Network Switching

Switches are great, they add network capacity and speed, but they are not a cure-all. How can you tell if your network will benefit from switching? And how do you add switches to your network design for the most benefit? This tutorial is written to answer these questions. Along the way we&#39;ll describe how switches work, and how they both harm and benefit your networking strategy. And we&#39;ll discuss different network types, so you can profile your network and gauge the potential benefit of switching for your environment.

What is a Switch?


Filtering of packets, and the regeneration of forwarded packets enables switching technology to split a network into separate collision domains. Regeneration of packets allows for greater distances and more nodes to be used in the total network design, and dramatically lowers the overall collision rates. In switched networks, each segment is an independent collision domain. In shared networks all nodes reside in one, big shared collision domain.

Easy to install, most switches are self learning. They determine the Ethernet addresses in use on each segment, building a table as packets are passed through the switch. This "plug and play" element makes switches an attractive alternative to hubs.

Switches can connect different networks types (such as Ethernet and Fast Ethernet) or networks of the same type. Many switches today offer high-speed links, like Fast Ethernet or FDDI, that can be used to link the switches together or to give added bandwidth to important servers that get a lot of traffic. A network composed of a number of switches linked together via these fast uplinks is called a "collapsed backbone" network.


Full duplex is another method to increase bandwidth to dedicated workstations or servers. To use full duplex, both network interface cards used in the server or workstation, and the switch must support full duplex operation. Full duplex doubles the potential bandwidth on that link, providing 20 Mbps for Ethernet and 200 Mbps for Fast Ethernet.

Network Congestion

As more users are added to a shared network or as applications requiring more data are added, performance deteriorates. This is because all users on a shared network are competitors for the Ethernet bus. A moderately loaded 10 Mbps Ethernet network is able to sustain utilization of 35% and throughput in the neighborhood of 2.5 Mbps after accounting for packet overhead, interpacket gaps and collisions. A moderately loaded Fast Ethernet shares 25 Mbps of real data in the same circumstances. With shared Ethernet and Fast Ethernet, the likelihood of collisions increases as more nodes and/or more traffic is added to the shared collision domain.


Collision rates measure the percentage of packets that are collisions. Some collisions are inevitable, with less than 10% common in well running networks.
The Factors Affecting Network Efficiency

 * ** Amount of traffic
 * ** Number of nodes
 * ** Size of packets
 * ** Network diameter
easuring Network Efficiency

 * ** Average to peak load devition
 * ** Collision Rate
 * ** Utilization Rate

Utilization rate is another widely accessible statistic about the health of a network. This statistic is available in Novell&#39;s Console monitor and WindowsNT performance monitor as well as any optional LAN analysis software. Utilization in an average network above 35% indicates potential problems. This 35% utilization is near optimum, but some networks experience higher or lower utilization optimums due to factors such as packet size and peak load deviation.

A switch is said to work at "wire speed" if it has enough processing power to handle full ethernet speed at minimum packet sizes. Most switches on the market are well ahead of network traffic capabilities supporting full "wire speed" of ethernet, 14,480 pps (packets per second)

Routers work in a manner similar to switches and bridges in that they filter out network traffic. Rather than doing so by packet addresses they filter by specific protocol. Routers were born out of the necessity for dividing networks logically instead of physically. An IP router can divide a network into various subnets so that only traffic destined for particular IP addresses can pass between segments. Routers recalculate the checksum, and rewrite the MAC header of every packet. The price paid for this type of intelligent forwarding and filtering is usually calculated in terms of latency, or the delay that a packet experiences inside the router. Such filtering takes more time than that exercised in a switch or bridge which only looks at the Ethernet address but in more complex networks network efficiency is improved. An additional benefit of routers is their automatic filtering of broadcasts, but overall they are complicated to setup.

Switch Benefits

 * ** Isolates traffic, relieving congestion
 * ** Saparates collision domains, reducing collisions
 * ** Segments, restarting distance and repeater rules
[/CODE]Switch Costs

* Price: currently 3 to 5 times the price of a hub
* Packet processing time is longer than in a hub
* Monitoring the network is more complicated

General Benefits of Switching


Switching in Your Network

The benefits of switching vary from network to network. Adding a switch for the first time has different implications than increasing the number of switched ports already installed. Understanding traffic patterns is very important to switching - the goal being to eliminate (or filter) as much traffic as possible. A switch installed in a location where it forwards almost all the traffic it receives will help much less than one that filters most of the traffic.

Networks that are not congested can actually be negatively impacted by adding switches. Packet processing delays, switch buffer limitations, and the retransmissions that can result sometimes slows performance compared with the hub based alternative. If your network is not congested, don&#39;t replace hubs with switches. How can you tell if performance problems are the result of network congestion? Measure utilization factors and collision rates.

[/CODE]Good Candidates for Performance Boosts from Switching

* Utilization more than 35%
* Collision rates more than 10%




Replacing a Central Hub with a Switch


Adding Switches to a Backbone Switched Network


Designing for Maximum Benefit</span>





Good Candidates for Performance Boosts from Switching

 * ** Important to know network demand per node
 * ** Try to group users with the nodes they communicate with most often on the same segment
 * ** Look for departmental traffic patterns
 * ** Avoid switch bottlenecks with fast uplinks
 * ** Move users switch between segments in an iterative process until all nodes seeing less than 35% utilization
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Advanced Switching Technology Issues


Managed or Unmanaged


Store-and-Forward vs. Cut-Through


Blocking vs. Non-Blocking Switches

Take a switch&#39;s specifications and add up all the ports at theoretical maximum speed, then you have the theoretical sum total of a switch&#39;s throughput. If the switching bus, or switching components cannot handle the theoretical total of all ports the switch is considered a "blocking switch". There is debate whether all switches should be designed non-blocking, but the added costs of doing so are only reasonable on switches designed to work in the largest network backbones. For almost all applications, a blocking switch that has an acceptable and reasonable throughput level will work just fine. Consider an eight port 10/100 switch. Since each port can theoretically handle 200 Mbps (full duplex) there is a theoretical need for 1600 Mbps, or 1.6 Gbps. But in the real world each port will not exceed 50% utilization, so a 800 Mbps switching bus is adequate. Consideration of total throughput versus total ports demand in the real world loads provides validation that the switch can handle the loads of your network.

Switch Buffer Limitations


<span style=\'color:red\'>Layer 3 Switching

A hybrid device is the latest improvement in internetworking technology. Combining the packet handling of routers and the speed of switching, these multilayer switches operate on both layer 2 and layer 3 of the OSI network model. The performance of this class of switch is aimed at the core of large enterprise networks. Sometimes called routing switches or IP switches, multilayer switches look for common traffic flows, and switch these flows on the hardware layer for speed. For traffic outside the normal flows, the multilayer switch uses routing functions. This keeps the higher overhead routing functions only where it is needed, and strives for the best handling strategy for each network packet. Many vendors are working on high end multilayer switches, and the technology is definitely a "work in process". As networking technology evolves, multilayer switches are likely to replace routers in most large networks. [/COLOR]