STP Terms STP Port Roles STP States CCNA CCNP switching

Here's an overview of the key terms, port roles, BPDU types, and states associated with the Spanning Tree Protocol (STP):

STP Terms
**Bridge Protocol Data Unit (BPDU)**: A type of message exchanged between switches to share information about bridge IDs, root paths, and port roles for STP.
**Root Bridge**: The switch with the lowest bridge ID (bridge priority + MAC address), which serves as the logical center of the network in STP.
**Bridge ID**: A unique identifier for each switch, composed of a priority value and the switch's MAC address.
**Path Cost**: The cost associated with traversing a particular link; lower costs are preferred for STP path selection.
**Port Priority**: A configurable value that can influence which port is selected for forwarding in cases where two ports have equal path costs.

STP Port Roles
**Root Port (RP)**: The port on each switch with the best path to the root bridge. There can be only one root port per switch.
**Designated Port (DP)**: The port on a network segment with the lowest path cost to the root bridge. It forwards traffic toward and away from that segment.
**Alternate Port (AP)**: A backup port with a link to the root bridge, ready to take over if the root port fails (typically seen in Rapid STP).
**Backup Port (BP)**: A redundant path to a network segment within the same switch. Rarely used in standard STP but exists in Rapid STP.

Root Bridge BPDU
**Configuration BPDU**: Sent by the root bridge, it contains essential information for building and maintaining the STP topology, including root bridge ID, sender bridge ID, path cost, and STP timers.
**Topology Change Notification (TCN) BPDU**: Used to signal topology changes, like a link failure or addition, prompting other switches to update their MAC address tables.

STP States
1. **Disabled**: Port is administratively down and does not participate in STP.
2. **Blocking**: Port does not forward frames but listens for BPDUs. Used to prevent loops by keeping non-designated ports inactive.
3. **Listening**: The port is actively listening to BPDUs to determine if it can transition to the forwarding state. No frames are forwarded, and no MAC addresses are learned.
4. **Learning**: The port learns MAC addresses from incoming frames to build the switch’s MAC table but does not forward frames.
5. **Forwarding**: The port actively forwards frames and continues to learn MAC addresses. This is the active state for designated and root ports.
6. **Broken**: Some STP implementations use this additional state for ports with configuration errors or issues.

These elements allow STP to maintain a loop-free topology and quickly respond to topology changes.