Cisco Three Layered Hierarchical Model
Welcome to a new lesson from our Cisco CCNA certification exam preparation series. We would like to present you today Cisco’s 3 Layered Hierarchical Model.
Understanding hierarchy is important and it’s simple too. In real life we have all kinds of hierarchical models. For example, when you were in school, you, as a student, were at the bottom of the hierarchy. At the next level of the hierarchy were the teachers. Above the teacher there was one or many directors.
In the networking world, hierarchy helps you separate some parts of your network from others. When you design your network, you should use a hierarchical model to separate your network in different layers. Every layer of your network should perform only some functions. Avoid using network equipments as all-in-one equipments. For example, a border router, connected to your service provider, should do only the process of routing and some basic filtering maybe. Don’t use this router to implement QoS policies or VoIP services. This way, your network will be more predictable and you will be able to troubleshoot problems faster.
After years in the networking industry, Cisco developed its own hierarchical model, Cisco’s Three Layered Hierarchical Model. This is probably the most basic model ever known, and Cisco even developed its network in accordance with this model. Benefits of using Cisco’s hierarchical model are network stability, reliability and cost-effectiveness.
Cisco’s 3 Layered model consist from the core, the distribution and the access layers.
The Core layer is actually the backbone, or the core, of your network. This is the most critical layer because its purpose is to provide fault isolation and backbone connectivity. The core layer must be able to switch traffic at the highest speeds possible in a timely fashion. Also, at the core layer, the network must have a level of redundancy, used in case of link or hardware failure. You should not use this layer to create services for your users.
Because it has to be a fast and reliable layer of your network, you should not implement at this layer anything that can slow down the traffic, like ACLs, Inter-VLAN routing, packet filtering. Also, don’t add support for group access and do not expand the core layer. If your current devices are not supporting the traffic, upgrade them, instead of adding more devices.
When you design the Core layer of your network you must make sure it will be reliable, meaning you have to use technologies able to carry large amounts of traffic and also provide a high level of redundancy, such as FDDI, Fast/Gigabit Ethernet or ATM. Also, a good practice is to use routing protocols with lower convergence times to avoid downtimes. Again, you have to design this with speed in mind. At the Core layer you should have very little latency.
Examples of network equipments used in the core layer are: high speed WAN routers and switches, multiplexers, ATM networks, such as Cisco 7000, 7200, 7500 and 12000 series for WAN and Cisco 4000, 5000 and 6000 series for LAN.
In smaller networks you may see high-speed routers instead of switches at the core layer.
The Distribution layer is used as a communication point between the access and the core layer. Basically, this layer handles the routing, packet filtering, WAN access and determines how packets can reach the core if needed. This layer includes all OSI Layer 3 devices, such as routers and layer 3 switches.
At the distribution layer, generally you would implement:
- Routing
- ACLs, packet filtering and QoS
- Security and network policies, including NAT and firewalls
- Route Reflectors (RRs) to distribute routes across your network
- Inter-VLAN routing
- Workgroup functions
Examples of network equipments used at the distribution layer are: LAN routers, layer 3 switches, firewalls, VPN access router.
Finally, the Access layer, also called the desktop layer, focuses on connecting client devices, such as workstations, laptops, servers and peripherals on the internal network (same broadcast domain). If a host must access a resource in some other network, the Distribution layer handles to traffic to reach that network. The access layer is used also to separate collision domains, filter MAC addresses and implement load balancing.
At this layer, the most used technology is Ethernet. Routing protocols are rarely used, instead static routing is often being seen.
As we already told you, this is the easiest hierarchical model and Cisco uses it in its network with great success. Although is not a big topic for your CCNA exam, you must know what are the layers of Cisco’s Three Layered Hierarchical Model and what is their purpose.
