ither just been powered on, or just taken out of the administratively down state. Blocking A port in the blocking state is essentially unused. It does not forward or receive frames, with the following exceptions: The port receives and processes BPDUs. The port receives and responds to messages relating to network management. Listening The listening state is similar to the blocking state, except that in this state, BPDUs are sent as well as received. Frame forwarding is still not allowed, and no addresses are learned. Learning A port in the learning state still does not forward frames, but it does analyze frames that come into the port and retrieve the MAC addresses from those frames for inclusion in the MAC address/CAM table. After the frames are ana- lyzed, they are discarded. Forwarding The forwarding state is what most people would consider the "normal" state. A port in this state receives and transmits BPDUs, analyzes incoming packets for MAC address information, and forwards frames from other switch ports. When a port is in the forwarding state, the device or network attached to the port is active and able to communicate. Disabled A port in the disabled state does not forward frames, and does not participate in spanning tree. It receives and responds to only network-management messages. Chapter 8: Spanning Tree Per-VLAN Spanning Tree Because VLANs can be pruned from trunks (as discussed in Chapter 6), it is possible that some VLANs may form loops while others do not. For this reason, Cisco switches now default to a multiple-VLAN form of spanning tree called Per-VLAN Spanning Tree (PVST). PVST allows for a spanning tree instance for each VLAN when used with ISL trunks. Per-VLAN Spanning Tree Plus (PVST+) offers the same features when used with 802.1Q trunks. By default, all VLANs will inherit the same values for all spanning tree configurations. However, each VLAN can be configured differently. For example, each VLAN may have a different spanning tree root bridge. This functionality is an advanced topic, and 