Virtual Trunking Protocol - Module 31

                                                       Overview 

     

31.1 Trunking   

31.1.1 History of trunking 

31.1.2 Trunking concepts 

31.1.3 Trunking operation 

31.1.4 VLANs and Trunking

31.1.5 Trunking implementation 

31.2 VTP   

31.2.1 History of VTP 

31.2.2 VTP concepts 

31.2.3 VTP operation 

31.2.4 VTP implementation 

31.2.5 VTP configuration 

31.3 Inter-VLAN Routing Overview  

31.3.1 VLAN basics 

31.3.2 Introducing inter-VLAN routing 

31.3.3 Inter-VLAN issues and solutions 

31.3.4 Physical and logical interfaces 

31.3.5 Dividing physical interfaces into subinterfaces 

31.3.6 Configuring inter-VLAN routing 

  Switching Basics and Intermediate Routing Case Study

  Summary 

   Overview   

Early VLANs were difficult to implement across networks. Each VLAN was manually configured on each switch. VLAN management over an extended network was a complicated task. To further complicate matters, each switch manufacturer had different VLAN capability methods. VLAN trunking was developed to solve these problems.

VLAN trunking allows many VLANs to be defined throughout an organization by the addition of special tags to frames that identify the VLAN to which they belong. This tagging allows many VLANs to be carried throughout a large switched network over a common backbone, or trunk. VLAN trunking is standards-based, with the IEEE 802.1Q trunking protocol now widely implemented. Inter-Switch Link (ISL) is a Cisco proprietary trunking protocol that can be implemented in all Cisco networks.

The manual configuration and maintenance of VLAN Trunking Protocol (VTP) on numerous switches can be a challenge. A key benefit of VTP is the automation of many VLAN configuration tasks after VTP is configured on a network.

This module explains VTP implementation in a switched network.

VLAN technology provides network administrators with many advantages. Among other things, VLANs help control Layer 3 broadcasts, improve network security, and can help to logically group network users. However, VLANs have an important limitation. They operate at Layer 2 which means that devices on different VLANs cannot communicate without the use of routers and network layer addresses.

This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams.  

Students who complete this module should be able to perform the following tasks:

• Explain the origins and functions of VLAN trunking
• Describe how trunking enables the implementation of VLANs in a large network
• Define IEEE 802.1Q
• Define Cisco ISL
• Configure and verify a VLAN trunk
• Define VTP
• Explain why VTP was developed
• Describe the contents of VTP messages
• List and define the three VTP modes
• Configure and verify VTP on an IOS-based switch
• Explain why routers are necessary for inter-VLAN communication
• Explain the difference between physical and logical interfaces
• Define subinterfaces
• Configure inter-VLAN routing with subinterfaces on a router port

    31.1  Trunking 

  31.1.1  History of trunking   

 

This page will explain the evolution of trunking.

The history of trunking goes back to the origins of radio and telephony technologies. In radio technology, a trunk is a single communications line that carries multiple channels of radio signals.

In the telephony industry, the trunking concept is associated with the telephone communication path or channel between two points. One of these two points is usually the Central Office (CO).  Shared trunks may also be created for redundancy between COs.

The concept used by the telephone and radio industries was then adopted for data communications. An example of this in a communications network is a backbone link between an MDF and an IDF. A backbone is composed of several trunks.

Currently, the same principle of trunking is applied to network switching technologies. A trunk is a physical and logical connection between two switches across which network traffic travels.

  31.1  Trunking 

  31.1.2  Trunking concepts   

 

This page will explain how trunks are used in a switched VLAN environment.

As mentioned before, a trunk is a physical and logical connection between two switches across which network traffic travels. It is a single transmission channel between two points. The two points are usually switching centers.

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. Figure  illustrates two VLANs shared across switches Sa and Sb. Each switch uses two physical links so that each port carries traffic for a single VLAN. This is a simple way to implement inter-switch VLAN communication, but it does not scale well.

The addition of a third VLAN would require the use of two more ports, one on each connected switch. This design is also inefficient in terms of load sharing. In addition, the traffic on some VLANs may not justify a dedicated link. 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.

