Trunking – Information Technology

Trunk is a computer terminology that is used to describe the communication line conveying multiple signal simultaneously thus facilitating the network access between two points. In simple terms trunk facilitate a connection in the switching center in a communication system (Garimella, et al., 2007). The signal that which is convey can be any type of communications data. In most cases, trunk consist of multiple wires strand bundled in a single physical cable or may consist of a single high-capacity link that allow signals to be multiplexed in order to optimize the availability of bandwidth. It is important to note that trunk can be in the form of cluster of broadcasting frequencies such as trunked radio system which allows a few radio frequency channels to be shared by a large group of users.

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            Analysis indicated that there are two type of trunks that data networks uses to convey the data from one point to another. The first type is referred to as a trunk port which conveys the data from virtual LANs (VLANs) or from multiple local area networks across a single interconnect between routers or switches. The second type is referred to as port trucking which joints together multiple physical links thus creating a single higher capacity. A trunk port identifies each frame with a special identifying tags which are determined by IEEE standard 802.1Q for virtual local areas network tags for Ethernet frames. As the signal passes between the switches, each frame is connected to the intended VLAN at the other end of the link (Garimella, et al., 2007). It is important to note that each Ethernet interface can only function as access port or trunk port but not serve the two functions at the same time. However, trunk port can have numerous VLAN routed on the interface. Therefore, trunk port can convey numerous traffic for numerous VLANs at the same time.

The purpose of Trunking

            Studies have shown that using VLAN trunking enhances real-time communication in organization’s telecommunication since it simplifies the communication within and between organizations. There are three main purpose of the trunking:

  • Saves money: The organizations that have deployed VLAN trunking have substantially experienced the reduction in cost especially on long distance services. The use of SIP trunk connection significantly reduces costs incurred in the communication system.
  • Eliminate IP-PSTN gateways: SIP trunks is linked directly to the ITSP hence eliminating the need for IP-PSTN gateways which is created when the connection is designed to traverse the public switched telephone network. SIP trunks also eliminate attendant cost as well as the complexity of the network. Studies have indicated that ITSPs can host a PBX hence eliminating both user administration and PBX hardware thus substantially reducing the cost of administration, maintenance and complexity.
  • Gets rid of a redundant network: Organizations that have deployed SIP trunking in their communication system have proved to have made a logical step towards creating a single IP-based network and eliminating data networks and redundant telephone.

Functions of trunking

            The main function of the trunk links is to convey VLAN information from one switch to another. This explains the reason why a port on a Cisco switch functions as either trunk port or access port. The access port does not provide identification tags on the frames that are conveyed between the switches and it belong to a single VLAN. In addition, access port function as carrier of traffic intended for a specific VLAN routed to the port (Garimella, et al., 2007). On the other hand, trunk port is generally a member of all VLANs that exists on the switches and function as conveyor of the traffic for all those VLANs between the switches. As oppose to access ports, trunk port uses special tags to mark the frames as they pass between the switches. In order for trunking to function appropriately, it must be enable on both sides of a link. For instance, when two switches are connected together, both switch ports must be configured for trunking. Both switch ports must have the same tagging mechanism (ISL or 802.1Q)

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Trunk configuration

            There are three major methods that Cisco’s switch trunk can be configured: actively dynamics, passively dynamic and statistically. However, most cisco switches by default are configured into a passively dynamic state which is commonly referred to as Dynamic Auto mode. The configuration process first requires trunking mode of a switch-port to be utilized in the combination with the Dynamic Trunking Protocol (DTP) that is utilized to facilitate port negotiation for trunking parameters (U. S Patent No. 6,445,715, 2002). It is important to understand that Dynamic Auto does not make the switch-ports to become trunks. The difference between the actively dynamic port which commonly referred to as Dynamic desirable and passively dynamic ports is that actively dynamic ports attempts to become a trunk without remote solicitation.

Trunking mechanisms           

There is only one primary mechanism that modern switches uses to trunk between devices. This primary mechanism is commonly referred to as IEEE 801.1Q. This mechanism permit the passage of traffic from numerous VLANs to across a trunk. The passage of traffic from multiple VLANs is accomplished using two methods: Tagging mechanism and a native VLANs (Garimella, et al., 2007). The difference between the two methods is traffic in the native VLAN does not change. There is addition of tags as in the case of tagging mechanism.

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