LAN Switching and VLANs     日期:2005-5-28    浏览次数:

A LAN switch is a device that provides much higher port density at a lower cost than traditional bridges. For this reason, LAN switches can accommodate network designs featuring fewer users per segment, thereby increasing the average available bandwidth per user. This chapter provides a summary of general LAN switch operation and maps LAN switching to the OSI reference model.

The trend toward fewer users per segment is known as microsegmentation. Micro-segmentation allows the creation of private or dedicated segments—that is, one user per segment. Each user receives instant access to the full bandwidth and does not have to contend for available bandwidth with other users. As a result, collisions (a normal phenomenon in shared-medium networks employing hubs) do not occur, as long as the equipment operates in full-duplex mode. A LAN switch forwards frames based on either the frame's Layer 2 address (Layer 2 LAN switch) or, in some cases, the frame's Layer 3 address (multilayer LAN switch). A LAN switch is also called a frame switch because it forwards Layer 2 frames, whereas an ATM switch forwards cells.

Figure 26-1 illustrates a LAN switch providing dedicated bandwidth to devices and illustrates the relationship of Layer 2 LAN switching to the OSI data link layer.

Figure 26-1: A LAN Switch Is a Data Link Layer Device
The earliest LAN switches were developed in 1990. They were Layer 2 devices (bridges) dedicated to solving desktop bandwidth issues. Recent LAN switches evolved to multilayer devices capable of handling protocol issues involved in high-bandwidth applications that historically have been solved by routers. Today, LAN switches are used to replace hubs in the wiring closet because user applications demand greater bandwidth.

LAN Switch Operation
LAN switches are similar to transparent bridges in functions such as learning the topology, forwarding, and filtering. These switches also support several new and unique features, such as dedicated communication between devices through full-duplex operations, multiple simultaneous conversations, and media-rate adaption.

Full-duplex communication between network devices increases file-transfer throughput. Multiple simultaneous conversations can occur by forwarding, or switching, several packets at the same time, thereby increasing network capacity by the number of conversations supported. Full-duplex communication effectively doubles the throughput, while with media-rate adaption, the LAN switch can translate between 10 and 100 Mbps, allowing bandwidth to be allocated as needed.

Deploying LAN switches requires no change to existing hubs, network interface cards (NICs), or cabling.

VLANs Defined
A VLAN is defined as a broadcast domain within a switched network. Broadcast domains describe the extent that a network propagates a broadcast frame generated by a station. Some switches may be configured to support a single or multiple VLANs. Whenever a switch supports multiple VLANs, broadcasts within one VLAN never appear in another VLAN. Switch ports configured as a member of one VLAN belong to a different broadcast domain, as compared to switch ports configured as members of a different VLAN.

Creating VLANs enables administrators to build broadcast domains with fewer users in each broadcast domain. This increases the bandwidth available to users because fewer users will contend for the bandwidth.

Routers also maintain broadcast domain isolation by blocking broadcast frames. Therefore, traffic can pass from one VLAN to another only through a router.

Normally, each subnet belongs to a different VLAN. Therefore, a network with many subnets will probably have many VLANs. Switches and VLANs enable a network administrator to assign users to broadcast domains based upon the user's job need. This provides a high level of deployment flexibility for a network administrator.

Advantages of VLANs include the following:

Segmentation of broadcast domains to create more bandwidth

Additional security by isolating users with bridge technologies

Deployment flexibility based upon job function rather than physical placement

Switch Port Modes
Switch ports run in either access or trunk mode. In access mode, the interface belongs to one and only one VLAN. Normally a switch port in access mode attaches to an end user device or a server. The frames transmitted on an access link look like any other Ethernet frame.

Trunks, on the other hand, multiplex traffic for multiple VLANs over the same physical link. Trunk links usually interconnect switches, as shown in Figure 26-2. However, they may also attach end devices such as servers that have special adapter cards that participate in the multiplexing protocol.

Figure 26-2: Switches Interconnected with Trunk Links
Note that some of the devices attach to their switch using access links, while the connections between the switches utilize trunk links.

To multiplex VLAN traffic, special protocols exist that encapsulate or tag (mark) the frames so that the receiving device knows to which VLAN the frame belongs. Trunk protocols are either proprietary or based upon IEEE 802.1Q. For example, a proprietary trunk protocol may be like Cisco's proprietary Inter-Switch Link (ISL), which enables Cisco devices to multiplex VLANs in a manner optimized for Cisco components. Or, an intervendor solution may be implemented, such as 802.1Q, which enables products from more than one vendor to multiplex VLANs on a trunk link.

Without trunk links, multiple access links must be installed to support multiple VLANs between switches. This is not cost-effective and does not scale well, so trunks are preferable for interconnecting switches in most cases.

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