evaluating the root ID field in the BPDUs that it receives. The unique BID is carried in the Root ID field of the BPDUs sent by each switch in the tree. When a switch first boots and begins sending BPDUs, it has no knowledge of a root ID, so it populates the Root ID field of outbound BPDUs with its own BID. The switch with the lowest numerical BID assumes the role of root bridge for that spanning tree instance. If a switch receives BPDUs with a lower BID than its own, it places the lowest value into the Root ID field of its outbound BPDUs. Spanning tree operation requires that each switch have a unique BID. In the original 802.1D standard, the BID was composed of the Priority Field and the MAC address of the switch, and all VLANs were represented by a CST. Because PVST requires that a separate instance of spanning tree run for each VLAN, the BID field is required to carry VLAN ID (VID) information, which is accomplished by reusing a portion of the Priority field as the extended system ID. To accommodate the extended system ID, the original 802.1D 16-bit Bridge Priority field is split into two fields, resulting in these components in the BID :

Bridge Priority: A 4-bit field that carries the bridge priority. Because of the limited bit count, priority is conveyed in discrete values in increments of 4096 rather than discrete values in increments of 1, as they would be in a full 16-bit field. The default priority, in accordance with IEEE 802.1D, is 32,768, which is the mid-range value.
Extended System ID: A 12-bit field that carries the VID for PVST.
MAC address: A 6-byte field with the MAC address of a single switch.

By virtue of the MAC address, a BID is always unique. When the priority and extended system ID are appended to the switch MAC address, each VLAN on the switch can be represented by a unique BID. If no priority has been configured, every switch has the same default priority and the election of the root for each VLAN is based on the MAC address. This is a fairly random means of selecting the ideal root bridge and, for this reason, it is advisable to assign a lower priority to the switch that should serve as root bridge. Only four bits are used to set the bridge priority. Because of the limited bit count, priority is configurable only in increments of 4096. A switch responds with the possible priority values if an incorrect value is entered: Switch(config)#spanning-tree vlan 1 priority 1234
% Bridge Priority must be in increments of 4096.
% Allowed values are:
0 4096 8192 12288 16384 20480 24576 28672
32768 36864 40960 45056 49152 53248 57344 61440 If no priority has been configured, every switch will have the same default priority of 32768. Assuming all other switches are at default priority, the spanning-tree vlan vlan-id root primary command sets a value of 24576. Also, assuming all other switches are at default priority, the spanning-tree vlan vlan-id root secondary command sets a value of 28672. The switch with the lowest BID becomes the root bridge for a VLAN. Specific configuration commands are used to determine which switch will become the root bridge. A Cisco Catalyst switch running PVST maintains an instance of spanning tree for each active VLAN that is configured on the switch. A unique BID is associated with each instance. For each VLAN, the switch with the lowest BID becomes the root bridge for that VLAN. Whenever the bridge priority changes, the BID also changes. This results in the recomputation of the root bridge for the VLAN. To configure a switch to become the root bridge for a specified VLAN, use the spanning-tree vlan vlan-ID root primary command. CAUTION: Spanning tree commands take effect immediately, so network traffic is disrupted while the reconfiguration occurs. A secondary root is a switch that may become the root bridge for a VLAN if the primary root bridge fails. To configure a switch as the secondary root bridge for the VLAN, use the command spanning-tree vlan vlan-ID root secondary. Assuming that the other bridges in the VLAN retain their default STP priority, this switch will become the root bridge in the event that the primary root bridge fails. This command can be executed on more than one switch to configure multiple backup root bridges. BPDUs are exchanged between switches, and the analysis of the BID and root ID information from those BPDUs determines which bridge is selected as the root bridge. and In the example shown, both switches have the same priority for the same VLAN. The switch with the lowest MAC address is elected as the root bridge. In the example, switch X is the root bridge for VLAN 1, with a BID of 0x8001:0c0011111111.

Content 3.1 Describing STP 3.1.6 Describing Port Roles On a nonroot bridge, the spanning tree determines each port’s role in the topology and the most desirable forwarding path for data frames as the switch receives BPDUs on the ports. There are four 802.1D port roles. and . Each Layer 2 port on a switch running STP exists in one of these five port states :

Blocking: The Layer 2 port is a nondesignated port and does not participate in frame forwarding. The port receives BPDUs to determine the location and root ID of the root switch and which port roles (root, designated, or nondesignated) each switch port should assume in the final active STP topology. By default, the port spends 20 seconds in this state (max age).
Listening: Spanning tree has determined that the port can participate in frame forwarding according to the BPDUs that the switch has received. At this point, the switch port is receiving BPDUs and also transmitting its own BPDUs and informing adjacent switches that the switch port is preparing to participate in the active topology. By default, the port spends 15 seconds in this state (forward delay).
Learning: The Layer 2 port prepares to participate in frame forwarding and begins to populate the CAM table. The port is still sending and receiving BPDUs. By default, the port spends 15 seconds in this state (forward delay).
Forwarding: The Layer 2 port is considered part of the active topology. It forwards frames and also sends and receives BPDUs.
Disabled: This is not really an STP state; rather it is the state resulting from administratively shutting down a switch port. In this state, the Layer 2 port does not participate in spanning tree and does not forward frames.

STP uses timers to determine how long to transition ports. STP also uses timers to determine the health of neighbor bridges and how long to cache MAC addresses in the bridge table. The timers operate as follows:

Hello timer: Determines how often root bridge sends configuration BPDUs. The default is 2 seconds.
Maximum Age (Max Age): Tells the bridge how long to keep ports in the blocking state before listening. The default is 20 seconds.
Forward Delay (Fwd Delay): Determines how long to stay in the listening state before going to the learning state, and how long to stay in the learning state before forwarding. The default is 15 seconds.

The root bridge informs the nonroot bridges of the time intervals to use and the STP timers can be tuned based on network size. The default parameters give STP ample opportunity to ensure a loop-free topology. Mistuning the parameters can cause serious network instability. Nonroot bridges place various ports in their proper roles by listening to BPDUs as they come in on all ports. Receiving BPDUs on multiple ports indicates a redundant path to the root bridge. The switch looks at the following components in the BPDU to determine which switch ports forward data and which block data :

Lowest path cost
Lowest sender BID
Lowest sender port ID

The switch looks at the path cost first, which is calculated on the basis of the link speed and the number of links the BPDU has traversed. Ports