Module 9: VLAN Trunking Protocol

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Module 9VLAN Trunking Protocol

• CCNA 3 Version 3.1

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Introduction to VLAN Trunking

• VLAN trunking uses tagged frames to allow
  multiple VLANs to be carried throughout a large
  switched network over shared backbones
• Two trunking protocols
• IEEE 802.1Q (widely implemented)
• Ciscos Inter-Switch Link (ISL) - a proprietary
  trunking protocol that can be implemented in
  Cisco networks
• Once a network is configured with VTP, many of
  the VLAN configuration tasks are automatic

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A Trunk is

• A physical and logical connection between two
  switches across which network traffic travels
• A single transmission channel between two points
  (usually switching centers)
• A point-to-point link that supports several VLANs

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Another Look at Trunking
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Before and After Trunking

• Before Each switch is uses two physical links
  so that each port carries traffic for a single
  VLAN. Simplest way of implementing inter-switch
  VLAN communication, but does not scale well.

• After Trunking bundles multiple virtual links
  over one physical link by allowing the traffic
  for several VLANs to travel over a single cable
  between the switches (conserves switch ports)

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Trunking Operation

• Two types of trunking mechanisms
• frame filtering (brief look at)
• frame tagging (focus is here)

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Frame Filtering (not as common)
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Frame Tagging (the standard)

• Frame Tagging
• adopted as the standard trunking mechanism by
  IEEE
• assigns an identifier to the frames makes
  management easier with faster delivery of frames
• Most common tagging schemes for Ethernet segments
  are
• ISL  Cisco proprietary Inter-Switch Link
  protocol.
• 802.1Q  IEEE standard that will be focused on in
  this section.

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Frame Tagging Encapsulation Methods

• Frame tagging places a unique identifier in the
  header of each frame as it is forwarded
  throughout the network backbone
• identifier is examined by each switch before any
  broadcasts or transmissions are made to other
  switches, routers, or end-station devices.
• Last switch removes the identifier before the
  frame is transmitted to the target end station
• Frame tagging functions at Layer 2 and requires
  little processing or administrative overhead

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IEEE 802.1Q vs. ISL

• Both are Ethernet frame tagging mechanisms
• Both protocols maintain VLAN information as
  traffic flows between the switches
• IEEE 802.1Q
• The VLAN ID (4 byte field) is inserted into the
  frame
• ISL
• The frame is encapsulated with a header that
  contains a VLAN ID

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Trunking Implementation

• To create or configure a VLAN trunk on a Cisco
  IOS command-based switch
• Step 1)
• configure the port first as a trunk

• Step 2)
• specify the trunk encapsulation

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Trunking Implementation

• Before attempting to configure a VLAN trunk on a
  port, determine what encapsulation the port can
  support with the following command

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Trunking Implementation

• To verify that trunking has been configured and
  to check the settings

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Fast Ethernet and Gigabit Ethernet Trunking Modes
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VTP Benefits

• VTP reduces the complexity of managing and
  monitoring VLAN networks
• VTP maintains VLAN configuration consistency
  across a common network administration domain
• VTP allows VLANs to be trunked over mixed media
• VTP provides for accurate tracking and monitoring
  of VLANs
• VTP provides Plug-and-Play configuration when
  adding new VLANs

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VTP Domain

• A VTP domain is made up of one or more
  interconnected devices that share the same VTP
  domain name
• A switch can be in one VTP domain only.
• 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
• Below shows the generic encapsulation for VTP
  within an ISL frame

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The VTP Header

• The VTP header varies (depending upon the type of
  VTP message)
• Generally four items are found in all VTP
  messages
• VTP protocol version Either Version 1 or 2
• VTP message type Indicates one of four types
  (well look at the types later)
• Management domain name length Indicates size of
  the name that follows
• Management domain name The name configured for
  the management domain

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VTP Modes

• VTP switches operate in one of three modes
• Server
• Client
• Transparent

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More on VTP Modes

• VTP servers
• can create, modify, and delete VLAN and VLAN
  configuration parameters for the entire domain
• VTP servers save VLAN configuration information
  in the switch NVRAM
• VTP servers send VTP messages out to all trunk
  ports.
• VTP clients
• cannot create, modify, or delete VLAN
  information.
• only role of VTP clients is to process VLAN
  changes and send VTP messages out all trunk ports
• VTP transparent mode
• forwards VTP advertisements but ignores
  information contained in the message.
• will not modify its database when updates are
  received
• Except for forwarding VTP advertisements, VTP is
  disabled on a transparent switch.

