The Cisco Three-Layered Hierarchical ModelBy SemSim.com http://www.mcmcse.com/cisco/guides/hierarchical_model.shtml Cisco has defined a hierarchical model known as the hierarchical internetworking model. This model simplifies the task of building a reliable, scalable, and less expensive hierarchical internetwork because rather than focusing on packet construction, it focuses on the three functional areas, or layers, of your network: Core layer: This layer is considered the backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by devices in this layer. Rather, this layer is concerned with speed and ensures reliable delivery of packets. Distribution layer: This layer includes LAN-based routers and layer 3 switches. This layer ensures that packets are properly routed between subnets and VLANs in your enterprise. This layer is also called the Workgroup layer. Access layer: This layer includes hubs and switches. This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network. This layer ensures that packets are delivered to end user computers. Figure INT.2.1 displays the three layers of the Cisco hierarchical model. the model allows you determine what will happen to the network when new stresses are placed on it. where only certain layers are susceptible to congestion. Its purpose is to reduce the latency time in the delivery of packets. each layer might comprise more than two devices or a single device might function across multiple layers.The benefits of the Cisco hierarchical model include: High Performance: You can design high performance networks. Policy creation: You can easily create policies and specify filters and rules.When you implement these layers. Scalability: You can grow the network easily by dividing your network into functional areas. Behavior prediction: When planning or managing a network. The factors to be considered while designing devices to be used in the core layer are: . The core layer is often known as the backbone or foundation network because all other layers rely upon it. Efficient management & troubleshooting: Allows you to efficiently organize network management and isolate causes of network trouble. Core Layer The core layer is responsible for fast and reliable transportation of data across a network. One way that core networks enable high data transfer rates is through load sharing. including obtaining route summaries. It is at this layer where you begin to exert control over network transmissions. then the device can quickly discover a new route. 5000. The distribution layer also performs queuing and provides packet manipulation of the network traffic. In a route summary. High reliability: Multiple data paths ensure high network fault tolerance. Access Layer Aggregation Point: The layer serves the aggregation point for the desktop layer switches. create virtual LANs. the command to obtain a routing summary is: show ip route summary . Switched Multimegabit Data Service (SMDS) Distribution Layer The distribution layer is responsible for routing. At the core layer. if one path experiences a problem. 7500. and conduct various management tasks. In Cisco routers. 7200. Frame relay connections. High data transfer rate: Speed is important at the core layer. Fewer and faster systems create a more efficient backbone. if necessary. Application Gateways: The layer allows you to create protocol gateways to and from different network architectures. You will also limit and create broadcast domains. efficiency is the key term. based on policies you set. you consolidate traffic from many subnets into a core network connection. and 12000 (for WAN use) Catalyst switches such as 6000. There are various equipments available for the core layer. Control Broadcast and Multicast: The layer serves as the boundary for broadcast and multicast domains. including what comes in and what goes out of the network. and 4000 (for LAN use) T-1 and E-1 lines. Examples of core layer Cisco equipment include: Cisco switches such as 7000. including: Packet filtering (firewalling): Processes packets and regulates the transmission of packets based on its source and destination information to create network borders. It also provides policy-based network connectivity. ATM networks. where traffic can travel through multiple network connections. Low latency period: The core layer typically uses high-speed low latency circuits which only forward packets and do not enforcing policy. QoS: The router or layer 3 switches can read packets and prioritize delivery. A collision domain describes a portion of an Ethernet network at layer 1 of the OSI model where any communication sent by a node can be sensed by any other node on the network. such as those found at the core layer. a switch is an advanced version of a hub. In the access layer. hub. This is different from a broadcast domain which describes any part of a network at layer 2 or 3 of the OSI model where a node can broadcast to any node on the network. Examples of Cisco-specific distribution layer equipment include 2600.4000. You can also determine how routers update each other’s routing tables by choosing specific routing protocols. At the access layer. Rather. you have the ability to expand or contract collision domains using a repeater. you can: Enable MAC address filtering: It is possible to program a switch to allow only certain systems to access the connected LANs. . a switch is not a high-powered device.com. Handle switch bandwidth: You can move data from one network to another to perform load balancing. Share bandwidth: You can allow the same network connection to handle all data.You can practice viewing routing information using a free CCNA exam router simulator available from SemSim. 