Now that you know which factors to consider when choosing a switch, let us examine which features are required at each layer in a hierarchical network. You will then be able to match the switch specification with its ability to function as an access, distribution, or core layer switch.
Access layer switches facilitate the connection of end node devices to the network. For this reason, they need to support features such as port security, VLANs, Fast Ethernet/Gigabit Ethernet, PoE, and link aggregation.
Port security allows the switch to decide how many or what specific devices are allowed to connect to the switch. All Cisco switches support port layer security. Port security is applied at the access layer. Consequently, it is an important first line of defense for a network. You will learn about port security in Chapter 2.
VLANs are an important component of a converged network. Voice traffic is typically given a separate VLAN. In this way, voice traffic can be supported with more bandwidth, more redundant connections, and improved security. Access layer switches allow you to set the VLANs for the end node devices on your network.
Port speed is also a characteristic you need to consider for your access layer switches. Depending on the performance requirements for your network, you must choose between Fast Ethernet and Gigabit Ethernet switch ports. Fast Ethernet allows up to 100 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. Gigabit Ethernet allows up to 1000 Mb/s of traffic per switch port. Most modern devices, such as workstations, notebooks, and IP phones, support Gigabit Ethernet. This allows for much more efficient data transfers, enabling users to be more productive. Gigabit Ethernet does have a drawback-switches supporting Gigabit Ethernet are more expensive.
Another feature requirement for some access layer switches is PoE. PoE dramatically increases the overall price of the switch across all Cisco Catalyst switch product lines, so it should only be considered when voice convergence is required or wireless access points are being implemented, and power is difficult or expensive to run to the desired location.
Link aggregation is another feature that is common to most access layer switches. Link aggregation allows the switch to use multiple links simultaneously. Access layer switches take advantage of link aggregation when aggregating bandwidth up to distribution layer switches.
Because the uplink connection between the access layer switch and the distribution layer switch is typically the bottleneck in communication, the internal forwarding rate of access layer switches does not need to be as high as the link between the distribution and access layer switches. Characteristics such as the internal forwarding rate are less of a concern for access layer switches because they only handle traffic from the end devices and forward it to the distribution layer switches.
In a converged network supporting voice, video and data network traffic, access layer switches need to support QoS to maintain the prioritization of traffic. Cisco IP phones are types of equipment that are found at the access layer. When a Cisco IP phone is plugged into an access layer switch port configured to support voice traffic, that switch port tells the IP phone how to send its voice traffic. QoS needs to be enabled on access layer switches so that voice traffic the IP phone has priority over, for example, data traffic.
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