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CCNA 3 Module 9

CCNA 3 Module 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.

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CCNA 3 Module 9

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  1. CCNA 3Module 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. Trunk Trunk VLAN1 VLAN2 VLAN1 B A The VLAN ID would be carried in frame the until the frame exits the trunking link. 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.

  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. Switch C is the VTP server for the domain 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.

  13. 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.

  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. 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 interfacecommand 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 192.168.1.1 255.255.255.0

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