4500 series routers Access Layer The access layer contains devices that allow workgroups and users to use the services provided by the distribution and core layers. Create separate collision domains: A switch can create separate collision domains for each connected node to improve performance. In regards to the access layer. or standard switch. https://supportforums.. 2975. . 2960/2960S is definitely an access switch. It doesn't have the functionality or capability to be a distro or even a core switch..com/thread/2061029 I have few questions: 1)What switches are commonly deployed as access layer switches? 2) what switches are commonly deployed as distribution layer switches? 3) what switches are commonly deployed as core layer switches? Let's start with the easy ones . 2350/2360 is also an access switch for servers.cisco. 3750/3750E/3750X can be used as all three. 2900/3500XL is access. Sup32 of the 6500 would be used for an access switch. 2940/2950/2955 is access. 3550 can be used as access and the 3508XL. 3550-12T or 12G can be used as a distribution switch.. Sup720 and the Sup2T can be used as either distribution or core switch. 2970 access switch. then it's a distribution.. If it has more than 6 SFP ports. Sup1 and Sup2 distribution or core. For the legacy systems .3560/3560E/3560X. Did I miss anyone? . My rule of thumb is simple: If the model has a PoE then it's an access switch. 2 Distribution layer o 2. distribution. Starting with the basics. Cisco both developed their system according to this model and recommend their end-users to follow the same philosophy. PCs and printers. The primary purpose of the core is to provide fault isolation and backbone connectivity. Cisco Systems has developed a three-layered model. on one hand. Contents [hide] 1 History 2 Description of Cisco layers o 2.3 Access layer 3 References [edit] History The Cisco three-layered model originates from the enterprise campus network [1] which has evolved over the last 20 years. the core must provide the appropriate level of redundancy to allow fault tolerance in case of . The hierarchy is Cisco's three-layered Model. the core must be highly reliable and switch traffic as fast as possible. [edit] Description of Cisco layers [edit] Core layer The core layer is literally the internet backbone. and access layers. Early LAN-based computer networks were made of a small number of simply connected servers. the simplest yet most critical layer. Cisco also highlighted the importance of the Cisco three-layered model in its famous CCNA certifications. Cisco borrowed the structured programming design principle from software engineering.Over years of building network equipment. The first generation of campus networks came into form by interconnecting these LANs. Cisco's three-layered model is a widely used network model. large complex Cisco systems must be built using a set of modularized components that can be assembled in a hierarchical and structured manner. Therefore.1 Core layer o 2. in other words. besides the OSI Layered Model and TCP/IP Layered Model[5]. To address the above problems. Based on two complementary principles: hierarchy and modularity. the Cisco network is traditionally defined as a three-tier hierarchical model comprising the core. Problems in one area of the network frequently impacted the entire network and a failure in one part of the campus often affected the entire campus network. Examples of Types of Service and Capabilities[1] Service Requirements Discovery and Configuration Services Service Features 802. [edit] Access layer The access layer is sometimes referred to as the desktop layer. various services are provided. The network resources the workgroup and users needed will be available locally. and 12000 (for WAN use) Catalyst switches such as 6000. Switched Multimegabit Data Service (SMDS) [edit] Distribution layer The distribution layer acts as an interface between the access layer and the core layer. The access layer is the edge of the entire network. ATM networks. Architects and engineers shouldn't implement complex policy services or attach user/server connections directly at this layer. While core layer and access layer are special purpose layers[4]. Examples of core layer Cisco equipment include [2]: Cisco switches such as 7000. and 4000 (for LAN use) T-1 and E-1 lines.1AF. printers. It is an aggregation point for all of the access layer switches and also participates in the core routing design. the distribution layer on the other hand serves multiple purposes. CDP. Frame relay connections. where a wide variety of types of consumer devices such as PCs. LLDP. As a result. It ensures that packets are properly routed between subnets and VLANs. if needed [3]. 7200. This layer includes LAN-based routers and OSI layer 3 switches. the access layer is most feature-rich layer of the Cisco three-layered model. The primary function of the distribution layer is to provide routing. the high-end switches and high-speed cables are implemented to achieve High data transfer rate and Low latency period. LLDP-MED . on the other hand. filtering.hardware or software failure or upgrade. 5000. The following table lists examples of the types of services and capabilities that need to be defined and supported in the access layer of the network. 7500. The core means to be simple and provides a very limited set of services. and dynamic configuration mechanisms implemented. cameras attach to the wired portion of the network. and WAN access and to determine how packets can access the core. PAgP/LACP. LoopGuard. 5. policing. DTP. MAB. Rapid PVST+. Cisco 3 Layer Model . 