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VTP Implementation

• With VTP, each switch advertises on its trunk
  ports the following
• its management domain
• configuration revision number
• the VLANs that it knows about
• certain parameters for each known VLAN
• These advertisement frames are sent to a
  multicast address so that all neighboring devices
  can receive the frames
• the frames are not forwarded by normal bridging
  procedures
• A new VLAN must be created and configured on one
  device only in the management domain
• All the other devices in the same management
  domain automatically learn the information

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VTP Advertisements

• Two types of VTP advertisements
• Requests from clients
• Responses from servers
• Configuration revision number
• Each advertisement starts as configuration
  revision number 0
• When changes are made, the configuration revision
  number is increased incrementally by one, (n
  1).
• Max revision number is 2,147,483,648 then counter
  will reset back to zero

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VTP Messages

• There are three types of VTP messages
• Advertisement requests
• Summary advertisements
• Subset advertisements

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Advertisement Requests

• VTP Clients use advertisement requests to
• request VLAN information from a VTP server
• The server then responds with summary and subset
  advertisements

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Summary Advertisements

• By default, server and client Catalyst switches
  issue summary advertisements every 5 minutes.
• Servers inform neighbor switches what they
  believe to be the current VTP configuration
  revision number
• Assuming the domain names match, the receiving
  server or client compares the configuration
  revision number
• If the revision number in the advertisement is
  higher than the current revision number in the
  receiving switch, the receiving switch then
  issues an advertisement request for new VLAN
  information.

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Subset Advertisements

• Subset advertisements contain detailed
  information about VLANs such as
• VTP version type
• domain name
• configuration revision number
• The following can trigger these advertisements
• Creating or deleting a VLAN
• Suspending or activating a VLAN
• Changing the name of a VLAN
• Changing the maximum transmission unit (MTU) of a
  VLAN

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Advertisement Information

• Advertisements may contain some or all of the
  following information
• Management domain name
• Advertisements with different names are ignored.
• Configuration revision number
• The higher number indicates a more recent
  configuration.
• Message Digest 5 (MD5)
• MD5 is the key that is sent with the VTP when a
  password has been assigned. If the key does not
  match, the update is ignored.
• Updater identity
• The updater identity is the identity of the
  switch that is sending the VTP summary
  advertisement

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VTP Version 1 and Version 2

• Two different versions of VTP are available,
  Version 1 and Version 2
• The two versions are not interoperable
• All switches in the same domain must be
  configured with the same VTP version.
• VTP Version 1 is the default.
• VTP Version 1 does not have Token Ring VLAN
  support (need version 2)

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Changing the VTP Version

• First enter VLAN database mode
• Switchvlan database
• Then change the VTP version to Version 2
• Switch(vlan)vtp v2-mode
• To create a management domain use the following
  command
• Switch(vlan)vtp domain cisco

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Adding a VTP Client to an ExistingVTP Domain

• First, always verify that its VTP configuration
  revision number is lower than the configuration
  revision number of the other switches in the VTP
  domain
• Use the show vtp status command
• Switches in a VTP domain always use the VLAN
  configuration of the switch with the highest VTP
  configuration revision number
• If a switch is added that has a revision number
  higher than the revision number in the VTP
  domain, it can erase all VLAN information from
  the VTP server and VTP domain
• Issue the no vtp file command to clear the vtp
  file

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Choosing the VTP Mode

• Choose one of the three available VTP modes for
  the switch
• If this is the first switch in the management
  domain and additional switches will be added, set
  the mode to server
• additional switches will be able to learn VLAN
  information from this switch
• NOTE There should be at least one server
• Command to set the VTP mode
• Switch(vlan)vtp client server transparent

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Verifying VTP Configuration

• To verify VTP configuration settings on a Cisco
  IOS command-based switch

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To Display Advertisement Stats

• To display statistics about advertisements sent
  and received on the switch

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Review of VLAN Basics
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VLAN Review

• A VLAN is a logical grouping of devices or users
  that can be grouped by function, department, or
  application regardless of their physical
  location.
• VLANs are configured at the switch through
  software.
• VLANs can span single building infrastructures or
  interconnected buildings.
• VLANs share the same broadcast domain or Layer 3
  network.

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More VLAN Review

• Network devices in different VLANs cannot
  directly communicate without the intervention of
  a Layer 3 routing device.
• A router is necessary to route the traffic
  between VLANs
• Without the routing device, inter-VLAN traffic
  would not be possible
• Put another waywhen a host on one VLAN wants to
  communicate with a host on another, a router must
  be involved

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InterVLAN Connectivity
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Traditional Approach (Multiple Links)
Subnet 3
Subnet 2
Subnet 1
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ISL and 802.1Q
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Trunk-Connected Router

• Primary advantage of using a trunk link is
  reduction in the number of router and switch
  ports used
• A single trunk link saves money and reduces
  configuration complexity

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Subinterfaces and VLANs
One FastEthernet Interface divided into 3
Subinterfaces
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Subinterface Preparation

• In order to route between VLANs with
  subinterfaces, a virtual subinterface must be
  created for each VLAN
• To define subinterfaces on a physical interface,
  perform the following tasks
• Identify the interface.
• Define the VLAN encapsulation.
• Assign an IP address to the interface.

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Configuring InterVLAN Routing
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Module 9VLAN Trunking Protocol

• CCNA 3 Version 3.1

THE END