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Module 9:VLAN 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)

– Cisco’s 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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• To verify that trunking has been configured and to check the settings:

Trunking Implementation

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Fast Ethernet and Gigabit Ethernet Trunking Modes

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• 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

VTP Benefits

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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 (we’ll 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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messages:

– Advertisement requests

– Summary advertisements

– Subset advertisements

VTP Messages

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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:

– Switch#vlan 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 way…when 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 #1Subnet #2Subnet #3

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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 9:VLAN Trunking Protocol

CCNA 3 – Version 3.1

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