4. Port Security. (CISF): port security.0 Architecture: Overview and Framework ^ The Cisco Three-Layered Hierarchical Model ^ Cisco Three Layer Hierarchical Model vs OSI ^ OSI & TCP-IP Tutorial ^ OSI Model v. etc. BackboneFast. MAB. Intelligent Network Control Services Physical Infrastructure Services PVST+. Web-Auth QoS marking. Web-Auth 802.1X. 2. DHCP snooping. BPDUGuard.1X). UDLD. ^ Cisco : Enterprise Campus 3.s. OSPF. UplinkFast. TCP/IP Model v. EIGRP. DAI.Security Services Network Identity and Access IBNS (802. RootGuard Power over Ethernet [edit] References [1] [2] [3] [4] [5] 1.s.1X. IPSG 802. queuing. Portfast. FlexLink. 3. deep packet inspection NBAR. such as those used to route between subnets or .http://archive. While the OSI model is concerned with how different systems communicate over networks. Each layer is meant to have its own roles and responsibilities. the Cisco hierarchical network design model. the Cisco hierarchical model is a blueprint of types that defines how networks should be designed in layers. The Distribution layer would contain intermediate switches and routers. The article provides an overview of the roles and responsibilities of each of the model’s 3 layers. its use can by all means be adapted to account for the switching and routing equipment of any vendor. and keeps required roles in their place. is manageable. The Core layer of the network would be considered along the same lines as the backbone – high speed and redundant.com/2004/0206.html By Dan DiNicolo This article focuses on another important network model. but the goal is to create a network that delivers high performance. and Access.networknewz. The model is made up of three layers. The diagram below shows each of these layers relative to one another. this model is used as the basis for designing Cisco networks for security and performance. Very different that the OSI model. While this model was designed by Cisco. Distribution. including Core. especially smaller ones. Switching provides much higher performance. Instead. To that end. A design that moves traffic over the Core layer when it isn’t necessary will not provide the best performance.VLANs. each layer of the model is actually home to multiple roles and responsibilities. The Access layer is literally where user’s PCs plug into their local switch. and not route it. That is. the core should also never be used to implement traffic filters such as access lists – these should be implement at other layers instead. and with a particular emphasis on how that layer should perform. mainly because a frame can travel across the backbone without needing to be recreated at each switch. but everything stays at OSI layers 1 and 2 instead of having to be considered at Layer 3. somewhere like an area wiring closet. The Layers Each layer in the model has a general level of responsibility. and meets performance requirements. and as such not all networks will necessarily look like this – many. it provides a general high-level overview. in terms of what capabilities should be implemented there. another path exists over which frames can travel. scalable. The Core Layer is also responsible for providing a degree of redundancy by providing multiple paths. Notice that the backbone is expected to switch traffic. Getting a little deeper into things. While this is a simplified view of the network. Routing can severely impact performance. To summarize. think of this model as one that outlines best practices to ensure that the network is reliable. mainly because each frame needs to be recreated as it passes through each router. In general. The Core layer is usually comprised of a relatively small number of high-end switches. Growth should not add devices. That’s not to say that the frame isn’t inspected at every switch (it will be to varying degrees). Each of the layers is outlined in more detail below. may not even be close. the Core Layer should: Be used to provide high-speed switching. but rather replace devices with higher-speed equipment as necessary. Remember that this is a model. you want to be sure that the only traffic that moves across the backbone is that which is moving between different Distribution-layer devices. We’ll consider this in a diagram shortly. . as we’ll look at a little later in the series. Core Layer The responsibility of the core layer is to act as a high-speed switched backbone. you want to be sure that even if a backbone link goes down. making these entries smaller. routers usually do not pass broadcasts. Broadcast domains. many subnets can be hidden behind a single routing table entry. and routing more efficient). and not more. such as mail servers. In some cases this traffic is localized. acting as the demarcation point between broadcast domains. there are often servers that need to be accessed by many subnets even within a given location. Distribution Layer The distribution layer acts as an intermediary between the Core and Access layers. which keeps it away from the backbone. Grow by using faster. Clients on one subnet may need to talk to servers on another. equipment. A broadcast domain is a layer 2 concept that defines how far a broadcast will travel on a given network. remember that a great deal of routing will usually happen on a network. An example of the type of interconnection here includes those between different types of media such as Ethernet and Token Ring. Never implement performance-decreasing elements such as access lists. The distribution layer would be responsible for this routing function. In all. this layer serves a number of purposes including the implementation of Security. The distribution layer is also where policies are usually implemented using Access Lists. This also acts as the intermediate point between where static and dynamic routing are used on the