A comparison for trunking is like a highway distributor.  The roads with different start and end points share a main national highway for a few kilometers then divide again to reach their particular destinations. This method is more cost effective than the construction of an entire road from start to end for every known or new destination.

31.1      Trunking 

  31.1.3  Trunking operation  

 

This page will explain how trunks manage frame transmissions between VLANs.

The switching tables at both ends of the trunk can be used to make forwarding decisions based on the destination MAC addresses of the frames. As the number of VLANs that travel across the trunk increase, the forwarding decisions become slower and more difficult to manage. The decision process becomes slower because the larger switching tables take longer to process.

Trunking protocols were developed to effectively manage the transfer of frames from different VLANs on a single physical line. The trunking protocols establish agreement for the distribution of frames to the associated ports at both ends of the trunk.

The two types of trunking mechanisms that exist are frame filtering and frame tagging. Frame tagging has been adopted as the standard trunking mechanism by the IEEE.  

Trunking protocols that use frame tagging achieve faster delivery of frames and make management easier.

The unique physical link between the two switches is able to carry traffic for any VLAN. In order to achieve this, each frame sent on the link is tagged to identify which VLAN it belongs to. Different tagging schemes exist. 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

  31.1  Trunking 

  31.1.4  VLANs and Trunking   

 

Specific protocols, or rules, are used to implement trunking. Trunking provides an effective method to distribute VLAN ID information to other switches.

The two standard trunking mechanisms are frame tagging and frame filtering. This page will explain how frame tagging can be used to provide a more scalable solution to VLAN deployment. The IEEE 802.1Q standard specifies frame tagging as the method to implement VLANs.

VLAN frame tagging was specifically developed for switched communications. Frame tagging places a unique identifier in the header of each frame as it is forwarded throughout the network backbone. The identifier is understood and examined by each switch before any broadcasts or transmissions are made to other switches, routers, or end stations. When the frame exits the network backbone, the switch removes the identifier before the frame is transmitted to the target end station. 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.

ISL is a protocol that maintains VLAN information as traffic flows between the switches. With ISL, an Ethernet frame is encapsulated with a header that contains a VLAN ID.

  31.1  Trunking 

  31.1.5  Trunking implementation  

 

This page will teach students how to create and configure a VLAN trunk on a Cisco IOS command-based switch. First configure the port as a trunk and then use the commands shown in Figure  to specify the trunk encapsulation.

Verify that trunking has been configured and verify the settings with the show interfacesFa0/port_num or show interfacestrunk commands from Privileged EXEC mode of the switch.

  31.2  VTP 

  31.2.1  History of VTP   

 

This page will introduce the VLAN Trunking Protocol (VTP).

VLAN Trunking Protocol (VTP) was created by Cisco to solve operational problems in a switched network with VLANs. It is a Cisco proprietary protocol.

Consider the example of a domain with several interconnected switches that support several VLANs. A domain is a logical group of users and resources under the control of one server, called the primary domain controller (PDC). To maintain connectivity within VLANs, each VLAN must be manually configured on each switch. As the organization grows and additional switches are added to the network, each new switch must be manually configured with VLAN information. A single incorrect VLAN assignment could cause two potential problems:

• Cross-connected VLANs due to VLAN configuration inconsistencies
• VLAN misconfiguration across mixed media environments such as Ethernet and Fiber Distributed Data Interface (FDDI)

With VTP, VLAN configuration is consistently maintained across a common administrative domain. Additionally, VTP reduces management and monitoring complexities of networks with VLANs.

  31.2  VTP 

  31.2.2  VTP concepts  

 

This page will explain how VTP is used in a network.

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. In IEEE 802.1Q frames, a 4-byte field is used to tag the frame.

While switch ports are normally assigned to only a single VLAN, trunk ports by default carry frames from all VLANs.

  31.2  VTP 

  31.2.3  VTP operation  

 

This page will explain how VTP messages are transmitted. Students will also learn about the three VTP switch modes.

