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October 8 2011
, Written by Cisco & Cisco Router, Network Switch
To select the appropriate switch for a layer in a particular network, you need to have specifications that detail the target traffic flows, user communities, data servers, and data storage servers. Company needs a network that can meet evolving requirements.
Traffic flow analysis is the process of measuring the bandwidth usage on a network and analysing the data for the purpose of performance tuning, capacity planning, and making hardware improvement decisions.
Network switches come in different sizes, features and functions, choosing a switch to match a particular network sometimes constitute a daunting task.
Consider what will happen if the HR or HQ department grows by five employees or more’ A solid network plan includes the rate of personnel growth over the past five years to be able to anticipate the future growth. With that in mind, you would want to purchase a switch that can accommodate more than 24 ports, such as stackable or modular switches that can scale.
When selecting a switch for the* access, **distribution, or ***core layer, consider the ability of the switch to support the port density, forwarding rates, and bandwidth aggregation requirements of your network.
Accesslayer switchesfacilitate the connection of end node devices to the network e.g. PC, Modems, IP phone, Printers etc. For this reason, they need to support features such as port security, VLANs, Fast Ethernet/Gigabit Ethernet, PoE(power over Internet, and link aggregation. Port security allows the switch to decide how many or what type of devices are permitted to connect to the switch. This is where most Cisco comes in, they all support port layer security. Most renowned network administrator knows this is the first line of defence.
DistributionLayer switchesplays a very important role on the network. They collect the data from all the access layer switches and forward it to the core layer switches. Traffic that is generated at Layer 2 on a switched network needs to be managed, or segmented into VLANs, Distribution layer switches provides the inter-VLAN routing functions so that one VLAN can communicate with another on the network.
Distribution layer switches provides advanced security policies that can be applied to network traffic using Access Control Lists (ACL). This type of security allows the switch to prevent certain types of traffic and permit others. ACLs also allow you to control, which network devices can communicate on the network.
Corelayer switches: These types of switches at the core layer of a topology, which is the high-speed backbone of the network and requires switches that can handle very high forwarding rates. The switch that operates in this area also needs to support link aggregation (10GbE connections which is currently the fastest available Ethernet connectivity.) to ensure adequate bandwidth coming into the core from the distribution layer switches.
Also, core layer switches support additional hardware redundancy features like redundant power supplies that can be swapped while the switch continues to operate. Because of the high workload carried by core layer switches, they tend to operate hotter than access or distribution layer switches, so they should have more sophisticated cooling options. Many true, core layer-capable switches have the ability to swap cooling fans without having to turn the switch off.
For example, it would be disruptive to shut down a switch at the core layer to change a power supply or a fan in the middle of the day when the network usage is at its Peak. To perform a hardware replacement, you could expect to have at least a 10 to 15 minute network shutdown, and that is if you are very fast at performing the maintenance. In more realistic circumstances, the switch could be down for 30 to 45 minutes or more, which most likely is not acceptable. With hot-swappable hardware, there is no downtime during switch maintenance.
Another characteristic one needs to put into consideration is port speed, which at times depend on performance requirements, choosing between fast Ethernet and Gigabit Ethernet Switch Ports.
Fast Ethernet allows up to 100 Mb/s of traffic per switch port while Gigabit Ethernet allows up to 1000 Mb/s of traffic per switch port. Fast Ethernet is adequate for IP telephony and data traffic on most business networks; however, performance is slower than Gigabit Ethernet ports..
Port density is the number of ports available on a single switch. Fixed configuration switches support up to 48 ports on a single device, with options for up to four additional ports.
High port densities allow for better use of space and power when both are in limited supply. If you have two switches that each contain 24 ports, you would be able to support up to 46 devices, because you lose at least one port per switch to connect each switch to the rest of the network. In addition, two power outlets are required. On the other hand, if you have a single 48-port switch, 47 devices can be supported, with only one port used to connect the switch to the rest of the network, and only one power outlet needed to accommodate the single switch.
Modular switches can support very high port densities through the addition of multiple switch port line cards, as shown in the figure. For example, the Cisco Catalyst 6500 switch can support in excess of 1,000 switch ports on a single device.
Switches have different processing capabilities at the rate in which they process data per second. Processing and forwarding data rates are very important when selecting a switch, the lower the processing, the slower the forwarding this results to the switch unable to accommodate full wire-speed communication across all its ports. A normal fast Ethernet port attains a 100Mb/s , while Gigabit Ethernet does 1000Mb/s.
For example, a 48-port gigabit switch operating at full wire speed generates 48 Gb/s of traffic. If the switch only supports a forwarding rate of 32 Gb/s, it cannot run at full wire speed across all ports simultaneously.
The more ports you have on a switch to support bandwidth aggregation, the more speed you have on your network traffic,. e.g. , consider a Gigabit Ethernet port, which carries up to 1 Gb/s of traffic in a network. If you have a 24-port switch, with all its ports capable of running at gigabit speeds, you could generate up to 24 Gb/s of network traffic. If the switch is connected to the rest of the network by a single network cable, it can only forward 1 Gb/s of the data to the rest of that network. Due to the contention for bandwidth, the data would forward more slowly. That results in 1 out of 24 wire speed available to each of the 24 devices connected to the switch.
Power over Ethernet (PoE)
Another characteristic you consider when choosing a switch is Power over Ethernet (PoE). This is the ability of the switch to deliver power to a device over the existing Ethernet cabling. IP phones and some wireless access points can use this feature, you can be able to install them anywhere you can run an Ethernet cable.
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