Switch Features.......

When selecting a switch for the access, distribution, or core layers, consider the ability of the switch to support the port density, forwarding rates, and bandwidth aggregation requirements of your network.
Port Density
Port density is the number of ports available on a single switch. Fixed configuration switches typically support up to 48 ports on a single device, with options for up to four additional ports for small form-factor pluggable (SFP) devices, as shown in the figure. 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 Catalyst 6500 switch can support in excess of 1,000 switch ports on a single device.
You must also address the issue of uplink bottlenecks. A series of fixed configuration switches may consume many additional ports for bandwidth aggregation between switches for the purpose of achieving target performance. With a single modular switch, bandwidth aggregation is less of an issue because the backplane of the chassis can provide the necessary bandwidth to accommodate the devices connected to the switch port line cards.
Forwarding Rates
Forwarding rates define the processing capabilities of a switch by rating how much data the switch can process per second. Switch product lines are classified by forwarding rates. Entry-layer switches have lower forwarding rates than enterprise-layer switches. Forwarding rates are important to consider when selecting a switch. If the switch forwarding rate is too low, it cannot accommodate full wire-speed communication across all of its switch ports. Wire speed is the data rate that each port on the switch is capable of attaining, either 100 Mb/s Fast Ethernet or 1000 Mb/s Gigabit Ethernet. 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. Fortunately, access layer switches typically do not need to operate at full wire speed because they are physically limited by their uplinks to the distribution layer. This allows you to use less expensive, lower performing switches at the access layer, and use the more expensive, higher performing switches at the distribution and core layers, where the forwarding rate makes a bigger difference.
Link Aggregation
As part of bandwidth aggregation, you should determine if there are enough ports on a switch to aggregate to support the required bandwidth. For example, consider a Gigabit Ethernet port, which carries up to 1 Gb/s of traffic. If you have a 24-port switch, with all 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 the network. Due to the contention for bandwidth, the data would forward more slowly. That results in 1/24th wire speed available to each of the 24 devices connected to the switch. Wire speed describes the theoretical maximum data transmission rate of a connection. For example, the wire speed of an Ethernet connection is dependent on the physical and electrical properties of the cable, combined with the lowest layer of the connection protocols.
Link aggregation helps to reduce these bottlenecks of traffic by allowing up to eight switch ports to be bound together for data communications, providing up to 8 Gb/s of data throughput when Gigabit Ethernet ports are used. With the addition of multiple 10 Gigabit Ethernet (10GbE) uplinks on some enterprise-layer switches, very high throughput rates can be achieved. Cisco uses the term EtherChannel when describing aggregated switch ports.


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