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. Figure  shows the generic encapsulation for VTP within an ISL frame. The VTP header varies based on the type of VTP message, but generally, the same four items are found in all VTP messages:

• VTP protocol version - Either version 1 or 2
• VTP message type - Indicates one of four types of messages
• Management domain name length - Indicates the size of the name that follows
• Management domain name - Name configured for the management domain

VTP switches operate in one of three modes:

• Server
• Client
• Transparent

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 clients cannot create, modify, or delete VLAN information. This mode is useful for switches that lack the memory to store large tables of VLAN information. The only role of VTP clients is to process VLAN changes and send VTP messages out all trunk ports.

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.

VLANs detected within the advertisements serve as notification to the switch that traffic with the newly defined VLAN IDs may be expected.

In Figure , Switch C transmits a VTP database entry with additions or deletions to Switch A and Switch B. The configuration database has a revision number that is incremented by one. A higher configuration revision number indicates that the VLAN information that is received is more current then the stored copy. Any time a switch receives an update that has a higher configuration revision number, the switch overwrites the stored information with the new information sent in the VTP update. Switch F will not process the update because it is in a different domain. This overwrite process means that if the VLAN does not exist in the new database, it is deleted from the switch. In addition, VTP maintains its own configuration in NVRAM. The erase startup-configuration command clears the configuration in the NVRAM, but not the VTP database revision number. To set the configuration revision number back to zero, the switch must be rebooted.

By default, management domains are set to a nonsecure mode. That means that the switches interact without the use of a password. To automatically set the management domain to secure mode, a password can be added. The same password must be configured on every switch in the management domain to use secure mode.

31.1      VTP 

  31.2.4  VTP implementation  

 

This page will describe the two types of VTP advertisements and the three types of VTP messages.

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. However, the frames are not forwarded by normal bridging procedures. All devices in the same management domain learn about any new VLANs configured in the transmitting device. A new VLAN must be created and configured on one device only in the management domain. All the other devices in the same management domain automatically learn the information.

Advertisements on factory-default VLANs are based on media types. User ports should not be configured as VTP trunks.

Each advertisement starts as configuration revision number 0. As changes are made, the configuration revision number is increased incrementally by one, or n + 1. The revision number continues to increment until it reaches 2,147,483,648. When it reaches that point, the counter will reset back to zero.

There are two types of VTP advertisements:

• Requests from clients that want information at bootup
• Response from servers

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. If the domain names match, 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.

Subset advertisements contain detailed information about VLANs such as VTP version type, domain name and related fields, and the configuration revision number. Certain actions can trigger subset advertisements:

• VLAN creation or deletion
• VLAN suspension or activation
• VLAN name change
• VLAN maximum transmission unit (MTU) change 

Advertisements can contain some or all of the following information:

• Management domain name - Advertisements with different names are ignored.
• Configuration revision number - The higher number indicates a more recent configuration.
• Message Digest 5 (MD5) - MD5 is the key that is sent with the VTP when a password has been assigned. If the key does not match, the update is ignored.
• Updater identity - The updater identity is the identity of the switch that sends the VTP summary advertisement.

  31.2  VTP 

  31.2.5  VTP configuration  

 

This page will teach students how to configure VTP.

Specific steps must be considered before VTP and VLANs are configured on the network:

1. Determine the version number of VTP that will be utilized.

2. 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. 

3. Choose a VTP mode for the switch.

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 version 2 can be implemented if the features required are not in version 1. The most common feature that is needed is Token Ring VLAN support.

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

The following command can be used to enter VLAN database mode and 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. If the management domain has been secured, configure a password for the domain.

The following command can be used to create the management domain.

Switch(vlan)#vtp domain cisco

The domain name can be between 1 and 32 characters in length. The password must be between 8 and 64 characters long.

To add 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. Use the show vtp status command. Switches in a VTP domain always use the VLAN configuration of the switch with the highest VTP configuration revision number. If a switch is added with a higher revision number than what is currently in the VTP domain, it can erase all VLAN information from the VTP server and VTP domain.

