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If one device at the higher level in the hierarchy fails, the lower level switch automatically fails over to the other switch. Redundancy is achieved at the distribution and core layers. Performance — it is recommended that core layer switches should have very fast switching abilities. The distribution switches should also be very fast and redundant. The result of using very fast core and distribution layer switches would guarantee very fast networks. Security — the security of the network is enhanced since at each layer of the model, there are several security measures that can be put in place; for example switch ports at the access layer can be configured with port security, segmentation of the distribution layer using VLANs is also another security feature.
Manageability is the ability to make configuration changes in the network, the use of the hierarchical model eases management of the switches. For example, making changes on one layer would be simplified since we can assume that the role of switches in that layer all perform similar functions, further, the modular design means that management does not mean that the network is down due to maintenance due to redundancy. Considerations when choosing a switch When deciding the switch we should implement for our LANs, there are several considerations that we need to take in mind.
These might be influenced by the organizational policies while others might be influenced by the technological needs. Switches with fixed configurations are switches that cannot be modified by adding additional modules, these are lower level switches and are ideal for the access layer functions. For more flexibility, we might need modular switches, these switches typically allow us to install modules such as more switching ports, these would be ideal for rapidly expanding networks that need to be changed frequently.
To provide high bandwidth, we may need to interconnect special types of switches which have a stackable ability using a backplane cable. These would be ideal for high bandwidth requirements in a large network at the core layer. Port density — this is the number of ports on a switch. In many cases you will find switches with 24 or 48 port switches. This can be a design consideration since you may need to consider the inter-switch connections. Forwarding rates are the processing capabilities of the switch.
The forwarding rate is measured by calculating how much data the switch can process in a second. This is different from the bandwidth that is available on its ports.
In most modern networks, the use of IP phones is prevalent, most of these devices get power over the LAN interfaces connected to switches using a technology called POE Power over Ethernet. As such, when deciding which devices to download, PoE should be a feature that should not be overlooked.
In recent times, switch designs have been changed so as to support layer 3 functionality, as you may already know, switches work at layer 2 of the OSI model, however, implementing layer 3 switches gives more options such as routing, IP addressing and other options. Access layer switch features There are several features that a switch at each level of the hierarchical model should have.
As we mentioned earlier, the access layer is the lowest level in the hierarchical LAN architecture, at this level user devices gain access to the network over a number of devices. As such, the features at this level include: VLAN support on the switches, Fast Ethernet and Gigabit Ethernet links, PoE and support for link aggregation so as to increase the switching speed. Security is important in our networks, at this layer, we can implemnent several security measures such as port security to control access to the network.
Link aggregation is the ability to use multiple links at the same time. This is a more effective way to use the bandwidth available on the switches. To support multiple devices on a single port, PoE is an important feature, it allows us to use the switch to power certain devices in our network such as IP phones and Wireless controllers.
The ports on access layer switches should be fast enough to support the evolving bandwidth needs of the enterprise.
As such, Fast Ethernet which offer speeds of up to Mbps and Gigabit Ethernet links which offer speeds of up to 1Gbps should be used. Distribution layer features At the distribution layer, communication across the various access layer switches should be supported, this means that these switches should offer more features than the access layer switches.
Features such as redundancy, faster ports than the access layer, layer 3 support should be implemented at this layer. The use of security policies is a security feature that should be implemented at the distribution layer, some of these may include the use of access lists.
Inter-vlan routing which is making communication between different VLANs possible should be available at this layer. The ports at this layer should be very fast, typically, Gigabit Ethernet and 10 gigabit Ethernet links should be used. VLANs allow you to segment the traffic on a switch into separate subnetworks.
For example, in a university you might separate traffic according to faculty, students, and guests. Core Layer The core layer of the hierarchical design is the high-speed backbone of the internetwork. The core layer is critical for interconnectivity between distribution layer devices, so it is important for the core to be highly available and redundant.
The core area can also connect to Internet resources. The core aggregates the traffic from all the distribution layer devices, so it must be capable of forwarding large amounts of data quickly.
Roll over the CORE button in the figure. Note: In smaller networks, it is not unusual to implement a collapsed core model, where the dis- tribution layer and core layer are combined into one layer. A Hierarchical Network in a Medium-Sized Business Let us look at the hierarchical network model applied to a business. In the figure, the access, distri- bution, and core layers are separated into a well-defined hierarchy. This logical representation makes it easy to see which switches perform which function.
It is much harder to see these hierar- chical layers when the network is installed in a business. Click the Physical Layout button in the figure.
The figure shows two floors of a building. The user computers and network devices that need net- work access are on one floor. The resources, such as e-mail servers and database servers, are lo- cated on another floor. To ensure that each floor has access to the network, access layer and distribution switches are installed in the wiring closets of each floor and connected to each of the devices needing network access.
