1 version 3.1.1 created by g.wright ccna 3 module 9 virtual trunk protocol

1Version 3.1.1 Created by G.Wright

CCNA 3Module 9

Virtual Trunk Protocol


Trunking

• A trunk is a physical and logical connection between two

switches across which network traffic travels.

• In a switched network, a trunk is a point-to-point link that

supports several VLANs.

• The purpose of a trunk is to conserve ports when a link

between two devices that implement VLANs is created.


Trunking

• Trunking bundles multiple virtual links over one physical

link.

• This allows the traffic of several VLANs to travel over a

single cable between the switches.


Trunking

• Trunking protocols were developed to effectively manage

the transfer of frames from different VLANs on a single

physical line.

• Frame tagging has been adopted as the standard trunking

mechanism by the IEEE.


Trunking

• The unique physical link between the two switches is able

to carry traffic for any VLAN.

• Each frame sent on the link is tagged so that it carries the

VLAN ID to identify which VLAN it belongs to.

• The two most common tagging schemes for Ethernet

segments are ISL and 802.1Q:– ISL – A Cisco proprietary protocol

– 802.1Q – An IEEE standard that is the focus of this section


Trunking

• Frame tagging places a unique identifier in the header of each frame as it is forwarded throughout the network backbone.

• When the frame exits the network backbone, the switch removes the identifier before the frame is transmitted to the target end station.

A B

Trunk Trunk

The VLAN ID would be carried in frame the until the frame exits the trunking link.

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VL

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Trunking

• Frame tagging functions at Layer 2 and does not require

much network resources or administrative overhead.

• It is important to understand that a trunk

link does not belong to a specific VLAN.

• A trunk link is a conduit for VLANs

between switches and routers.


Trunking

• To configure 802.1q trunking on a 2950 switch, first

determine which ports on the switches will be used to

connect the two switches together.

• Then in the Global configuration mode enter the following

commands on both switches:Switch_A(config)#interface fastethernet interface #

Switch_A(config-if)#switchport mode trunk

• The 2950 only does dot1q trunking, otherwise the following

command would have had to also been entered:Switch_A(config-if)#switchport trunk encapsulation dot1q


Trunking

• To verify that trunking has been configured and verify the settings use the following commands from Privileged EXEC mode of the switch:

show interfaces Fa0/port_num

show interfaces trunk


Virtual Trunking Protocol (VTP)

• The role of VTP is to maintain VLAN configuration consistency across a common network administration domain.

• VTP is a messaging protocol that uses Layer 2 trunk frames to add, delete, and rename VLANs on a single domain.

• VTP also allows for centralized changes that are communicated to all other switches in the network.

• VTP messages are encapsulated in either ISL or IEEE 802.1Q protocol frames, and passed across trunk links to other devices.


Virtual Trunking Protocol (VTP)

• 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.

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


VTP Servers

• 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.

Switch C is the VTP server for the domain


VTP Client

• VTP clients cannot create, modify, or delete VLAN information.

• The only role of VTP clients is to process VLAN changes and send VTP messages out all trunk ports.

Switch C is the VTP server for the domain


VTP Transparent Mode

• Switches in VTP transparent mode forward VTP advertisements but ignore information contained in the message.

• A transparent switch will not modify its database when updates are received, or send out an update that indicates a change in its VLAN status.

• Except for forwarding VTP advertisements, VTP is disabled on a transparent switch.

• Switches in VTP transparent mode can create and remember VLANs, but only of local significance.

• VLANs created in the transparent mode will not be sent to other switches.


VTP Transparent Mode

• With VTP, each switch advertises on its trunk ports its management domain, configuration revision number, the VLANs that it knows about, and certain parameters for each known VLAN.

• These advertisement frames are sent to a multicast address so that all neighbor devices can receive the frames.

• A new VLAN must be created and configured on one device (VTP server) only in the management domain.

• All the other devices (VTP clients) in the same management domain automatically learn the information.


VTP Advertisements

• Each advertisement starts as configuration revision number 0.

• As changes are made, the configuration revision number is increased incrementally by one, or n + 1.

• Only the advertisement with the highest revision number is maintained.

