Hierarchical Network Design | Cisco Hierarchical Model

Hierarchical Network Design | Cisco hierarchical Model

This post describe the Hierarchical Network Design and principles of structured engineering for network design . You will learn the three layers of Cisco hierarchical Model and how they are used in network design. Network design start from analyzing the network component, it is useful to categorize the networks according to the number of devices served:

• Small network : provides services for up to 200 devices.
• Medium network : provides services for 200 to 1000 devices.
• Large network : provides services for more than 1000 devices.

Network designs vary according to the size and needs of organizations. There are many variables to consider when designing a network. Normally a large business network consisting of a main campus that connects small, medium and large sites. Network design is an expanding area and requires a lot of knowledge and experience. The objective of this section is to present widely accepted network design concepts.

PRINCIPLES OF STRUCTURED ENGINEERING

Regardless of the size or requirements of the network, a fundamental factor for the correct implementation of any network design is to follow good principles of structured engineering:

• Hierarchy : a hierarchical network model is a useful high-level tool for designing a reliable network infrastructure. Divide the complex problem of network design into smaller and easier to manage areas.
• Modularity : by separating in modules the various functions that exist in a network, it is easier to design. Cisco identified several modules, including the business campus, the service block, the data center and the Internet perimeter.
• Resistance : the network must be available so that it can be used both in normal conditions (maintenance periods) and abnormal conditions (hardware or software failures).
• Flexibility : the ability to modify parts of the network, add new services or increase capacity without the need for major updates (i.e. replace major hardware devices).

To meet these fundamental design objectives, the network must be built on the basis of a hierarchical network architecture that allows flexibility and growth.

Cisco Hierarchical Model

In network technology, a hierarchical design involves dividing the network into independent layers . Each layer (or level) in the hierarchy provides specific functions that define its function within the general network.
This helps the network designer and architect to optimize and select the appropriate network features, hardware and software to perform the specific functions of that network layer. Hierarchical models apply to LAN and WAN design.
A typical design of a corporate campus hierarchical LAN network includes the following three layers:

• Access layer : provides network access for workgroups and users.
• Distribution layer : provides policy-based connectivity and controls the boundary between the access and core layers.
• Core layer : provides fast transport between distribution switches within the business campus.

The benefit of dividing a flat network into smaller and easier to manage blocks is that local traffic remains local. Only traffic destined for other networks is moved to a higher layer.
Layer 2 devices in a flat network provide few opportunities to control broadcasts or filter unwanted traffic. As more devices and applications are added to a flat network, response times degrade until the network becomes unusable.

In Image, another example of a three-layer hierarchical network design is shown. Note that each building uses the same hierarchical network model that includes the access, distribution and core layers.

ACCESS LAYER

In a LAN environment, the access layer grants access to the network for the terminals. In the WAN environment, you can provide access to the business network for remote workers or remote sites through WAN connections.
As shown in Image, the access layer for a small business network usually incorporates Layer 2 switches and access points that provide connectivity between workstations and servers.

The access layer performs several functions, including the following:

• Layer 2 Switching
• High availability
• Port security
• Classification and marking of QoS, and confidence limits
• Address Resolution Protocol (ARP) Inspection
• Virtual access control lists (VACL)
• Expansion tree
• Auxiliary Ethernet and VLAN power for VoIP

DISTRIBUTION LAYER

The distribution layer aggregates the data received from the access layer switches before they are transmitted to the core layer for routing to its final destination. In Image 4, the distribution layer is the boundary between the layer 2 domains and the layer 3 routed network.
The distribution layer device is the center in wiring cabinets. To segment workgroups and isolate network problems in a campus environment, a multilayer router or switch is used.
A distribution layer switch can provide upstream services for many access layer switches.
The distribution layer can provide the following:

• LAN or WAN link aggregation.
• Policy-based security in the form of access control lists (ACLs) and filtering.
• Routing services between LAN and VLAN networks, and between routing domains (eg, EIGRP to OSPF).
• Redundancy and load balancing.
• A limit for aggregation and summarization of routes that is configured in the interfaces to the core layer.
• Broadcast domain control, since neither routers nor multilayer switches resend broadcasts. The device works as a demarcation point between broadcast domains.

Cisco CORE LAYER

The core layer is also known as " network backbone ." The core layer consists of high-speed network devices, such as Cisco Catalyst 6500 or 6800 switches. These are designed to switch packets as quickly as possible and interconnect various campus components, such as distribution modules, service modules, the center of data and the perimeter of the WAN.
As shown in above Image, the core layer is essential for interconnectivity between the distribution layer devices; for example, interconnects the distribution block to the perimeter of the WAN and the Internet.
The core must have high availability and must be redundant. The kernel aggregates traffic from all devices in the distribution layer, so it must be able to send large amounts of data quickly.
Some of the considerations regarding the core layer include the following:

• You must provide high speed switching (i.e. fast transport).
• It must provide reliability and fault tolerance.
• You must achieve scalability through faster teams, not more teams.
• You must avoid packet handling that implies a high demand for the CPU because of security, inspection, quality of service (QoS) classification or other processes.

TWO-LEVEL CONTRACTED CORE DESIGN

There are no absolute rules about how a campus network should be physically assembled. While it is true that many campus networks are built with three physical levels of switches, it is not a strict requirement. On a smaller campus, the network can have two levels of switches in which the core and distribution elements are combined into a physical switch. This is called "contracted core design."
The three-tier hierarchical design maximizes performance, network availability and the ability to scale the network design.
However, there are many small business networks that do not grow much over time. Therefore, a two-level hierarchical design in which the core and distribution layers are combined into a single layer is usually more practical. There is a "contracted core" when the functions of the distribution layer and the core layer are implemented by a single device. The main motivation to choose the contracted core design is the reduction of network costs, while maintaining the majority of the benefits of the three-tier hierarchical model.