network. Routing. and is usually where the routing functions (and more) on a well-designed network are found. However. Almost all routing is done at this layer. . such as with departmental file or database servers. in the form of Access Lists and filtering. A boundary for route aggregation and summarization (for example. To get a feel for the function of the distribution layer. By default. Provide reliability and fault tolerance. Canada. Dan is the author of a number of technical books including the soon-to-be-released CCNA/CCDA Study Guide. About the Author: Dan DiNicolo is a technical trainer. How it fits together The diagram below shows how a typical network might be configured to account for the Cisco hierarchical network design model.com. Remember that the Core layer switches might be geographically dispersed. . He can be reached by email at [email protected]. this layer is usually used to define network collision domains. As such.Access Layer The Access Layer acts as the point as which end stations connect to the network. consultant. and author as well as the owner and managing editor of 2000Trainers. The Access layer is also sometimes used to define additional network security policies and filtering if necessary. When he's not busy travelling the world as an IT volunteer with organizations like Geekcorps Dan makes his home in hockey-crazed Toronto. and that the distribution layer routers might be connected to the core via a WAN link of similar. usually by plugging into Layer 2 switches or hubs. Why Use a Hierarchical Network Design Model Networks that grow unheeded without any plan in place tend to develop in an unstructured format. A broadcast packet interrupts the CPU on each device within the broadcast domain. in a large flat (switched) network. Free Download Hierarchical Network Software and View All Examples A typical hierarchical topology is A core layer of high-end routers and switches that are optimized for availability and performance. When network devices communicate with many other devices.com/Hierarchical-Network-Design. you might need to recommend a network topology consisting of many interrelated components. For example. medium-speed routers can connect buildings at each campus. and demands processing time on every device for which a protocol understanding for that broadcast is installed. Dr. broadcast packets are burdensome. allowing you to choose the right systems and features for the layer. Each layer can be focused on specific functions. . and servers. and switches can connect user devices and servers within buildings. workstations. Welcher explains the disadvantages of a fur-ball topology by pointing out the problems that too many CPU adjacencies cause. the author of network design and technology articles for Cisco World and other publications. A distribution layer of routers and switches that implement policies. the workload required of the CPUs on the devices can be burdensome. refers to unplanned networks as fur-ball network. For example. An access layer that connects users via lower-end switches and wireless access points.php To meet a customer's business and technical goals for a corporate network design. Peter Welcher. This includes routers.Hierarchical Network Design http://www. high-speed WAN routers can carry traffic across the enterprise WAN backbone.edrawsoft. This task is made easier if you can "divide and conquer" the job and develop the design in layers. Network design experts have developed the hierarchical network design model to help you develop a topology in discrete layers. thus avoiding spending money on unnecessary features for a layer. Fault isolation is improved because network technicians can easily recognize the transition points in the network to help them isolate possible failure points. Replacing one device can affect numerous networks because of the complex interconnections. Network management responsibility and network management systems can be distributed to the different layers of a modular network architecture to control management costs. besides broadcast packets. remote site. How Can You Tell When You Have a Good Design Here are some wise answers from Peter Welcher that are based on the tenets of hierarchical. WAN link. In large flat or meshed network architectures. Modularity lets you keep each design element simple and easy to understand. As elements in a network require change. Using a hierarchical model can help you minimize costs. changes tend to impact a large number of systems. You can purchase the appropriate internetworking devices for each layer of the hierarchy. the cost of making an upgrade is contained to a small subset of the overall network. A hierarchical network design methodology lets you design a modular topology that limits the number of communicating routers. When new additions cause only local change. modular network design: When you already know how to add a new building. floor. a hierarchical topology is recommended because modularity in a design enables creating design elements that can be . When troubleshooting is easy because there are no complex protocol interactions to wrap your brain around. Simplicity minimizes the need for extensive training for network operations personnel and expedites the implementation of a design. is the CPU workload required for routers to communicate with many other routers and process numerous route advertisements. and so on. the modular nature of the hierarchical design model enables accurate capacity planning within each layer of the hierarchy. Also.Another potential problem with nonhierarchical networks. When scalability is a major goal. When your network can double or triple in size without major design changes. Hierarchical