• There are two types of VTP advertisements:– Requests from clients that want information at bootup – Response from servers


VTP Messages

• There are three types of VTP messages:– Advertisement requests

– Summary advertisements

– Subset advertisements

• With advertisement requests, clients request VLAN information and the server responds with summary and subset advertisements.

• By default, server and client Catalyst switches issue summary advertisements every five minutes.

• Servers inform neighbor switches what they believe to be the current VTP revision number.


VTP Messages

• The server or client compares the configuration revision number that it received.

• If the switch receives a revision number that is higher than the current revision number in that switch, it issues an advertisement request for new VLAN information.

• Advertisements can contain some or all of the following information:– Management domain name

– Configuration revision number

– Message Digest 5 (MD5)

– Updater identity


VTP Configuration

• Specific steps must be considered before VTP and VLANs

are configured on the network: – Determine the version number of VTP that will be utilized.

– Decide if the switch will be a member of a management domain that

already exists, or if a new domain should be created. If a

management domain exists, determine the name and password of

the domain.

– Choose a VTP mode for the switch.


VTP Configuration

• Two different versions of VTP are available, Version 1 and Version 2.

• The two versions are not interoperable. • If a switch is configured in a domain for VTP Version 2, all

switches in the management domain must be configured for VTP Version 2.

• VTP Version 1 is the default.


VTP Configuration

• To configure the VTP version on a Cisco IOS command-based switch, first enter VLAN database mode.

• Then configure the VTP version number.

Switch#vlan database

Switch(vlan)#vtp v2-mode • If the switch is the first switch in the network, the

management domain should be created. • The command can be used to create the management

domain.

Switch(vlan)#vtp domain cisco


VTP Configuration

• Before adding a VTP client to a VTP domain that already

exists, verify that its VTP configuration revision number is

lower than the configuration revision number of the other

switches in the VTP domain.

• The VTP mode should also be set to the proper mode.

• The following command can be used to set the correct

mode of the switch:

Switch(vlan)#vtp {client | server | transparent}


Inter-VLAN Routing

• If a VLAN spans across multiple devices a trunk is used to interconnect the devices.

• A trunk carries traffic for multiple VLANs. – a trunk can connect a switch to another switch– a switch to the inter-VLAN router– a switch to a server with a special NIC installed that supports

trunking.

• Remember that when a host on one VLAN wants to communicate with a host on another, a router must be involved.


Inter-VLAN Routing

• In a traditional situation, a network with four VLANs would require four physical connections between the switch and the external router.

• The router only supports one VLAN per interface.• This does not scale very well.


Inter-VLAN Routing

• Networks with many VLANs must use VLAN trunking to assign multiple VLANs to a single router interface.

• The router can support many logical interfaces on individual physical links through the use of subinterfaces.

• The primary advantage of using a trunk link is a reduction in the number of router and switch ports used.


Inter-VLAN Routing

• A subinterface is a logical interface within a physical interface.

• Each subinterface supports one VLAN, and is assigned one IP address.

• In order to route between VLANs with subinterfaces, a subinterface must be created for each VLAN.


Inter-VLAN Routing

• 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.

• To identify the interface, use the interface command in global configuration mode.

Router(config)#interface fastethernet port-number subinterface-number

Router_A(config-if)#interface fastethernet 0/0.1


Inter-VLAN Routing

• The router must be able to talk to the switch using a standardized trunking protocol (encapsulation).

• To define the VLAN encapsulation, enter the encapsulation command in interface configuration mode.

Router(config-if)#encapsulation dot1q vlan-number

• The vlan-number identifies the VLAN for which the subinterface will carry traffic.


Inter-VLAN Routing

• To assign the IP address to the subinterface, enter the following command in subinterface configuration mode.

Router_A(config-subif)# ip address ip-address subnet-mask

Router_A(config)#interface fastethernet 0/0

Router_A(config-if)#no shutdown

Router_A(config-if)#interface fastethernet 0/0.1

Router_A(config-subif)#encapsulation dot1q 1

Router_A(config-subif)#ip address 192.168.1.1 255.255.255.0

1 version 3.1.1 created by g.wright ccna 3 module 9 virtual trunk protocol

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