The three-tier hierarchical model is a widely adopted network design approach, particularly in enterprise environments. It involves dividing the network into three distinct layers: the access layer, distribution layer, and core layer, where each layer has specific functions and responsibilities. This model provides scalability, modular design, and efficient traffic management.
Commonly implemented due to its effectiveness, the three-tier hierarchical model is typically found in large corporate networks, data centers, and campus environments. Benefits include simplified management, ease of troubleshooting, and improved scalability. It allows for flexible scaling of network resources, ensuring a stable and responsive network infrastructure. The three-tier hierarchical model offers a structured approach to network design, enhancing performance, reliability, and security in complex networking environments.
However, the three-tier hierarchical model can be overly complex and expensive, particularly in smaller networks. As an alternative, consider deploying a collapsed core architecture. It is a network design where the distribution and core layers of the network are combined into a single layer. It is commonly deployed in small to medium-sized networks, such as branch offices or small businesses, where simplicity and cost-effectiveness are prioritized over scalability and redundancy.
Access Layer
The access layer is the entry point for end-user devices into the network. Its primary responsibilities include providing network connectivity to end devices like computers, printers, and IP phones, controlling user access, and enforcing security policies.
Access layer devices, such as switches, are pivotal in segmenting broadcast domains, creating separate collision domains, and ensuring efficient data transmission within their respective VLANs. They also facilitate PoE (Power over Ethernet) to power IP phones and access points, enhancing network flexibility.
Additionally, access points extend network connectivity by providing wireless access to devices like smartphones, tablets, and laptops. This ensures that both wired and wireless devices can connect securely to the network. The access layer is crucial for connecting end-users to the distribution layer and is key to ensuring a well-organized, scalable, and secure network infrastructure.
Although primarily focused on network access control, the access layer provides several functions, such as port security and VLANs, access control lists (ACL), and Address Resolution Protocol (ARP) inspection to regulate and secure device access. Additionally, the access layer handles Quality of Service (QoS) tasks, such as traffic classification and marking, ensuring efficient data handling.
Distribution Layer
The distribution layer is the network's backbone and is responsible for routing, filtering, security, QoS, and directing traffic between different access layer segments. It often serves as the initial routing point in the physical network and is central in configuring Layer 3 features, including route summarization, DHCP relay, and ACLs.
Although its key functionalities include routing and inter-VLAN routing for efficient communication across the network, the distribution layer also ensures high availability and fault tolerance through redundancy by using a First Hop Redundancy Protocol such as Hot Standby Router Protocol (HSRP), Virtual Router Redundancy Protocol (VRRP), or Gateway Load Balancing Protocol (GLBP).
This layer is vital because it aggregates and controls traffic from multiple access layer switches, providing a centralized point for network management and security policy enforcement. Typical devices in the distribution layer include Layer 3 switches and routers to facilitate intelligent routing and traffic management.
Core Layer
The core layer is responsible for high-speed, efficient packet forwarding and forms the network's backbone. Its primary functionalities include fast, reliable packet switching and routing between distribution layer switches, ensuring rapid data transport across the network. The core layer is crucial because it provides high-speed connectivity to support data transfer without introducing delays or bottlenecks.
Typically, this layer employs devices like high-speed routers and Layer 3 switches, capable of forwarding traffic at the network's maximum capacity. Redundancy and load balancing mechanisms are often implemented in the core to enhance fault tolerance and maximize network uptime, ensuring uninterrupted communication across the entire network.
The core layer of the network should maintain simplicity by avoiding complex policy services and direct connections to users or servers. This approach ensures that the core remains manageable, operates at high speed, and maintains security, as it primarily focuses on efficient packet forwarding and routing without getting involved in user-level or application-specific functions.