The figure shows a small rack of switches. The access layer switch and distribution layer switch are stacked one on top of each other in the wiring closet. Although the core and other distribution layer switches are not shown, you can see how the physi- cal layout of a network differs from the logical layout of a network. Benefits of a Hierarchical Network There are many benefits associated with hierarchical network designs. Scalability Hierarchical networks scale very well.
The modularity of the design allows you to replicate design elements as the network grows. Because each instance of the module is consistent, expansion is easy to plan and implement. For example, if your design model consists of two distribution layer switches for every 10 access layer switches, you can continue to add access layer switches until you have 10 access layer switches cross-connected to the two distribution layer switches before you need to add additional distribution layer switches to the network topology.
Also, as you add more distribution layer switches to accommodate the load from the access layer switches, you can add additional core layer switches to handle the additional load on the core.
Redundancy As a network grows, availability becomes more important. You can dramatically increase availabil- ity through easy redundant implementations with hierarchical networks. Access layer switches are connected to two different distribution layer switches to ensure path redundancy.
If one of the dis- tribution layer switches fails, the access layer switch can switch to the other distribution layer switch. Additionally, distribution layer switches are connected to two or more core layer switches to ensure path availability if a core switch fails. Typically, end node devices, such as PCs, printers, and IP phones, do not have the ability to connect to multiple access layer switches for redundancy.
If an access layer switch fails, just the devices connected to that one switch would be affected by the outage. The rest of the net- work would continue to function unaffected. Performance Communication performance is enhanced by avoiding the transmission of data through low-per- forming, intermediary switches. Data is sent through aggregated switch port links from the access layer to the distribution layer at near wire speed in most cases.
The distribution layer then uses its high performance switching capabilities to forward the traffic up to the core, where it is routed to its final destination. Because the core and distribution layers perform their operations at very high speeds, there is less contention for network bandwidth. As a result, properly designed hierarchical networks can achieve near wire speed between all devices. Security Security is improved and easier to manage. Access layer switches can be configured with various port security options that provide control over which devices are allowed to connect to the net- work.
You also have the flexibility to use more advanced security policies at the distribution layer. You may apply access control policies that define which communication protocols are deployed on your network and where they are permitted to go. For example, if you want to limit the use of HTTP to a specific user community connected at the access layer, you could apply a policy that blocks HTTP traffic at the distribution layer.
Restricting traffic based on higher layer protocols, such as IP and HTTP, requires that your switches are able to process policies at that layer.
Some access layer switches support Layer 3 functionality, but it is usually the job of the distribution layer switches to process Layer 3 data, because they can process it much more efficiently. Manageability Manageability is relatively simple on a hierarchical network. Each layer of the hierarchical design performs specific functions that are consistent throughout that layer. Therefore, if you need to change the functionality of an access layer switch, you could repeat that change across all access layer switches in the network because they presumably perform the same functions at their layer.
Deployment of new switches is also simplified because switch configurations can be copied be- tween devices with very few modifications. Consistency between the switches at each layer allows for rapid recovery and simplified troubleshooting. In some special situations, there could be con- figuration inconsistencies between devices, so you should ensure that configurations are well doc- umented so that you can compare them before deployment.
Maintainability Because hierarchical networks are modular in nature and scale very easily, they are easy to main- tain. With other network topology designs, manageability becomes increasingly complicated as the network grows. Also, in some network design models, there is a finite limit to how large the net- work can grow before it becomes too complicated and expensive to maintain.
In the hierarchical design model, switch functions are defined at each layer, making the selection of the correct switch easier. Adding switches to one layer does not necessarily mean there will not be a bottle- neck or other limitation at another layer.
For a full mesh network topology to achieve maximum performance, all switches need to be high-performance switches, because each switch needs to be capable of performing all the functions on the network. In the hierarchical model, switch functions are different at each layer. You can save money by using less expensive access layer switches at the lowest layer, and spend more on the distribution and core layer switches to achieve high perform- ance on the network.
These simple guidelines will help you differentiate between well-designed and poorly designed hierarchical networks. This section is not intended to provide you with all the skills and knowledge you need to design a hierarchical network, but it offers you an opportunity to begin to practice your skills by transforming a flat network topology into a hierarchical network topology.
Network Diameter When designing a hierarchical network topology, the first thing to consider is network diameter. Diameter is usually a measure of distance, but in this case, we are using the term to measure the number of devices. Network diameter is the number of devices that a packet has to cross before it reaches its destination. Keeping the network diameter low ensures low and predictable latency be- tween devices. Roll over the Network Diameter button in the figure.
