Cisco Cram or: How I Learned to Stop Lolling and Love the Book: CCIE 400-101: Layer 2 Technologies

The first IEEE-standardized STP, also often called the “legacy” STP, was originally described in 802.1D.
The Rapid STP (RSTP) was standardized in 802.1w, while Multiple STP (MSTP) was covered in 802.1s.
The latest 802.1D-2004 standard no longer includes the legacy STP at all (which is considered obsolete), and instead, it covers the RSTP originally found in 802.1w.
The 802.1s MSTP is integrated into 802.1Q-2005 and later revisions.
Therefore, RSTP is covered in 802.1D while MSTP is covered in 802.1Q, and legacy STP has been dropped.

STP uses special messaging between switches to establish a loop-free topology by logically blocking redundant links.
STP protocol messages are called Bridge Protocol Data Units (BPDUs).
STP operation is based on the ability to compare any two arbitrary Configuration BPDUs and determine which one of them is better, or superior.
For a detailed breakdown of a BPDU, see: BPDU Format and STP Timers.
The determine the superior BPDU, they are compared in the following sequence of values, looking for the first occurrence of a lower value:
- Root Bridge ID (RBID)
- Root Path Cost (RPC)
- Sender Bridge ID (SBID)
- Sender Port ID (SPID)
- Receiver Port ID (RPID; not included in the BPDU, evaluated locally; very uncommon)
Only Configuration BPDUs are compared.
Each port in STP stores (that is, remembers) the superior BPDU it has either sent or received.
Essentially, each port stores the Designated Port’s BPDU — whether it is the port itself that is Designated or it is a neighbor’s port.
Should a port store a received BPDU, it must be received again within a time interval of MaxAge-MessageAge seconds; otherwise it will expire after this period.
To determine which ports forward and block, STP follows a three-step process:
1. Elect the root switch: the switch with the lowest bridge ID.
2. Determine root port on each switch: the port on each non-root switch that receives the best BPDU.
3. Determine the designated port for each segment: the switch that sends the superior BPDU on the segment.

Each non-root switch chooses exactly one port as its Root Port, i.e. the port that receives the best resulting BPDU.
This means that the switch adds the port's cost to the advertised Root Path Cost value, yielding a resulting BPDU.
BPDUs received on the Root Port of a non-root switch are forwarded out its designated ports after updating the Root Path Cost, Sender Bridge ID, Send Port ID, and MessageAge fields accordingly.
BPDUs received on other ports of a non-root switch are processed but they are not forwarded.
Many people think of STP costs as being associated with a segment; however, the cost is actually associated with interfaces. Good design practices dictate using the same STP cost on each segment, but the values can be different.
On recent Catalyst switches, the default costs correspond to the 802.1D-1998 version of the standard if PVST or Rapid PVST is used, and to the 802.1D-2004 version if MSTP is used.
With PVST and Rapid PVST, the 802.1D-2004 costs can be activated using the spanning-tree pathcost method long global configuration command.
By default, spanning-tree pathcost method short is configured, causing the switch to use the older revision of the costs.

Designated switch = switch forwarding BPDUs on a LAN segment.
Designated Port = the port that the designated switch uses to forward frames onto the segment.
All other ports are neither Root or Designated will be moved to Blocking state (Non-Designated ports).
The become the Designated Port, the switch must send superior BPDUs on the segment.
The tiebreakers during Designated Port selection are the same as before (how to find the superior BPDU).
Neither Root Ports nor Blocking ports send BPDUs.

The root switch is the switch that has the lowest Bridge ID in the topology.
On each non-root switch, a Root Port is the port receiving the best (that is, superior) resulting BPDUs from all received BPDUs on all ports.
On each connected segment, a Designated Port is the port sending the best (that is, superior) BPDUs on the segment.
All ports that are neither Root Ports nor Designated Ports are superfluous in an active topology and will be put into the Blocking state.
Configuration BPDUs are sent out only from Designated Ports.
Each port stores the best (that is, superior) BPDU it has received or sent itself.
Designated Ports store the BPDU they send; Root and Blocking ports store the best BPDU they receive.
The stored BPDU determines the role of the port and is used for comparisons.
Received superior stored BPDUs will expire in MaxAge-MessageAge seconds if not received within this time period.

The MessageAge field is an estimation of the BPDU’s age since it was originated by the root bridge.
At the root bridge, the MessageAge is set to 0. Any other switch will increment this value, usually by 1, before forwarding the BPDU further.
The remaining lifetime of a BPDU after being received by a switch is MaxAge-MessageAge.
Finally, the remaining fields carry the values of STP timers: MaxAge, HelloTime, ForwardDelay. These timer values always reflect the timer settings on the root switch.
Timers configured on a non-root switch are not used and would become effective only if the switch itself became the root switch.

CCIE Routing and Switching v5.0 Official Cert Guide, Volume 1

Cisco Cram or: How I Learned to Stop Lolling and Love the Book