Layer 2 vs Layer 3 Design: Core and Distribution Layer Demarcation

The Hierarchical Network Design Model

In modern enterprise networks, a hierarchical design approach is widely adopted to ensure scalability, performance, and manageability. This model typically consists of three primary layers:

1. The Core Layer

The core layer, also known as the backbone, is responsible for providing optimal transport between sites or distribution layers. Its primary function is to switch packets as quickly as possible, without performing resource-intensive operations like access control lists (ACLs) or packet filtering, which could slow down packet forwarding.

2. The Distribution Layer

The distribution layer serves as the demarcation point between the access and core layers. It plays a crucial role in defining and differentiating the core network. This layer facilitates several key functions, including:

• Address or area aggregation
• Media transitions (e.g., Ethernet to WAN)
• Broadcast/multicast domain definition
• Departmental or workgroup access
• Security policies (ACLs, firewalling)
• Virtual LAN (VLAN) routing

In non-campus environments, the distribution layer may act as a redistribution point between routing domains or the demarcation between static and dynamic routing protocols, enabling remote site access to the corporate network.

3. The Access Layer

The access layer serves as the entry point for end-users into the network. It may implement additional access controls or filters to optimize user experience and enforce policies. In a campus environment, access layer functions may include:

• MAC layer filtering
• Microsegmentation
• Shared bandwidth management
• Switched bandwidth allocation

In non-campus scenarios, the access layer provides remote sites with connectivity to the corporate network via wide-area technologies like Frame Relay, ISDN, or leased lines.

Layer 2 or Layer 3 Boundary: Core and Distribution

The demarcation between the core and distribution layers is a critical design consideration. Traditionally, this boundary has been implemented at Layer 3 (the network layer) using routing protocols. However, with the advent of modern campus networks and the prevalence of Layer 2 technologies like VLANs and Layer 2 Ethernet, some organizations opt for a Layer 2 boundary between the core and distribution layers.

Layer 3 Boundary (Routing)

Implementing a Layer 3 boundary between the core and distribution layers offers several advantages, including:

• Better scalability and hierarchical structure
• Easier traffic engineering and load balancing
• Improved security through inter-VLAN routing and firewall integration
• Simplified management and troubleshooting

Layer 2 Boundary (Switching)

In scenarios where Layer 2 adjacency is preferred between the core and distribution layers, network designers may leverage technologies like Spanning Tree Protocol (STP), Virtual Switching System (VSS), or Virtual Port Channels (vPCs) to create a loop-free Layer 2 topology.

The choice between a Layer 2 or Layer 3 boundary depends on various factors, including network size, performance requirements, security considerations, and operational complexity.

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