Choose one of the three available VTP modes for the switch. If this is the first switch in the management domain and additional switches will be added, set the mode to server. The additional switches will be able to learn VLAN information from this switch. There should be at least one server.

VLANs can be created, deleted, and renamed at will without the switch propagating changes to other switches. VLANs can overlap if several people configure devices within a network. For example, the same VLAN ID can be used for VLANs with dissimilar purposes.

The following command can be used to set the correct mode of the switch:

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

Figure  shows the output of the show vtp status command. This command is used to verify VTP configuration settings on a Cisco IOS command-based switch.

Figure  shows an example of the show vtp counters command. This command is used to display statistics about advertisements sent and received on the switch.

  31.3  Inter-VLAN Routing Overview 

  31.3.1  VLAN basics   

 

This page will review what a VLAN is and how it is used.

A VLAN is a logical grouping of devices or users that can be grouped by function, department, or application regardless of their physical location.

VLANs are configured at the switch through software. The number of competing VLAN implementations can require the use of proprietary software from the switch vendor. Grouping ports and users into communities of interest, referred to as VLAN organizations, may be accomplished by the use of a single switch or more powerfully among connected switches within the enterprise. By grouping the ports and users together across multiple switches, VLANs can span single building infrastructures or interconnected buildings. VLANs assist in the effective use of bandwidth as they share the same broadcast domain or Layer 3 network. VLANs optimize use of bandwidth. VLANs contend for the same bandwidth although the bandwidth requirements may vary greatly by workgroup or department.    The following are some VLAN configuration issues:

• A switch creates a broadcast domain
• VLANs help manage broadcast domains
• VLANs can be defined on port groups, users or protocols
• LAN switches and network management software provide a mechanism to create VLANs

VLANs help control the size of broadcast domains and localize traffic. VLANs are associated with individual networks. Therefore, network devices in different VLANs cannot directly communicate without the intervention of a Layer 3 routing device.

When a node in one VLAN needs to communicate with a node in another VLAN, a router is necessary to route the traffic between VLANs. Without the routing device, inter-VLAN traffic would not be possible.

  31.3  Inter-VLAN Routing Overview 

  31.3.2  Introducing inter-VLAN routing   

 

This page will explain how routers operate between VLANs.

When a host in one broadcast domain wishes to communicate with a host in another broadcast domain, a router must be involved.

Port 1 on a switch is part of VLAN 1, and port 2 is part of VLAN 200.  If all of the switch ports were part of VLAN 1, the hosts connected to these ports could communicate. In this case however, the ports are part of different VLANs, VLAN 1 and VLAN 200. A router must be involved if hosts from the different VLANs need to communicate.

The most important benefit of routing is its proven history of facilitating networks, particularly large networks. Although the Internet serves as the obvious example, this point is true for any type of network, such as a large campus backbone. Because routers prevent broadcast propagation and use more intelligent forwarding algorithms than bridges and switches, routers provide more efficient use of bandwidth. This simultaneously results in flexible and optimal path selection. For example, it is very easy to implement load balancing across multiple paths in most networks when routing. On the other hand, Layer 2 load balancing can be very difficult to design, implement, and maintain.

If a VLAN spans across multiple devices a trunk is used to interconnect the devices. A trunk carries traffic for multiple VLANs. For example, a trunk can connect a switch to another switch, a switch to the inter-VLAN router, or 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.

  31.3  Inter-VLAN Routing Overview 

  31.3.3  Inter-VLAN issues and solutions   

 

This page will describe some logical and physical connectivity issues that occur between VLANs.

When VLANs are connected together, several technical issues will arise. Two of the most common issues that arise in a multiple-VLAN environment are:

• The need for end user devices to reach non-local hosts
• The need for hosts on different VLANs to communicate

When a router needs to make a connection to a remote host, it checks its routing table to determine if a known path exists. If the remote host falls into a subnet that it knows how to reach, then the system checks to see if it can connect along that interface. If all known paths fail, the system has one last option, the default route. This route is a special type of gateway route, and it is usually the only one present in the system. On a router, an asterisk (*) indicates a default route in the output of the show ip route command. For hosts on a local area network, this gateway is set to whatever machine has a direct connection to the outside world, and it is the Default Gateway listed in the workstation TCP/IP settings. If the default route is being configured for a router which itself is functioning as the gateway to the public Internet, then the default route will point to the gateway machine at an Internet service provider (ISP) site. Default routes are implemented using the ip route command.