In the figure, PC1 communicates with PC3. There could be up to six interconnected switches be- tween PC1 and PC3. In this case, the network diameter is 6. Each switch in the path introduces some degree of latency. Network device latency is the time spent by a device as it processes a packet or frame. Each switch has to determine the destination MAC address of the frame, check its MAC address table, and forward the frame out the appropriate port. Even though that entire process happens in a fraction of a second, the time adds up when the frame has to cross many switches.
In the three-layer hierarchical model, Layer 2 segmentation at the distribution layer practically eliminates network diameter as an issue. In a hierarchical network, network diameter is always going to be a predictable number of hops between the source and destination devices. Bandwidth Aggregation Each layer in the hierarchical network model is a possible candidate for bandwidth aggregation.
Bandwidth aggregation is the practice of considering the specific bandwidth requirements of each part of the hierarchy.
After bandwidth requirements of the network are known, links between spe- cific switches can be aggregated, which is called link aggregation. Link aggregation allows multiple switch port links to be combined so as to achieve higher throughput between switches. Cisco has a proprietary link aggregation technology called EtherChannel, which allows multiple Ethernet links to be consolidated. A discussion of EtherChannel is beyond the scope of this course. Roll over the Bandwidth Aggregation button in the figure.
In the figure, computers PC1 and PC3 require a significant amount of bandwidth because they are used for developing weather simulations. The network manager has determined that the access layer switches S1, S3, and S5 require increased bandwidth. Following up the hierarchy, these ac- cess layer switches connect to the distribution switches D1, D2, and D4. The distribution switches connect to core layer switches C1 and C2. Notice how specific links on specific ports in each switch are aggregated.
In this way, increased bandwidth is provided for in a targeted, specific part of the network. Note that in this figure, aggregated links are indicated by two dotted lines with an oval tying them together.
In other figures, aggregated links are represented by a single, dotted line with an oval. Redundancy can be provided in a number of ways. For example, you can double up the network connections between devices, or you can double the devices themselves.
This chapter explores how to employ redundant network paths between switches. A discussion on doubling up network devices and employing special net- work protocols to ensure high availability is beyond the scope of this course. Implementing redundant links can be expensive. Imagine if every switch in each layer of the net- work hierarchy had a connection to every switch at the next layer.
It is unlikely that you will be able to implement redundancy at the access layer because of the cost and limited features in the end devices, but you can build redundancy into the distribution and core layers of the network.
Roll over the Redundant Links button in the figure. In the figure, redundant links are shown at the distribution layer and core layer. At the distribution layer, there are two distribution layer switches, the minimum required to support redundancy at this layer. The access layer switches, S1, S3, S4, and S6, are cross-connected to the distribution layer switches. This protects your network if one of the distribution switches fails. In case of a fail- ure, the access layer switch adjusts its transmission path and forwards the traffic through the other distribution switch.
Some network failure scenarios can never be prevented, for example, if the power goes out in the entire city, or the entire building is demolished because of an earthquake.
Redundancy does not at- tempt to address these types of disasters. Start at the Access Layer Imagine that a new network design is required. Design requirements, such as the level of perform- ance or redundancy necessary, are determined by the business goals of the organization. Once the design requirements are documented, the designer can begin selecting the equipment and infra- structure to implement the design. When you start the equipment selection at the access layer, you can ensure that you accommodate all network devices needing access to the network.
After you have all end devices accounted for, you have a better idea of how many access layer switches you need. The number of access layer switches, and the estimated traffic that each generates, helps you to determine how many distribu- tion layer switches are required to achieve the performance and redundancy needed for the net- work.
After you have determined the number of distribution layer switches, you can identify how many core switches are required to maintain the performance of the network. A thorough discussion on how to determine which switch to select based on traffic flow analysis and how many core switches are required to maintain performance is beyond the scope of this course. For a good introduction to network design, read this book that is available from Cisco- press.
Small and medium-sized businesses are embracing the idea of running voice and video services on their data networks. Legacy Equipment Convergence is the process of combining voice and video communications on a data network. There were high network costs associated with convergence because more expensive switch hardware was required to support the additional bandwidth re- quirements.
Converged networks also required extensive management in relation to Quality of Ser- vice 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 conver- gence 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.
Click Advanced Technology button in the figure. 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 download. The figure shows a high-end VoIP phone and switch combination suitable for a medium-sized business of employees. The figure also shows a Cisco Catalyst Express switch and a Cisco G 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 sepa- rate 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 net- work 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 net- work.
With a converged network, you have one group managing both the voice and data networks. Click New Options button in the figure. New Options Converged networks give you options that had not existed previously.
You can now tie voice and video communications directly into an employees personal computer system, as shown in the fig- ure. There is no need for an expensive handset phone or videoconferencing equipment. You can ac- complish 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 per- son 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 downloading 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.