Spanning Tree Protocol (STP)

802.1d

Spanning Tree Protocol (STP)

Importance of STP

• Allows switches to use redundant connections without causing loops

• Prevents broadcast storms and helps maintain stable network connectivity

Network without Spanning Tree Protocol (STP)

• May cause switching loops and broadcast storms

  Broadcast storm: When the same frames are continuously sent around the network, using up bandwidth and overwhelming the network

Spanning Tree Protocol (STP) Functionality

• Operates using a root bridge and a non-root bridge

• Root Bridge: A switch is elected as the central refrence point for the spanning tree; chosen based on lowest bridge ID (BID)

• Bridge ID (BID): Consists of a priority value and the switches MAC address; the switch with the lowest BID becomes the root bridge

• Non-Root Bridges: All other switches in the STP topology that are not elected as the root bridge

Spanning Tree Protocol (STP) Types

• Root Port: Found on non-root bridges; this port has the lowest cost path to the root bridge; only one root port per switch (non-root); if multiple paths have the same cost, STP uses the lowest port number

• Designated Port: Present on each network segment; it is the port with the best (lowest cost) path to the root bridge for that segment; all ports on the root bridge are designated ports

• Non-Designated Port: these ports are places in a blocking state to prevent loops; they do not forward traffic

Spanning Tree Protocol

• Blocking: Does not forward frames; listens for BPDU (Bridge Protocol Data Units) to detect network changes

• Listening: Prepares to participate in STP; does not forward frames or learn MAC addresses yet; Listens for BPDUs

• Learning: Learns MAC addresses; still does not forward frames; builds the MAC address table

• Forwarding: Actively forwards frames; occurs on root and designated ports

Link Cost

• Determined by the speed of the link

  Higher speed = lower cost

  Lower speed = higher cost

• used by STP to decide the best path to the root bridge