Distribution Layer Functionality

This section describes distribution layer functions and the interaction of the distribution layer with the core and access layers.

The Role of the Distribution Layer

The distribution layer represents both a separation between the access and core layers and a connection point between the diverse access sites and the core layer. The distribution layer determines department or workgroup access and provides policy-based connectivity.

Following are the characteristics of the distribution layer:

■ Distribution layer devices control access to resources that are available at the core layer and must therefore use bandwidth efficiently.

■ In a campus environment, the distribution layer aggregates wiring closet bandwidth by concentrating multiple low-speed access links into a high-speed core link and using switches to segment workgroups and isolate network problems to prevent them from affecting the core layer.

Similarly, in a WAN environment, the distribution layer aggregates WAN connections at the edge of the campus and provides policy-based connectivity.

■ This layer provides redundant connections for access devices. Redundant connections also provide the opportunity to load-balance between devices.

■ The distribution layer represents a routing boundary between the access and core layers and is where routing and packet manipulation are performed.

■ The distribution layer allows the core layer to connect diverse sites while maintaining high performance. To maintain good performance in the core, the distribution layer can redistribute between bandwidth-intensive access-layer routing protocols and optimized core routing protocols. Route filtering is also implemented at the distribution layer.

■ The distribution layer can summarize routes from the access layer to improve routing protocol performance. For some networks, the distribution layer offers a default route to access-layer routers and runs dynamic routing protocols only when communicating with core routers.

■ The distribution layer connects network services to the access layer and implements policies for QoS, security, traffic loading, and routing. For example, the distribution layer addresses different protocols' QoS needs by implementing policy-based traffic control to isolate backbone and local environments. Policy-based traffic control prioritizes traffic to ensure the best performance for the most time-critical and time-dependent applications.

■ The distribution layer is often the layer that terminates access layer VLANs (broadcast domains); however, this can also be done at the access layer.

■ This layer provides any media transitions (for example, between Ethernet and ATM) that must occur.

Policy-Based Connectivity

Policy-based connectivity means implementing the policies of the organization (as described in Chapter 2, "Applying a Methodology to Network Design"). Methods for implementing policies include the following:

■ Filtering by source or destination address

■ Filtering based on input or output ports

■ Hiding internal network numbers by route filtering

■ Providing specific static routes rather than using routes from a dynamic routing protocol

■ Security (for example, certain packets might not be allowed into a specific part of the network)

■ QoS mechanisms (for example, the precedence and type of service [ToS] values in IP packet headers can be set in routers to leverage queuing mechanisms to prioritize traffic)

Distribution Layer Example

Figure 3-4 shows a sample network with various features of the distribution layer highlighted.

Figure 3-4 Example of Distribution Layer Features

Figure 3-4 Example of Distribution Layer Features

Following are the characteristics of the distribution layer in the routed campus network shown in Figure 3-4:

■ Multilayer switching is used toward the access layer (and, in this case, within the access layer).

■ Multilayer switching is performed in the distribution layer and extended toward the core layer.

■ The distribution layer performs two-way route redistribution to exchange the routes between the Routing Information Protocol version 2 (RIPv2) and Enhanced Interior Gateway Routing Protocol (EIGRP) routing processes.

■ Route filtering is configured on the interfaces toward the access layer.

■ Route summarization is configured on the interfaces toward the core layer.

■ The distribution layer contains highly redundant connectivity, both toward the access layer and toward the core layer.

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Responses

  • ROY
    Which two function are appropriate for the distribution layer in this design?
    4 months ago
  • REILLY
    What is the function purposes of the distribution layer?
    2 months ago
  • AMBER
    What protocol running between access to distribution?
    2 months ago
  • Wegahta
    What is the function aggregate trafic in distrubtion layer?
    2 months ago

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