Router(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.1

In this example, 192.168.1.1 is the gateway. Inter-VLAN connectivity can be achieved through either logical or physical connectivity.

Logical connectivity involves a single connection, or trunk, from the switch to the router. That trunk can support multiple VLANs. This topology is called a router on a stick because there is a single connection to the router. However, there are multiple logical connections between the router and the switch.

Physical connectivity involves a separate physical connection for each VLAN. This means a separate physical interface for each VLAN.

Early VLAN designs relied on external routers connected to VLAN-capable switches. In this approach, traditional routers are connected via one or more links to a switched network. The router-on-a-stick designs employ a single trunk link that connects the router to the rest of the campus network.  Inter-VLAN traffic must cross the Layer 2 backbone to reach the router where it can move between VLANs. Traffic then travels back to the desired end station using normal Layer 2 forwarding. This out-to-the-router-and-back flow is characteristic of router-on-a-stick designs.

  31.3  Inter-VLAN Routing Overview 

  31.3.4  Physical and logical interfaces   

 

This page will explain how physical and logical interfaces are added to a network design.

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

As technologies such as Inter-Switch Link (ISL) became more common, network designers began to use trunk links to connect routers to switches.  Although any trunking technology such as ISL, 802.1Q, 802.10, or LAN emulation (LANE) can be used, Ethernet-based approaches such as ISL and 802.1Q are most common.

The Cisco Proprietary protocol ISL as well as the IEEE multivendor standard 802.1q are used to trunk VLANs over Fast Ethernet links.

The solid line in the example refers to the single physical link between the Catalyst Switch and the router. This is the physical interface that connects the router to the switch.

As the number of VLANs increases on a network, the physical approach of having one router interface per VLAN quickly becomes unscalable. Networks with many VLANs must use VLAN trunking to assign multiple VLANs to a single router interface.

The dashed lines in the example refer to the multiple logical links running over this physical link using subinterfaces. The router can support many logical interfaces on individual physical links. For example, the Fast Ethernet interface FastEthernet 1/0 might support three virtual interfaces numbered FastEthernet 1/0.1, 1/0.2 and 1/0.3.

The primary advantage of using a trunk link is a reduction in the number of router and switch ports used. Not only can this save money, it can also reduce configuration complexity. Consequently, the trunk-connected router approach can scale to a much larger number of VLANs than a one-link-per-VLAN design.

  31.3  Inter-VLAN Routing Overview 

  31.3.5  Dividing physical interfaces into subinterfaces   

 

This page will introduce subinterfaces.

A subinterface is a logical interface within a physical interface, such as the Fast Ethernet interface on a router.

Multiple subinterfaces can exist on a single physical interface.

Each subinterface supports one VLAN, and is assigned one IP address. In order for multiple devices on the same VLAN to communicate, the IP addresses of all meshed subinterfaces must be on the same network or subnetwork. For example, if subinterface FastEthernet 0/0.1 has an IP address of 192.168.1.1 then 192.168.1.2, 192.168.1.3, and 192.1.1.4 are the IP addresses of devices attached to subinterface FastEthernet 0/0.1.

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

  31.3  Inter-VLAN Routing Overview 

  31.3.6  Configuring inter-VLAN routing   

 

This page will demonstrate the commands that are used to configure inter-VLAN routing between a router and a switch.

This section demonstrates the commands necessary to configure inter-VLAN routing between a router and a switch. Before any of these commands are implemented, each router and switch should be checked to see which VLAN encapsulations they support. Catalyst 2950 switches have supported 802.1q trunking since the release of Cisco IOS release 12.0(5.2)WC(1), but they do not support Inter-Switch Link (ISL) trunking. In order for inter-VLAN routing to work properly, all of the routers and switches involved must support the same encapsulation.

