Core Layer Functionality

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

The Role of the Core Layer

The function of the core layer is to provide fast and efficient data transport. Characteristics of the core layer include the following:

■ The core layer is a high-speed backbone that should be designed to switch packets as quickly as possible to optimize communication transport within the network.

■ Because the core is critical for connectivity, core layer devices are expected to provide a high level of availability and reliability. A fault-tolerant network design ensures that failures do not have a major impact on network connectivity. The core must be able to accommodate failures by rerouting traffic and responding quickly to changes in network topology. The core must provide a high level of redundancy. A full mesh is strongly suggested, and at least a well-connected partial mesh with multiple paths from each device is required.

■ The core layer should not perform any packet manipulation, such as checking access lists or filtering, which would slow down the switching of packets.

■ The core layer must be manageable.

■ The core devices must be able to implement scalable protocols and technologies, and provide alternative paths and load balancing.

Switching in the Core Layer

Layer 2 switching or multilayer switching (routing) can be used in the core layer. Because core devices are responsible for accommodating failures by rerouting traffic and responding quickly to network topology changes, and because performance for routing in the core with a multilayer switch incurs no cost, most implementations have multilayer switching in the core layer. The core layer can then more readily implement scalable protocols and technologies, and provide alternate paths and load balancing.

Figure 3-5 shows an example of Layer 2 switching in the campus core.

Figure 3-5 Layer 2 Switching in the Campus Core

Layer 2 Multilayer Layer 2

Figure 3-5 shows an example of Layer 2 switching in the campus core.

Layer 2 Multilayer Layer 2

Routing Protocol at the Distribution Layer Computes Primary and Alternative Paths

Redundant Layer 2 Links to Provide Resilience

Redundant Component to Avoid Network Outages in Case of a Device Failure

Routing Protocol at the Distribution Layer Computes Primary and Alternative Paths

Redundant Layer 2 Links to Provide Resilience

Redundant Component to Avoid Network Outages in Case of a Device Failure

Access

Core

In Figure 3-5, a typical packet between access sites follows these steps:

Step 1 The packet is Layer 2-switched toward a distribution switch.

Step 2 The distribution switch performs multilayer switching toward a core interface.

Step 3 The packet is Layer 2-switched across the LAN core.

Step 4 The receiving distribution switch performs multilayer switching toward an access layer LAN.

Step 5 The packet is Layer 2-switched across the access layer LAN to the destination host.

Figure 3-6 shows an example of multilayer switching in the campus core.

Figure 3-6 Multilayer Switching in the Campus Core

Distribution Layer Routers Peer with Core Routers and Exchange Routing Information

Redundant Links and Core Devices Provide

High Level of Resilience

In Figure 3-6, a typical packet between access sites follows these steps:

Step 1 The packet is Layer 2-switched toward a distribution switch.

Step 2 The distribution switch performs multilayer switching toward a core interface.

Step 3 The packet is multilayer-switched across the LAN core.

Step 4 The receiving distribution switch performs multilayer switching toward an access LAN.

Step 5 The packet is Layer 2-switched across the access layer LAN to the destination host.

Figure 3-6 Multilayer Switching in the Campus Core

Distribution Layer Routers Peer with Core Routers and Exchange Routing Information

Redundant Links and Core Devices Provide

High Level of Resilience

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