Cisco's Hierarchical Three-Layer Model

Cisco's three-layer model contains three layers, core, distribution, and access, responsible for different functions involved in designing, implementing, and maintaining a hierarchical Cisco internetwork. The core layer provides the backbone while the distribution layer is responsible for routing, filtering, and network access as well as determining the proper route for packets to access the core. The access layer, also known as the desktop layer, is responsible for switching and controlling access to internetwork resources.

The Core Layer

The core layer occupies the top level of the hierarchy where its primary job involves transporting high traffic volume as fast as possible. Traffic moving through the core is intended for most users, so core errors can affect all users. The data intended for users is processed at the distribution layer. Because of high traffic volumes at the core level, speed, reliability, and latency are important.

When designing the core, it's important to:

• Design for speed and reliability – Design the core with minimal latency and consider including data-link technologies such as Fast Ethernet, FDDI, and ATM which provide for both speed and redundancy.
• Choose low convergence routing protocols.
• Avoid packet filtering, routing between VLANs, access lists, and other processes that can slow network traffic.
• Avoid supporting workgroup access at the core layer.
• Choose upgrades over expansion as the internetwork grows or core performance suffers.

The Distribution Layer

The distribution layer serves as the communication channel between the core and access layers. It determines the fastest routes for network requests, provides routing and filtering, and enables network access. Once the distribution layer receives a service request, if needed, it forwards it to the core which transports it to the appropriate service. Network policies are implemented at the distribution layer.

The following steps are typically designed at the distribution layer:

• Network routing and VLAN routing
• Network and security policy implementation such as firewalls and address translation
• Network tool implementation such as queuing, access lists, and packet filtering
• Broadcast and multicast domain definitions
• Routing protocol redistribution

The Access Layer

The access layer is responsible for controlling workgroup and user access to a Cisco internetwork's resources. Most resources are locally available and handled by the access layer with the distribution layer handling requests for remote resources.

When designing the access layer, the following are typically included:

• Policy and access control continued from the distribution layer
• Segmentation – separate collision domain creation
• Distribution / workgroup connectivity

In addition, the access layer offer incorporates Ethernet switching, static routing, and DDR.

About CBT Planet:

For an in-depth exploration of the three-layer model and comprehensive Cisco internetworking instruction, enroll in Cisco CCNA certification training from CBT Planet. Certified instructors have been carefully prescreened by CBT Planet to ensure quality instruction. These instructors have maintained a classroom instruction evaluation rating of over 95 percent and are featured in live training classes, CCNA training videos, onsite training classes, and CCNA boot camps. The courses prepare students for the Cisco Certified Network Associate certification exams and equip them with the knowledge and skills required for working with Cisco internetworks. Whether you need a deeper understanding of Cisco internetworks or want to earn your CCNA credential, CBT Planet's IT certification training offers a convenient, flexible way to prepare.