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September 5th, 2014 07:00

An Introduction to VLAN and Trunk

An Introduction to VLAN and Trunk

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Introduction

A virtual local-area network (VLAN) can be created on a Layer 2 switch to reduce the size of broadcast domains, similar to Layer 3 device. VLANs are commonly incorporated into network design, making it easier for a network to support the goals of an organization.


Detailed Information

What is a VLAN

Within a switched internetwork, VLANs provide segmentation and organizational flexibility. VLANs provide a way to group devices within a LAN. A group of devices within a VLAN communicate as if they were attached to the same wire. VLANs are based on logical connections, instead of physical connections.

A common topology can be seen in the following figure in which two switches are separated by a router. In this case, a group of nodes are connected to each switch. The nodes on a particular switch share a common IP addressing scheme. There are two networks, 192.168.1.0 and 192.168.2.0.

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Nonlocal network traffic must be sent to the router for forwarding. Routers will not forward Layer 2 unicast, multicast and broadcast frames.

In the following figure, a single switch with multiple VLANs. A VLAN operates in the same way as a Layer 3 IP-based network.  Thus, nodes on the 192.168.1.0 network must go to the router when trying to communicate with nodes on the 192.168.2.0 network even though all of the computers are connected to the same switch. In order to communicate between VLANs, routing functionality must be part of the topology. Layer 2 unicast, multicast and broadcast traffic will not cross VLAN boundaries, therefore traffic generated on VLAN 1 will not be seen by nodes on VLAN 2.

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VLAN Trunks

A trunk is a point-to-point link between two network devices that carries more than one VLAN. A VLAN trunk extends VLANs across an entire network. VLANs would not be very useful without VLAN trunks. VLAN trunks allow all VLAN traffic to propagate between switches so that devices that are in the same VLAN, but connected to different switches, can communicate without the intervention of a router.

Trunk links are most beneficial when switches are connected to other switches or switches are connected to routers. A trunk link is not assigned to a specific VLAN. Instead, one, many, or all active VLANs can be transported between switches using a single physical trunk link.

Connecting two switches with separate physical links for each VLAN is possible. The top half of the following figure shows how two switches might be connected in this fashion.

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As VLANs are added to a network, the number of links can grow quickly. A more efficient use of physical interfaces and cabling involves the use of trunking. The bottom half of the figure shows how one trunk link can replace many individual VLAN links.

When a trunk line is installed, a trunking protocol is used to modify the Ethernet frames as they travel across the trunk line. By default, all ports are called “access ports.”When a port is used to interconnect switches and convey VLAN information, the operation of the port is changed to a trunk.

In the following figure we’ll go through an example of two nodes communicating over a trunk line. There are several steps to the process (in addition to host routing) so the following figure is labeled based on the steps listed.

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PC1 sends traffic to PC2 after processing its host routing table. These nodes are in the same VLAN but they are connected to different switches. The basic process:

·         The Ethernet frame leaves PC1 and is received by Switch 1.

·         The Switch 1 SAT(source address table) indicates that the destination is on the other end of the trunk line.

·         Switch 1 uses the trunking protocol to modify the Ethernet frame by adding the VLAN id.

·         The new frame leaves the trunk port on Switch1 and is received by Switch 2.

·         Switch2 reads the VLAN id and strips off the trunking protocol.

·         The original frame is forwarded to the destination (port 4) based on the SAT of Switch 2.






Author:Jiawen

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