design facilitates changes. to the directly connected devices. e-commerce service. thus reducing wasted bandwidth. Testing a network design is made easy because there is clear functionality at each layer. The lack of hierarchy makes troubleshooting difficult. routing protocols can converge quickly. planning a campus network for a new site might simply be a matter of replicating an existing campus network design. and it is easy to maintain. a flat network is undesirable. such as token passing or carrier sense multiple access with collision detection (CSMA/CD) to control access to the shared bandwidth. resulting in significant delay and a higher probability of failure. With a flat network design. and the network is not divided into layers or modules. however.replicated as the network grows. expansion is easy to plan and implement. Flat WAN Topologies A wide-area network (WAN) for a small company can consist of a few sites connected in a loop. however. Because each instance of a module is consistent. you should recommend a hierarchical topology instead of a loop. Rather than being able to concentrate troubleshooting efforts in just one area of the network. some sites are isolated from others. If your analysis of traffic flow indicates that routers on opposite sides of a loop topology exchange a lot of traffic. When more than one link fails. The PCs and servers implemented a media-access control process. (As long as only one link fails. Flat LAN Topologies In the early and mid-1990s. A loop topology can mean that there are many hops between routers on opposite sides of the loop. A flat network topology is easy to design and implement. redundant routers or switches can be placed at upper layers of the hierarchy. communication recovers. For example. a typical design for a LAN was PCs and servers attached to one or more hubs in a flat topology. To avoid any single point of failure.) A flat loop topology is generally not recommended for networks with many sites. you may need to inspect the entire network. and communication with any other site can recover when a link fails. Each internetworking device has essentially the same job. As long as the WAN is small (a few sites). The devices were all part of the same . Flat Versus Hierarchical Topologies A flat network topology is adequate for very small networks. Each site has a WAN router that connects to two other adjacent sites via point-to-point links. as long as the network stays small. there is no hierarchy. When the network grows. A partial-mesh network has fewer connections. the devices do compete for service by the switching hardware and software. every router or switch is connected to every other router or switch.bandwidth domain and had the ability to negatively affect delay and throughput for other devices. In a WAN design. Inc. the access layer consists of the routers at the edge of the campus networks. (However. and offers good performance because there is just a single-link delay between any two sites. The three-layer model permits traffic aggregation and filtering at three successive routing or switching levels. traffic loading. the network is segmented into small bandwidth domains so that a limited number of devices compete for bandwidth at any one time. and routing. The distribution layer connects network services to the access layer. In a full-mesh topology. The core layer provides optimal transport between sites. so it is important to understand the performance characteristics of candidate switches. Each layer of the hierarchical model has a specific role. . These days. the access layer provides switches or hubs for end-user access. and implements policies regarding security. This makes the three-layer hierarchical model scalable to large international internet works. The Classic Three-Layer Hierarchical Model Literature published by Cisco Systems. Mesh Versus Hierarchical-Mesh Topologies Network designers often recommend a mesh topology to meet availability requirements. In a campus network. Although the model was developed at a time when routers delineated layers. network designers usually recommend attaching the PCs and servers to data link layer (Layer 2) switches instead of hubs. To reach another router or switch in a partial-mesh network might require traversing intermediate links. In this case. A full-mesh network provides complete redundancy. the model can be used for switched networks as well as routed networks. and other networking vendors talks about a classic three-layer hierarchical model for network design topologies. A controlled network diameter also makes troubleshooting and network documentation easier.Guidelines for Hierarchical Network Design This section briefly describes some guidelines for hierarchical network design. and then finally the core layer. It also helps you predict routing paths. By starting with the access layer. three major layers are sufficient: 1. The access layer Controlling the network diameter provides low and predictable latency. followed by the distribution layer. Finally. you can more accurately perform capacity planning for the distribution and core layers. The distribution layer 3. traffic flows. . Following these simple guidelines will help you design networks that take advantage of the benefits of hierarchical design. The core layer 2. The first guideline is that you should control the diameter of a hierarchical enterprise network topology. and capacity requirements. one other guideline for hierarchical network design is that you should design the access layer first. In most cases. .You can also recognize the optimization techniques you will need for the distribution and core layers.
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