Access Layer Switch Features

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. 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 ayer 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.

Switch Features

Switch Form Factors

What are the key features of switches that are used in hierarchical networks? When you look up the specifications for a switch, what do all of the acronyms and word phrases mean? What does "PoE" mean and what is "forwarding rate"? In this topic, you will learn about these features.

When you are selecting a switch, you need to decide between fixed configuration or modular configuration, and stackable or non-stackable. Another consideration is the thickness of the switch expressed in number of rack units. For example, the Fixed Configuration Switches shown in the figure are all 1 rack unit (1U). These options are sometimes referred to as switch form factors.

Fixed Configuration Switches

Fixed configuration switches are just as you might expect, fixed in their configuration. What that means is that you cannot add features or options to the switch beyond those that originally came with the switch. The particular model you purchase determines the features and options available. For example, if you purchase a 24-port gigabit fixed switch, you cannot add additional ports when you need them. There are typically different configuration choices that vary in how many and what types of ports are included.

Modular Switches

Modular switches offer more flexibility in their configuration. Modular switches typically come with different sized chassis that allow for the installation of different numbers of modular line cards. The line cards actually contain the ports. The line card fits into the switch chassis like expansion cards fit into a PC. The larger the chassis, the more modules it can support. As you can see in the figure, there can be many different chassis sizes to choose from. If you bought a modular switch with a 24-port line card, you could easily add an additional 24 port line card, to bring the total number of ports up to 48.

Stackable Switches

Stackable switches can be interconnected using a special backplane cable that provides high-bandwidth throughput between the switches. Cisco introduced StackWise technology in one of its switch product lines. StackWise allows you to interconnect up to nine switches using fully redundant backplane connections. As you can see in the figure, switches are stacked one atop of the other, and cables connect the switches in daisy chain fashion. The stacked switches effectively operate as a single larger switch. Stackable switches are desirable where fault tolerance and bandwidth availability are critical and a modular switch is too costly to implement. Using cross-connected connections, the network can recover quickly if a single switch fails. Stackable switches use a special port for interconnections and do not use line ports for inter-switch connections. The speeds are also typically faster than using line ports for connection switches.

Considerations for Hierarchical Network Switches

Traffic Flow Analysis

To select the appropriate switch for a layer in a hierarchical network, you need to have specifications that detail the target traffic flows, user communities, data servers, and data storage servers.

Companies need a network that can meet evolving requirements. A business may start with a few PCs interconnected so that they can share data. As the business adds more employees, devices, such as PCs, printers, and servers, are added to the network. Accompanying the new devices is an increase in network traffic. Some companies are replacing their existing telephone systems with converged VoIP phone systems, which adds additional traffic.

When selecting switch hardware, determine which switches are needed in the core, distribution, and access layers to accommodate the bandwidth requirements of your network. Your plan should take into account future bandwidth requirements. Purchase the appropriate Cisco switch hardware to accommodate both current needs as well as future needs. To help you more accurately choose appropriate switches, perform and record traffic flow analyses on a regular basis.

Traffic Flow Analysis

Traffic flow analysis is the process of measuring the bandwidth usage on a network and analyzing the data for the purpose of performance tuning, capacity planning, and making hardware improvement decisions. Traffic low analysis is done using traffic flow analysis software. Although there is no precise definition of network traffic flow, for the purposes of traffic flow analysis we can say that network traffic is the amount of data sent through a network for a given period of time. All network data contributes to the traffic, regardless of its purpose or source. Analyzing the various traffic sources and their impact on the network, allows you to more accurately tune and upgrade the network to achieve the best possible performance.

Traffic flow data can be used to help determine just how long you can continue using existing network hardware before it makes sense to upgrade to accommodate additional bandwidth requirements. When you are making your decisions about which hardware to purchase, you should consider port densities and switch forwarding rates to ensure adequate growth capability. Port density and forwarding rates are explained later in this chapter.

There are many ways to monitor traffic flow on a network. You can manually monitor individual switch ports to get the bandwidth utilization over time. When analyzing the traffic flow data, you want to determine future traffic flow requirements based on the capacity at certain times of the day and where most of the data is generated and sent. However, to obtain accurate results, you need to record enough data. Manual recording of traffic data is a tedious process that requires a lot of time and diligence. Fortunately, there are some automated solutions.

Analysis Tools

Many traffic flow analysis tools that automatically record traffic flow data to a database and perform a trend analysis are available. In larger networks, software collection solutions are the only effective method for performing traffic flow analysis. The figure displays sample output from Solarwinds Orion 8.1 NetFlow Analysis, which monitors traffic flow on a network. While the software is collecting data, you can see just how every interface is performing at any given point in time on the network. Using the included charts, you can identify traffic flow problems visually. This is much easier than having to interpret the numbers in a column of traffic flow data.

User Communities Analysis

User community analysis is the process of identifying various groupings of users and their impact on network performance. The way users are grouped affects issues related to port density and traffic flow, which, in turn, influences the selection of network switches. Port density is explained later in this chapter.

In a typical office building, end users are grouped according to their job function, because they require similar access to resources and applications. You may find the Human Resource (HR) department located on one floor of an office building, while Finance is located on another floor. Each department has a different number of users and application needs, and requires access to different data resources available through the network. For example, when selecting switches for the wiring closets of the HR and Finance departments, you would choose a switch that had enough ports to meet the department needs and was powerful enough to accommodate the traffic requirements for all the devices on that floor. Additionally, a good network design plan factors in the growth of each department to ensure that there are enough open switch ports that can utilized before the next planned upgrade to the network.

