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NOT ALL SWITCHES PERFORM THE SAME

Not all Ethernet switching engines are designed the same and therefore their characteristics are a performance factor that must be considered in the configuration of an extended LAN. Switching modules are designed to interconnect LAN segments in much the same way that a telephone conversation is linked using a PBX. The LAN switching engine itself provides for the full wire speed interconnection of a LAN’s segments. Although the terminology has changed in the past few years, two prevalent switching designs may be found in Ethernet switches: cut-through switching and store-and-forward switching.

Cut-Through Switching

Under this switching architecture, the switch has been designed to forward packets to their destination before a packet is fully received and before the collision window passes. This type of architecture does not limit the end-to-end throughput as do store-and-forward bridges, for example. Cisco and IBM switches use this design characteristic.

Store-and-Forward Architecture

Under this switching architecture, the whole packet is fully received before the forwarding process begins. Each packet is buffered in memory and the switch examines the entire packet. Because the packets can be inspected, more advanced management capabilities are available.

The forwarding method used by Ethernet switches varies and is based on whether there is bridging or routing software. Some vendors (e.g., 3Com, Cisco, and Performance Technologies, Inc.) have methods that combine techniques from cut-through and store and forward. Depending on error thresholds, these switches may switch from store and forward to a form of adaptive cut-through.

Using proprietary software, a vendor will incorporate one of these designs into a proprietary switching matrix. This software switching matrix is integrated into specially designed hardware that will support back-to-back packets in a F-DX mode. Most switch designs offer multiple ports (four to eight) to support simultaneous Ethernet connections between connected switches. Through this design, some Ethernet switches can offer as high as 40M-bps throughput with a transit delay as low as 70 microseconds.

In some switches, filters are incorporated into the design of the switch to filter out packet fragments or runts generated as the result of the Ethernet collision process. Some Ethernet switches support both broadcast and multicast frames at as many as 59,520 packets per second. Although port designs vary among manufacturers, most use an RJ-45 interface and provide support for several 10Base-T ports as well as multimedia (i.e., 10Base-2, 10Base-5). In some designs, provisions may exist for as many as 1,024 address per port with buffer sizes of as high as 1,500 packets per port. Other features include intrusion control and bridging security features, redundant clocking, and power supply modules.

MULTIPROTOCOL (WORKGROUP) SWITCHING

One of the greatest applications of Ethernet switches is workgroup switching. This may be a result of the fact that many large networks evolve as an amalgamation of several different smaller LANs, each with a different protocol. The key to successful network management lies in the ability of the network administrator to filter network traffic as it arrives at specific points along the network. For this reason, protocol switches have come on the market that combine wire speed connectivity with the ability to filter multiprotocol Ethernet traffic (e.g., IP, IPX, DEC, and AppleTalk).

Protocol switches often serve as workgroup accelerators or collapsed backbones. Protocol switches, when properly deployed, can support the organization of virtual LANs. Filtering is accomplished at the port level, where in some switches there can be as much as four tiers of wire speed filtering: broadcast groups, internal LANs, protocol filtering, and MAC address filtering. Through this filtering process, these switches can block or filter the propagation of unwanted traffic across a network by forming a firewall.


Exhibit 2.  University of California, San Diego Campus, Network Operations.

Firewalls

For example, using this process an administrator might block IPX traffic from crossing onto a DEC network segment while IP traffic might be allowed to pass. MAC address filtering can be used to establish a secure means to restrict traffic to specific terminals. Broadcast domains can be established that restrict broadcast packets to specific segments or to ports that are members of the same broadcast group. In this way, specific broadcasts can be restricted to those segments of which they are members.

Exhibit 2 shows the application of an inexpensive protocol switch, the Nebula 2000,which is used on the campuswide network at the UCSD. The Nebula 2000 links many diverse servers and local networks (e.g., Office LAN, mail servers, and Sun servers) by providing a high-speed protected link between networks.

Exhibit 3. Factors Effecting End-to-End Throughput

Design Characteristic

Feature Cut-Through Store/Forward

Filter corrupt and fragmented packets No Yes
Low latency Yes No
Support for redundant links No Yes
Full duplex Yes Yes
Smooth handling of broadcast packets No Yes

In this configuration, the Nebula 2000 has been divided into three separate partitions that comprise a collapsed backbone for the UCSD campus network. Protocol filtering and MAC address filtering provide a means to restrict certain types of traffic to specific segments while MAC address filtering ensures complete security against unauthorized access to privileged information. Here, the establishment of specific broadcast domains insulate all of the attached networks from broadcast saturation on unauthorized segments.

The Nebula 2000 SGNMS is used by the LAN administrator to reconfigure network segments and filtering patterns (firewalls) to suit any change in requirements. There is a complete diagnostic subsystem incorporated within the StarGazer system that allows the Network Administrator to monitor the entire system. A WAN port on the Nebula 2000 provides the support for remote diagnostics. This capability allows the network administrator to access the Nebula 2000 remotely to monitor the activity on individual ports as well as reconfigure or resegment the network.

Network Considerations

Network configurations will be affected by the switching characteristics of a switch. For example, multiple nonblocking paths between individual pairs of ports on connected switches provide for full use of all available bandwidth. With cut-through design, a packet’s latency is dramatically reduced because the leading edge of a packet exits the switch before the trailing edge enters. In this way, packets can be forwarded 20 times faster than conventional store-and-forward bridges.


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