Spanning Tree Protocol (STP / IEEE 802.1D) Simulation
Steady State STP: Switch_Root (Lowest BID) coordinates topology; Switch_C blocks Gi0/2 to eliminate Layer 2 loops
Status: Step 1 -> Switch_Root transmits Hello BPDUs every 2 seconds across all Designated Ports.
Cisco IOS CLI Console - Switch_A (Root Bridge)
Switch_A> enable
Switch_A#
Switch_A#
What is Spanning Tree Protocol (STP)?
Spanning Tree Protocol (STP) defined under IEEE 802.1D is a Layer 2 network protocol designed to prevent broadcast storms, duplicate frame transmission, and MAC address table instability in redundant bridged Ethernet networks. STP continuously monitors network topology and logically blocks redundant link paths to ensure a loop-free forwarding topology.
STP Election Hierarchy & Operational Logic
Root Bridge Selection: The switch with the lowest Bridge ID (BID) becomes the Root Bridge. BID consists of Priority (Default: 32768) + System ID Extension (VLAN ID) + Base MAC Address.
Root Port (RP): The port on non-root switches with the lowest cumulative Path Cost to reach the Root Bridge. Every non-root switch elects exactly one Root Port.
Designated Port (DP): The port on a network segment (link) that advertises the lowest Root Path Cost toward the Root Bridge. All active ports on the Root Bridge are Designated Ports.
Alternate / Blocking Port (BLK): Non-designated ports that receive BPDUs but are placed in a discarding/blocking state to prevent active Layer 2 loops.
STP Port States & Convergence Timers
Blocking (20s Max Age): Listens for BPDUs; does not learn MAC addresses or forward data frames.
Listening (15s Forward Delay): Transmits/receives BPDUs to determine active topology; does not update MAC table or forward frames.
Learning (15s Forward Delay): Processes BPDUs and populates the MAC address table; does not forward user traffic.
Forwarding: Fully operational; learns MAC addresses and forwards data frames across the network.
Disabled: Administratively shut down by network administrator.