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Virtual Trunk Protocol

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1 Virtual Trunk Protocol
CCNA 3 Module 9 Virtual Trunk Protocol

2 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.

3 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.

4 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.

5 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

6 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 The VLAN ID would be carried in frame the until the frame exits the trunking link. VLAN1 VLAN2

7 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.

8 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

9 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

10 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.

11 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

12 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

13 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

14 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.

15 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.

16 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

17 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.

18 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

19 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.

20 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.

21 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

22 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}

23 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.

24 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.

25 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.

26 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.

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

28 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.

29 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

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