Download CCNA 3 Module 3 Single-Area OSPF

CCNA 3 v3.0 Module 8
Virtual LANs
Cisco Networking Academy
© 2003, Cisco Systems, Inc. All rights reserved.
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Objectives
• VLAN concepts
• VLAN configuration
• Troubleshooting VLANs
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Benefits of VLANs
• Easily move workstations on the LAN
• Easily add workstations to the LAN
• Easily change the LAN configuration
• Easily control network broadcast traffic
• Improve security
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VLANs
VLANs logically segment switched networks based on an
organization's functions, project teams, or applications
as opposed to only a physical or geographical basis.
However, geographic VLANs (local VLANs) are becoming
more common.
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Local VLANs
• VLANs are more frequently being created around
geographic boundaries (wiring closet) rather than
commonality (application) boundaries.
– traffic flow patterns utilize the new 20/80 rule
– the user must cross a Layer 3 device in order to
reach 80 percent of the resources
– considerably easier to manage and conceptualize
than VLANs that span different geographic areas
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3 Switches, 3 Interfaces, 3 Broadcast Domains
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Example with 1 Switch, 3 VLANs and
3 Broadcast Domains
In this example, 3 different FastEthernet
interfaces are being used. However,
subinterfaces can be created instead to
achieve the same design goal.
Router(config)# int fa0/0.1
Router(config)# int fa0/0.2
Router(config)# int fa0/0.3
One physical interface
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VLAN Configuration
VLANs can be configured as static or dynamic.
• Statically:
–Network administrators configure port-by-port
–Each port is associated with a specific VLAN
–Network admin. is responsible for keying in the
mappings between the switchports and VLANs
• Dynamically:
–Switchports are able to dynamically configure their
VLAN association
–Uses a software database of MAC addresses to VLAN
mappings (which the network admin. must set up first)
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Static VLANs
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Dynamic VLANs
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Dynamic VLANs
• With a VLAN Management Policy Server (VMPS),
you can assign switch ports to VLANs
dynamically.
– When you enable VMPS, a MAC address-to-VLAN
mapping database downloads from a Trivial File
Transfer Protocol (TFTP) server and VMPS begins to
accept client requests.
Note: The VLAN Trunking Protocol (VTP) management domain and the
management VLAN of VMPS clients and the VMPS server must be the same.
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VLAN Types
•
•
Port Based:
–
Most common configuration method
–
Ports assigned individually, in groups or across
multiple switches
–
Simple to use/administer
MAC address:
–
Rarely implemented today
–
Each address must be entered into the switch
and configured individually
–
More administrative overhead
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Communicating Between VLANs
One physical interface
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Interswitch Communication and VTP
• In order for switches to communicate between
each other, a trunk link must be established from
switch to switch using a trunking protocol.
• Trunk links carry frames from all VLANs.
• Trunking ports tag frames with a VLAN ID before
the frame is forwarded to another switch.
• The VLAN tag is then removed before the frame
is forwarded out an access port.
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Access/Trunk Links
ISL or 802.1q
An access link is a link on the switch that is a member of only one VLAN.
–referred to as the native VLAN of the port
A trunk link is capable of supporting multiple VLANs.
–typically used to connect switches to other switches or routers
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VLAN Trunking Protocols
• The switch has two methods of identifying the
VLAN that a frame belongs to when the
switch receives the frame on a trunk link.
–Cisco proprietary ISL standard
–IEEE 802.1Q standard
• There are other trunking encapsulation types
but we will focus on these two.
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VLAN Frame Identification
Identification
Method
Encapsulation
Tagging (insertion
into frame)
Media
802.1Q
No
Yes
Ethernet
ISL
Yes
No
Ethernet
802.10
No
No
FDDI
LANE
No
No
ATM
http://www.cisco.com/en/US/tech/tk389/tk390/technologies_tech_note09186a0080094665.shtml
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VLAN Trunking Protocol
• A VTP domain is made up of one or more
interconnected devices that share the same
VTP domain name.
– A switch can be configured to be in one VTP domain
only.
– Global VLAN information is propagated across the
network by way of connected switch trunk ports.
– When transmitting VTP messages to other switches in
the network, the VTP message is encapsulated in a
trunking protocol frame such as ISL or IEEE 802.1Q.
– In order to share VTP information, switches must be in
the same VTP domain.
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End-to-End VLANs
• Users are grouped into
VLANs independent of
physical location, but
dependent on group or job
function.
• All users in a VLAN should
have the same 80/20 traffic
flow patterns.
• As a user moves around
the campus, VLAN
membership for that user
should not change.
• Each VLAN has a common
set of security
requirements for all
members
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VLAN Configuration
The following guidelines must be followed when
configuring VLANs on Cisco 29xx switches:
• The maximum number of VLANs is switch dependent.
• VLAN 1 is one of the factory-default VLANs.
• VLAN 1 is the default Ethernet VLAN.
• Cisco Discovery Protocol (CDP) and VLAN Trunking Protocol
(VTP) advertisements are sent on VLAN 1.
• The Catalyst 29xx IP address is in the VLAN 1 broadcast domain
by default.
• The switch must be in VTP server mode to create, add, or delete
VLANs.
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Creating and Deleting VLANs
To Create VLANs:
Switch# vlan database
Switch(vlan)# vlan 2
Switch(vlan)# vlan 3 name Accounting
Switch(vlan)# no vlan 4
To group a switchport to a VLAN:
Switch(config)# int range fa0/1 - 4
Switch(config-range-if)# switchport mode access
Switch(config-range-if)# switchport access vlan 2
Switch(config-range-if)# no switchport access vlan 2
Switch(config-range-if)# int g0/1
Switch(config-if)# switchport mode trunk
The Catalyst 2900XL will do both
ISL and 802.1q encapsulation so
you must specify at the switchport.
2900XL:
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport encapsulation isl | dot1q
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Verifying VLAN Configuration
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Common Problems in Troubleshooting
VLANs
• Ports grouped to incorrect VLAN
• Trunk link between switches may not have the
same encapsulation on both sides
• VTP Domain name different
• Duplex and speed mismatch
• Intervlan routing configured incorrectly at router
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VLAN Problem Isolation
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Problem Isolation in Catalyst Networks
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Preventing Broadcast Storms
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Catalyst IOS show vlan Command
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VLAN Troubleshooting Scenarios
A trunk link cannot be established between
a switch and a router
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VLAN Troubleshooting Scenarios
VTP is not properly propagating VLAN configuration
changes between switches.
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