Layer 3 Designs

Designs that incorporate Layer 3/4 aware hardware rather than Layer 2-only aware hardware have many proven benefits including, first and foremost, a reduced reliance on spanning tree for redundancy, followed by more intelligent routing or treatment of traffic. Cisco's current recommendations for switching designs focus on the three-tiered core/distribution/access model, implementing Layer 3 connections between the core and distribution devices, and Layer 2 connections between the distribution and access layers. As Layer 3 switching capabilities become common even in the least expensive access layer switch platforms, the three-tiered model is likely to adopt a Layer 3 switching approach everywhere, not just in the core and distribution layers. Figure 11-1 illustrates a relatively simple three-layer core, distribution, and access design model.

In the first few examples, such as Figure 11-1, each access layer switch is assigned a single VLAN for user data, and that VLAN does not extend beyond that access layer switch. This is similar to the configuration examples in Chapter 7, "Configuring Switches." Although it often makes sense to assign more than one VLAN to an access-layer switch, in an ideal design those VLANs would not be trunked between access-layer switches. Although not trunking, these VLANs might be more difficult to do with switched infrastructures that have evolved over a period of years; new designs should adopt this best practice, and existing designs can be converted incrementally. One of the main benefits of assigning data VLAN on a per-switch basis is an elimination of Layer 2 loops in spanning tree, resulting in a much-simplified troubleshooting process.

In contrast to the configuration examples in Chapter 7, each access layer switch in Figure 11-1 has a Layer 2 connection to both SW-D1 and SW-D2. Like the examples in Chapter 7, the connections between the distribution and core switches are routed Layer 3 connections and use VLAN numbers 901?905 that are not utilized for any access layer devices. No Layer 2 spanning-tree loops exist in the design illustrated in Figure 11-1. Even though no spanning-tree loops exist in this configuration, spanning tree is not disabled. Spanning tree is enabled by default and should remain on even in loop-free topologies as a loop-prevention mechanism.

Before finalizing and implementing any switching design, it is vitally important to understand as much as possible about the anticipated traffic flow. Not understanding the traffic flow might result in high-bandwidth connections being underutilized and lower bandwidth connections being overrun with traffic. Figure 11-1 represents an almost fully meshed design from a core and distribution perspective, with each core and distribution switch connecting to every other with the exception of switches SW-D1 and SW-D2.