLAN identified by VLAN name.
The VLAN name is an ASCII string from 1 to 32 characters
name vlan-name
Display VLAN summary information summary
Show interface command
Show interface |interface-id | vlan vlan-id | switchport
Valid interface include physical port(type, module and port number)
and port channel. The port channel range is 1 to 6
Interface-if
VLAN identification. The range is 1 to 4094 van vlan-id
Display the administrative and operational status of a switching port,
including port blocking and port protection settings
switchport
25. VLAN Trunks: Configuring IEEE
802.1q Trunk Links
Cisco Switch IOS Commands
Enter global configuration mode S1#config t
Enter interface configuration S1#(config)interface interface_id
Force the link to be trunk link S1#(config-if)switchport mode trunk
Specify a native VLAN for untagged
frames
S1#(config-if)switchport trunk native vlan vlan-id
Specify the list of VLANS to be allowed
on the trunk link
S1#(config-if)switchport trunk allowed vlan vlan-
list
Return to the privileged EXEC mode S1#(config-if)end
27. Resetting the Trunk to Default State
Cisco switch IOS commands
Enter global
configuration mode
S1#config t
Enter interface
configuration mode
S1#(config)interface
interface_id
Set trunk to allow all
VLANs
S1#(config-if)no
switchport trunk
allowed vlan
Reset native VLAN to
default
S1#(config-if)no
switchport trunk native
vlan
Return to the privileged
EXEC mode
S1#(config-if)end
28. Troubleshoot VLANs and TrunksIP
Addressing Issues with VLANs
• It is a common practice to associate a VLAN with an IP network.
• Because different IP networks only communicate through a router, all devices
within a VLAN must be part of the same IP network to communicate.
• The figure displays that PC1 cannot communicate to the server because it has a
wrong IP address configured.
29. Troubleshoot VLANs and TrunksIP
Addressing Issues with VLANs
• Missing VLANs
• If all the IP address mismatches have been solved, but the device still cannot
connect, check if the VLAN exists in the switch.
• If the VLAN to which a port belongs is deleted, the port becomes inactive. All
ports belonging to the VLAN that was deleted are unable to communicate with
the rest of the network.
• Not functional until the missing VLAN is created using thevlanvlan_id global
configuration.
31. Introduction to Troubleshooting
Trunks
• Note: To solve a native VLAN mismatch, configure the native VLAN
to be the same VLAN on both sides of the link.
• Common Problems with Trunks are usually associated with incorrect
configurations. The most common type of trunk configuration errors
are:
• Native VLAN mismatches
• Trunk mode mismatches
• Allowed VLANs on trunks
• If a trunk problem is detected, the best practice guidelines recommend
to troubleshoot in the order shown above.
32. Introduction to Troubleshooting
Trunks
Problem Result Example
Native VLAN
mismatches
Poses a security risk and
creates unintended results
One port is defined as VLAN 99
and the other is defined as VLAN
110
Trunk Mode
mismatches
Causes loss of network
connectivity
Both local and peer switchport
modes are configured as dynamic
duo
Allowed VLANs on
Trunks
Causes unexpected traffic or
no traffic to be sent over the
trunk
The list of allowed VLANs does
not support current VLAN
trunking requirements
33. Incorrect VLAN List
• VLANs must be allowed in the trunk before their frames can be transmitted
across the link.
• Use the switchport trunk allowed vlan command to specify which VLANs are
allowed in a trunk link.
• Use the show interfaces trunk command to ensure the correct VLANs are
permitted in a trunk.
34. Inter-VLAN Routing Operation
• What is Inter-VLAN Routing?
• Layer 2 switches cannot forward traffic between VLANs without the assistance
of a router.
• Inter-VLAN routing is a process for forwarding network traffic from one
VLAN to another, using a router.
35. Router-on-a-Stick Inter-VLAN
Routing
• The router-on-a-stick approach uses only one of the router’s physical interface.
• One of the router’s physical interfaces is configured as a 802.1Q trunk port so
it can understand VLAN tags.
• Logical subinterfaces are created; one subinterface per VLAN.
• Each subinterface is configured with an IP address from the VLAN it
represents.
• VLAN members (hosts) are configured to use the subinterface address as a
default gateway.
36. Router-on-a-Stick Inter-VLAN
Routing
• Router interface configured to operate as a trunk link and is connected to a
trunked switch port. The router performs inter-VLAN routing by accepting
VLAN-tagged traffic on the trunk interface coming from the adjacent switch,
and then, internally routing between the VLANs using subinterfaces. The
router then forwards the routed traffic, VLAN-tagged for the destination
VLAN, out the same physical interface as it used to receive the traffic.
37. Configure Router-on-a Stick:
Preparation
• An alternative to legacy inter-VLAN routing is to use VLAN trunking and subinterfaces.
• VLAN trunking allows a single physical router interface to route traffic for multiple VLANs.
• The physical interface of the router must be connected to a trunk link on the adjacent
switch.
• On the router, subinterfaces are created for each unique VLAN.
• Each subinterface is assigned an IP address specific to its subnet or VLAN and is also
configured to tag frames for that VLAN.
41. Configure Router-on-a Stick:
Verifying Routing
• Access to devices on remote VLANs can be tested using the ping command.
• The ping command sends an ICMP echo request to the destination address.
• When a host receives an ICMP echo request, it responds with an ICMP echo
reply.
• Tracert is a useful utility for confirming the routed path taken between two
devices.
