 unpredictable results may occur. Virtual Port Channel A common requirement in networking is to dual-home devices. Switches connected to two other switches form a Layer-2 loop that can be disastrous for a network unless the loop is broken. Traditionally, spanning tree was used to find these loops and block one of the legs, thus preventing problems like broadcast storms. While Spanning Tree Pro- tocol (STP) works well, it is slow and prohibits the use of one-half of the available bandwidth from the dual-homed switch. Figure 18-17 shows a traditional dual-home STP-based design. Notice that one of each of the links from the bottom switches is unavailable, since STP has blocked those links. Chapter 18: Cisco Nexus Figure 18-17. Traditional spanning tree design In a 6500 environment using VSS supervisors, we can create a single logical switch from two physical switches. This new logical switch is called a VSS cluster (see Chapter 17 for more information). Since the two links from each bottom switch connect to a single logical switch, STP is not needed. A VSS cluster design is shown in Figure 18-18. The bottom switches see the VSS cluster as a single endpoint and can make a single port channel to that device. Cisco describes this as two switches with a single control plane. Single Logical Switch VSS Cluster Figure 18-18. A 6500 VSS design This solution is much cleaner, since spanning tree is not required. Each link can now be fully utilized, doubling the uplink speed from the bottom switches. Nexus does not support VSS technology and instead uses vPCs. With vPCs, the top switches are not bonded into a single logical switch, but the bottom switches still see them as a single endpoint when creating port channels. Figure 18-19 shows two switches connected to two Nexus 7000s using vPCs for all connections. Cisco describes the Nexus vPC arrangement as two switches having one data plane and two control planes. Let's walk though the steps needed to configure a vPC. Nexus Design Features Figure 18-19. vPC design First, you mus