orward 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 analyzed, 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. 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. See more detail on different types of spanning tree in Cisco switches at the end of this chapter. Managing Spanning Tree Spanning tree is enabled by default. To see its status, use the show spanning-tree com- mand in IOS and NX-OS: Cat-3550#sho spanning-tree VLAN0001 Spanning tree enabled protocol ieee Managing Spanning Tree Root ID Priority 24577 Address 0009.43b5.0f80 20 (FastEtherneto/20) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 000f.8f5c.5a00 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio. Nbr Type Fa0/13 Altn BLK 1