Collapsed Core

A collapsed core block is one where the core layer of the hierarchy is collapsed into the distribution layer. Here, both distribution and core functions are provided within the same switch devices. This situation is usually found in smaller campus networks, where a separate core layer (and additional cost or performance) is not warranted.

Figure 2-9 shows the basic collapsed core design. Although the distribution and core layer functions are performed in the same device, keeping these functions distinct and properly designed is important. Note also that the collapsed core is not an independent building block, but is integrated into the distribution layer of the individual standalone switch blocks.

In the collapsed core design, each access layer switch has a redundant link to each distribution/ core layer switch. All Layer 3 subnets present in the access layer terminate at the Layer 3 ports of the distribution switches, as in the basic switch block design. The distribution/core switches are connected to each other by one or more links, completing a path to be used during a redundancy failover. Spanning Tree will keep one of the redundant links to the access layer blocked to prevent Layer 2 bridging loops.

Figure 2-9 Collapsed Core Block Design

Switch block 1 Switch block 2

Access layer

Distribution/Core > layer

Figure 2-9 Collapsed Core Block Design

Switch block 1 Switch block 2

Distribution/Core > layer

Core links

Core links

However, at Layer 3, redundancy is provided through HSRP for IP. The two distribution switches will share a common default gateway address, but only one will be active at any time. In the event of a distribution/core switch failure, connectivity to the core will be maintained as the redundant Layer 3 switch takes over.

Why be concerned about the differences between Layer 2 and Layer 3 redundancy? Although the distribution/core switches have Layer 3 functionality, as in the case of MLS, understanding how MLS works in switches like the Catalyst 5000 and 6000 families is useful. A Layer 3 routing decision is first made on a traffic flow (either by an integrated or external router). The switches cache this information, and Layer 3 switching begins after a path is determined by the routing decision. Therefore, both Layer 2 and Layer 3 operations are still occurring on the distribution/core switches, each with different redundancy requirements. Chapter 8 covers MLS in more detail.

Dual Core

A dual core connects two or more switch blocks in a redundant fashion. Although the collapsed core can connect two switch blocks with some redundancy, the core is not scalable when more switch blocks are added. Figure 2-10 illustrates the dual core. Notice that this core appears as an independent module and is not merged into any other block or layer.

Normally, the dual core is built with Layer 2 switches to provide the simplest and most efficient throughput. Building a dual core with Layer 3 is possible, as discussed in the section "Layer 3 Core" later in the chapter. The dual core uses two identical switches to provide redundancy. Redundant links connect the distribution layer portion of each switch block to each of the dual core switches. Note the absence of any links between the two core switches. In a Layer 2 core, the switches are not linked to avoid any bridging loops.

Figure 2-10 Dual Network Core Design

Switch block 1

Switch block 2

Figure 2-10 Dual Network Core Design

Switch block 1

Switch block 2

> Access

^Distribution

Subnet A Subnet B

> Access

^Distribution

Subnet A Subnet B

In the dual core, each distribution switch has two equal-cost paths to the core, providing twice the available bandwidth. Both paths remain active because the distribution layer uses Layer 3 devices that can manage equal-cost paths in routing tables. In fact, the Layer 3 path determination across the core occurs without any reliance on Spanning Tree at all. Designing the core without links between the core switches removes any possibility of loops and eliminates the need for Spanning Tree in the core. The routing protocol in use determines the availability or loss of a neighboring Layer 3 device. Therefore, if one core switch fails, the routing protocol will reroute traffic using an alternate path through the remaining core switch.

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