Can I Use Layer 2 Distribution Switches

This chapter covers the best practice design that places Layer 3 switches at both the core and distribution layers. What would happen if you could not afford Layer 3 switches at the distribution layer?

Figure 2-5 shows a dual-core campus network with Layer 2 distribution switches. Notice how each access VLAN extends not only throughout the switch block but also into the core. This is because the VLAN terminates at a Layer 3 boundary present only in the core. As an example, VLAN A's propagation is shaded in the figure.

Figure 2-5 Design Using Layer 2 Distribution Switches

Switch Block 1 Switch Block 2

Figure 2-5 Design Using Layer 2 Distribution Switches

Switch Block 1 Switch Block 2

Here are some implications of this design:

■ Redundant Layer 3 gateways still can be used in the core.

■ Each VLAN propagates across the redundant trunk links from the access to the core layers. Because of this, Layer 2 bridging loops form.

■ The STP must run in all layers to prevent Layer 2 loops. This causes traffic on some links to be blocked. As a result, only one of every two access-layer switch uplinks can be used at any time.

■ When Layer 2 uplinks go down, the STP can take several seconds to unblock redundant links, causing downtime.

■ Access VLANs can propagate from one end of the campus to the other, if necessary.

■ Broadcast traffic on any access-layer VLAN also reaches into the core layer. Bandwidth on uplinks and within the core can be wasted unnecessarily.

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