tus Status Consistency Reason Active vlans success success 1-10,50-90 ,94-97 success success 1-10,50-90 ,94-97 success success 3-7,9 Virtual Port Channel success success 3-7,9 success success 3-7,9 down* success success If you think all of this configuration matching looks like a pain, you're absolutely right. One of the biggest problems I've seen with Nexus vPCs is in environments where they need to be changed often. I worked in a data center where VLANs were constantly being added or removed from trunks as new virtual machines were added to blade servers. To add a VLAN to a trunk, we would issue the switchport trunk allowed vlan vlan-number command. As you can see from the preceding output, the number of allowed VLANs is a consistency parameter for vPCs. This means when we added it to the NX-5K-1 switch, the vPC would break until we could get the command added to the other switch in the pair. Even if we set up two SSH windows, one to each switch, and hit Enter at the same time, the vPCs would break. Here's a similar example, where I changed the spanning tree port type on the vPC. Before I could get to the second switch in the pair, the vPC broke with the following message: 2010 May 11 19:51:38 NX-5k-1 %ETHPORT-5-IF_ERROR_VLANS_SUSPENDED: VLANs 3-7,9 on Interface port-channel14 are being suspended. (Reason: vPC type-1 configuration incompatible - STP interface port type inconsistent) Cisco's answer to this problem is a feature called config-sync. See Chapter 18 for more information. Chapter 7: Link Aggregation CHAPTER 8 Spanning Tree The Spanning Tree Protocol (STP) is used to ensure that no Layer-2 loops exist in a LAN. As you'll see in this chapter, Layer-2 loops can cause havoc. Spanning tree is designed to prevent loops among bridges. A bridge is a device that connects multiple segments within a single collision domain. Switches are considered bridges-hubs are not. While the spanning tree documentation always refers to bridges generically, my examples will show switches. Switches are generally the devices in