On a router, an interface can be logically divided into multiple, virtual subinterfaces. Subinterfaces provide a flexible solution for routing multiple data streams through a single physical interface. 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 fastethernetport-number subinterface-number

The port-number identifies the physical interface, and the subinterface-number identifies the virtual interface.

The router must be able to talk to the switch using a standardized trunking protocol. This means that both devices that are connected together must understand each other. In the example, 802.1Q is used. 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. A VLAN ID is added to the frame only when the frame is destined for a nonlocal network. Each VLAN packet carries the VLAN ID within the packet header.

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

Router(config-if)#ip address ip-address subnet-mask

The ip-address and subnet-mask are the 32-bit network address and mask of the specific interface.

In the example, the router has three subinterfaces configured on Fast Ethernet interface 0/0. These three interfaces are identified as 0/0.1, 0/0.2, and 0/0.3. All interfaces are encapsulated for 802.1Q. Interface 0/0.1 is routing packets for VLAN 1, whereas interface 0/0.2 is routing packets for VLAN 20 and 0/0.3 is routing packets for VLAN 30.

  Summary  

 

This page summarizes the topics discussed in this module.

A trunk is a physical and logical connection between two switches across which network traffic travels. The concept of trunking goes back to the origins of radio and telephony technologies. In the context of a VLAN switching environment, a trunk is a point-to-point link that supports several VLANs.

The purpose of a trunk is to conserve ports when creating a link between two devices implementing VLANs. Trunking will bundle multiple virtual links over one physical link by allowing the traffic for several VLANs to travel over a single cable between the switches.

Switching tables at both ends of the trunk can be used to make port forwarding decisions based on frame destination MAC addresses. This process slows as the number of VLANs traveling across the trunk increases. To effectively manage the transfer of frames from different VLANs on a single physical line trunking protocols were developed. The trunking protocols establish agreement for the distribution of frames to the associated ports at both ends of the trunk.

There are two types of trunking mechanisms, fame filtering and frame tagging. Trunking protocols that use a frame tagging mechanism assign an identifier to the frames. This provides better management and faster delivery. Frame tagging functions at Layer 2 and requires little processing or administrative overhead. ISL, the Cisco proprietary Inter-Switch Link protocol and 802-1Q, the IEEE standard are the most common tagging schemes for Ethernet segments.

Before trunking can be implemented, determine what encapsulation the port can support by using the show port capabilities command. To verify that trunking has been configured use the show trunk [mod_num/port_num ] command from Privileged mode on the switch.

VLAN Trunking Protocol (VTP) was created to solve operational problems in a switched network with VLANs. The two most common problems include cross-connected VLANs caused by configuration inconsistencies and misconfiguration across mixed media environments.

With VTP, VLAN configuration is consistently maintained across a common administrative domain. 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. They include server which can create, modify, and delete VLAN and VLAN configuration parameters for the entire domain, client which processes VLAN changes and sends VTP messages out all trunk ports, and transparent which forwards VTP advertisements but ignores information contained in the message.

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.

There are two types of VTP advertisements; client requests and server responses. They generate three types of VTP messages including an advertisement request, summary advertisement, and a subset advertisement. 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. That number is compared and if there are differences, requests new VLAN information. Subset advertisements contain detailed information about VLANs such as VTP version type, domain name and related fields, and the configuration revision number.

Before configuring VTP and VLAN on a network, determine the version number of VTP, if anew domain should be created, and the VTP mode. There should be at least one server. To set the correct mode of the Cisco IOS command-based switch, use the Switch(vlan)#vtp {client | server | transparent} command.

Use the show vtp status command to verify the VTP configuration revision number is lower than the configuration revision number on the other switches in the VTP domain before adding a client.

When a host in one broadcast domain wishes to communicate with a host in another broadcast domain, a router must be involved. On a router, an interface can be logically divided into multiple, virtual subinterfaces. Subinterfaces provide a flexible solution for routing multiple data streams through a single physical interface.