As shown in the figure, the HR department requires 20 workstations for its 20 users. That translates to 20 switch ports needed to connect the workstations to the network. If you were to select an appropriate access layer switch to accommodate the HR department, you would probably choose a 24 port switch, which has enough ports to accommodate the 20 workstations and the uplinks to the distribution layer switches.

Future Growth
But this plan does not account for future growth. Consider what will happen if the HR department grows by five employees. 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.

As well as looking at the number of devices on a given switch in a network, you should investigate the network traffic generated by end-user applications. Some user communities use applications that generate a lot of network traffic, while other user communities do not. By measuring the network traffic generated for all applications in use by different user communities, and determining the location of the data source, you can identify the effect of adding more users to that community.

A workgroup-sized user community in a small business is supported by a couple of switches and typically connected to the same switch as the server. In medium-sized businesses or enterprises, user communities are supported by many switches. The resources that medium-sized business or enterprise user communities need could be located in geographically separate areas. Consequently, the location of the user communities influences where data stores and server farms are located.



If the Finance users are using a network-intensive application that exchanges data with a specific server on the network, it may make sense to locate the Finance user community close to that server. By locating users close to their servers and data stores, you can reduce the network diameter for their communications, thereby reducing the impact of their traffic across the rest of the network.

One complication of analyzing application usage by user communities is that usage is not always bound by department or physical location. You may have to analyze the impact of the application across many network switches to determine its overall impact.

what is converged network

Small and medium-sized businesses are embracing the idea of running voice and video services on their data networks. Let us look at how voice and video over IP (VoIP) affect a hierarchical network.

Legacy Equipment

Convergence is the process of combining voice and video communications on a data network. Converged networks have existed for a while now, but were only feasible in large enterprise organizations because of the network infrastructure requirements and complex management that was involved to make them work seamlessly. There were high network costs associated with convergence because more expensive switch hardware was required to support the additional bandwidth requirements. Converged networks also required extensive management in relation to Quality of Service (QoS), because voice and video data traffic needed to be classified and prioritized on the network. Few individuals had the expertise in voice, video, and data networks to make convergence feasible and functional. In addition, legacy equipment hinders the process. The figure shows a legacy telephone company switch. Most telephone companies today have made the transition to digital-based switches. However, there are many offices that still use analog phones, so they still have existing analog telephone wiring closets. Because analog phones have not yet been replaced, you will also see equipment that has to support both legacy PBX telephone systems and IP-based phones. This sort of equipment will slowly be migrated to modern IP-based phone switches.

Advanced Technology

Converging voice, video, and data networks has become more popular recently in the small to medium-sized business market because of advancements in technology. Convergence is now easier to implement and manage, and less expensive to purchase. The figure shows a high-end VoIP phone and switch combination suitable for a medium-sized business of 250-400 employees. The figure also shows a Cisco Catalyst Express 500 switch and a Cisco 7906G phone suitable for small to medium-sized businesses. This VoIP technology used to be affordable only to enterprises and governments.

Moving to a converged network can be a difficult decision if the business already invested in separate voice, video, and data networks. It is difficult to abandon an investment that still works, but there are several advantages to converging voice, video, and data on a single network infrastructure.

One benefit of a converged network is that there is just one network to manage. With separate voice, video, and data networks, changes to the network have to be coordinated across networks. There are also additional costs resulting from using three sets of network cabling. Using a single network means you just have to manage one wired infrastructure.

Another benefit is lower implementation and management costs. It is less expensive to implement a single network infrastructure than three distinct network infrastructures. Managing a single network is also less expensive. Traditionally, if a business has a separate voice and data network, they have one group of people managing the voice network and another group managing the data network. With a converged network, you have one group managing both the voice and data networks.



New Options

Converged networks give you options that had not existed previously. You can now tie voice and video communications directly into an employee's personal computer system, as shown in the figure. There is no need for an expensive handset phone or videoconferencing equipment. You can accomplish the same function using special software integrated with a personal computer. Softphones, such as the Cisco IP Communicator, offer a lot of flexibility for businesses. The person in the top left of the figure is using a softphone on the computer. When software is used in place of a physical phone, a business can quickly convert to converged networks, because there is no capital expense in purchasing IP phones and the switches needed to power the phones. With the addition of inexpensive webcams, videoconferencing can be added to a softphone. These are just a few examples provided by a broader communications solution portfolio that redefine business processes today.

Separate Voice, Video and Data Networks

As you see in the figure, a voice network contains isolated phone lines running to a PBX switch to allow phone connectivity to the Public Switched Telephone Network (PSTN). When a new phone is added, a new line has to be run back to the PBX. The PBX switch is typically located in a Telco wiring closet, separate from the data and video wiring closets. The wiring closets are usually separated because different support personnel require access to each system. However, using a properly designed hierarchical network, and implementing QoS policies that prioritize the audio data, voice data can be converged onto an existing data network with little to no impact on audio quality.

In this figure, videoconferencing equipment is wired separately from the voice and data networks. Videoconferencing data can consume significant bandwidth on a network. As a result, video networks were maintained separately to allow the videoconferencing equipment to operate at full speed without competing for bandwidth with voice and data streams. Using a properly designed hierarchical network, and implementing QoS policies that prioritize the video data, video can be converged onto an existing data network with little to no impact on video quality.

The data network interconnects the workstations and servers on a network to facilitate resource sharing. Data networks can consume significant data bandwidth, which is why voice, video, and data networks were kept separated for such a long time. Now that properly designed hierarchical networks can accommodate the bandwidth requirements of voice, video, and data communications at the same time, it makes sense to converge them all onto a single hierarchical network.