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 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.

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.

• 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.

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.

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.

VTP ConfigurationSwitch# configure terminalSwitch(config)# vtp mode [ server | client | transparent ]Switch(config)# vtp domain domain-nameSwitch(config)# vtp passwordSwitch(config)# vtp pruningSwitch(config)# exit

Switch(config)# vtp domain ICNDChanging VTP domain name to ICNDSwitch(config)# vtp mode transparentSetting device to VTP TRANSPARENT mode.Switch(config)# exit

Switch#show vtp statusVTP Version : 2Configuration Revision : 0Maximum VLANs supported locally : 64Number of existing VLANs : 17VTP Operating Mode : TransparentVTP Domain Name : ICNDVTP Pruning Mode : DisabledVTP V2 Mode : DisabledVTP Traps Generation : DisabledMD5 digest : 0x7D 0x6E 0x5E 0x3D 0xAF 0xA0 0x2F 0xAAConfiguration last modified by 10.1.1.4 at 3-3-93 20:08:05Switch#

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