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HomeBlogSpanning Tree Protocol: An Essential Component of IEEE 802.1D Standard

Spanning Tree Protocol: An Essential Component of IEEE 802.1D Standard

IEEE 802.1D-1998: Spanning Tree Protocol

The Spanning Tree Protocol (STP) is one of the most important protocols that run in an aggregation layer based on the IEEE 802.1D standard; it is used to maintain loop-free topologies in Ethenet networks and also contributes a lot to network stability and performance. This is the most comprehensive article on STP I have written till now which is in context to IEEE 802.1D standard which is also the most used version by most switches running STP.

IEEE 802.1D

IEEE 802.1D – StandardANSI/IEEE standard developed by the Institute of Electrical and Electronics Engineers for MAC bridges and the operation of network switches.

Fundamental ConceptIt defines the way Ethernet frames are forwarded and provides a loop free topology with the help of STP.

Spanning Tree Protocol (STP)

Description: Spanning Tree Protocol STP Primary Function is to break Loops in network to prevent broadcast storm to prevent network Congestion.

Mechanism : It selectively disables redundant paths by logical blockage to makes single common active Path between all segments of the network.

STP-KeyTerms and Features

Root Bridge Election:

When STP is activated, an election to elect a root bridge-the central reference point in the network for path calculations.

BPDU (Bridge Protocol Data Units):

Stp uses these frames to share information on network topology between switches.

Port States and Roles:

Blocking: To achieve non loop forwarded that block (do not forward frames).

Listening, it prepares to forward frames but does not actually forwarding any frame.

Learning: Memorizes MAC addresses without sending frames.

Forwarding: Passes the frame on and keeps track of MAC addresses.

Shutdown: Manual administrative shutdown of the port.

Root Port: It is the type of port that helps towards the best way to reach the root bridge.

Designated Port Best path of a bridge to the root bridge on all network links.

Non-designated Thing: Normally in a blocking state to stop loops from happening

Extensions and Versions of STP

Rapid Spanning Tree Protocol (RSTP, IEEE 802.1w): This provides a much faster convergence times compared to traditional STP, quickly adjusting to network changes such as link failures.

Multiple Spanning Tree Protocol (MSTP-IEEE 802.1s) – This protocol can let the multiple spanning tree at the same time coexist, which can make the redundant link from being the best use and the different VLAN maps to different multiple spanning tree.

Per-VLAN Spanning Tree Protocol (PVST) and PVST+ : These are the Cisco-specific take on the 802.1d standard but the difference is that they run an independent instance of STP for each VLAN allowing for a more control adjacent network path.

Integration in the OSI Model

STP works at Layer 2 of the OSI model and controls bridges and switches in the data link layer (2).


The Spanning Tree Protocol (STP), one of the critical components of the IEEE 802.1D standard, is designed to help prevent loops within a network in order to allow for a stable and redundant Ethernet network to maintain normal operation. STP has undergone improvements like RSTP and MSTP to provide faster and more efficient network topology management.



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