Content Overview Early VLANs were difficult to implement across networks. Most VLANs were defined on each switch, which meant that defining VLANs over an extended network was a complicated task. Every switch manufacturer had a different idea of the best ways to make their switches VLAN capable, which further complicated matters. VLAN trunking was developed to solve these problems. VLAN trunking allows many VLANs to be defined throughout an organization by adding special tags to frames to identify the VLAN to which they belong. This tagging allows many VLANs to be carried across a common backbone, or trunk. VLAN trunking is standards-based, with the IEEE 802.1Q trunking protocol now widely implemented. Cisco’s Inter-Switch Link (ISL) is a proprietary trunking protocol that can be implemented in all Cisco networks. VLAN trunking uses tagged frames to allow multiple VLANs to be carried throughout a large switched network over shared backbones. Manually configuring and maintaining VLAN Trunking Protocol (VTP) on numerous switches can be challenging. The benefit of VTP is that, once a network is configured with VTP, many of the VLAN configuration tasks are automatic. This module explains VTP implementation in a VLAN switched LAN environment. VLAN technology provides network administrators with many advantages. Among other things, VLANs help control Layer 3 broadcasts, they improve network security, and they can help logically group network users. However, VLANs have an important limitation. They operate at Layer 2, which means that devices on one VLAN cannot communicate with users on another VLAN without the use of routers and network layer addresses. Students completing this module should be able to:
Content 9.1 Trunking 9.1.1 History of trunking The history of trunking goes back to the origins of radio and telephony technologies. In radio technologies, 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 that had been 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 a number of trunks. At present, 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.
Content 9.1 Trunking 9.1.2 Trunking concepts 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. Those points are usually switching centers. 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. Figure illustrates two VLANs shared across two switches, (Sa and Sb). Each switch is using two physical links so that each port carries traffic for a single VLAN. This is the simplest way of implementing inter-switch VLAN communication, but it does not scale well. Adding a third VLAN would require using two additional 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 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. A comparison for trunking is like a Highway Distributor. The roads with different starting and ending points share a main national highway for a few kilometers then will divide again to reach their particular destinations. This method is more cost effective than building an entire road from start to end for every existing or new destination. Web Links Understanding and Configuring VLAN Trunk Protocol (VTP) http://www.cisco.com/en/US/tech/
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Content 9.1 Trunking 9.1.3 Trunking operation The switching tables at both ends of the trunk can be used to make port forwarding decisions based on frame destination MAC addresses. As the number of VLANs traveling across the trunk increases, 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. Currently two types of trunking mechanisms exist, frame filtering and frame tagging. Frame tagging has been adopted as the standard trunking mechanism by IEEE. Trunking protocols that use a frame tagging mechanism assign an identifier to the frames to make their management easier and to achieve a faster delivery of the frames. 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 most common tagging schemes for Ethernet segments are listed below: Interactive Media Activity Fill in the Blanks: Trunking Operation When the student has completed this activity, the student will how using trunk links can the number of physical interfaces needed on a switch. Web Links Understanding and Configuring VLAN Trunk Protocol (VTP) http://www.cisco.com/en/US/ tech/tk389/ tk689/technologies_tech_ note09186a0080094c52.shtml
Content 9.1 Trunking 9.1.4 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. Using frame tagging as the standard trunking mechanism, as opposed to frame filtering, provides a more scalable solution to VLAN deployment. Frame tagging is the way to implement VLANs according to IEEE 802.1Q. VLAN frame tagging is an approach that has been 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-station devices. 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 requires little processing or administrative overhead. It is important to understand that a trunk link does not belong to a specific VLAN. The responsibility of a trunk link is to act as a conduit for VLANs between switches and routers. ISL is a protocol that maintains VLAN information as