Virtual Trunk Protocol

TrunkingA 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.

TrunkingTrunking bundles multiple virtual links over one physical link. This allows the traffic of several VLANs to travel over a single cable between the switches.

TrunkingTrunking 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.

TrunkingThe 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

TrunkingFrame 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.

TrunkingTo 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 trunkThe 2950 only does dot1q trunking, otherwise the following command would have had to also been entered:Switch_A(config-if)#switchport trunk encapsulation dot1q

TrunkingTo 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_numshow 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 ServersVTP 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 ClientVTP 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 ModeSwitches 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 ModeWith 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 AdvertisementsEach 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 Configuration

Switch# 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 RoutingIf 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 switcha switch to the inter-VLAN routera 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 RoutingIn 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 RoutingNetworks 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 RoutingA 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 RoutingTo 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 RoutingThe 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 RoutingTo 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