Spanning Tree Protocol Explained

Spanning Tree Protocol (STP) was invented by Dr. Radia Perlman whilst she worked for Digital Equipment Corporation (DEC) based on an algorithm she designed to eliminate loops in computer networks which became her thesis at MIT. Her thesis was “Network Layer Protocols with Byzantine Robustness” submitted to the Massachusetts Institute of Technology (MIT) in August 1988.

“Abstract”

“The Network Layer of a network architecture is a distributed protocol that facilitates packet delivery across multiple hops. One of its chief functions is the calculation of routes throughout the network. Traditional Network Layer protocols have addressed robustness in the face of simple failures, i.e. nodes or links becoming inoperative. This thesis examines Network Layer protocol designs that are robust in the presence of Byzantine failures, i.e. nodes that through malice or malfunction exhibit arbitrary behaviour such as corrupting, forging, or delaying routing protocol messages.”

- Dr. Radia Perlman - “Network Layer Protocols with Byzantine Robustness” – 04 August 1988

The Institute of Electrical Engineers (IEEE) then formalised the algorithm as the IEEE 802.1D standard for Spanning Tree Protocol which was later replaced by IEEE 802.1W Rapid Spanning Tree Protocol which was newer and faster than IEEE 802.1D.

The focus of the thesis was on the Network Layer (Layer 3 in the OSI Model or the Network Access Layer 1 in the TCP/IP Model). The reason being the Network layer is responsible for delivery of packets across multiple hops and this layer is a protocol in which all nodes participate.

The problem was in computer networks that broadcast messages were being broadcast throughout the network and causing loops and switches and bridges could not stop the endless loops. The idea was to create an algorithm that guarantees that there is one and only one active path between two network devices.

You configure Spanning Tree Protocol in switches and bridges that are IEEE 802.1D compliant. The idea as mentioned above is to avoid loops and bringing your network or LAN or VLAN to a grinding halt. Another important factor concerning STP is to use it where you need redundant links, but not loops. A failure of your primary link activates your backup links. Without STP such failures can cause an eternal loop in your network traffic.

How STP Works

STP creates a tree that spans all the switches in an extended network. The key in STP is to create a root bridge that becomes the central node point of your network. In a switching network the root bridge is referred to as a root switch. All switches exchange information in the root switch for configuration of the network. Bridge Protocol Data Units (BPDU) carries this routing information.

STP calculates the path cost to the root bridge based on the medium transport speed and the port cost of each port forwarding frame. If the speed of the network transport medium is changed or the switch is changed from half-duplex to full duplex or vice versa the Spanning Tree Protocol recalculates the path cost because a change in the path cost can change the Spanning Tree Topology.

By default the Spanning Tree Protocol runs on every port in a switch and cannot be turned off on a per-port basis. Although it is not recommended you can turn off STP globally on the switch or on a per-VLAN basis, you should take extreme care as this creates Layer 2 loops in the network.

In VLAN’s one root switch is identified per-VLAN and this is caused by Bridge Protocol Data Units (BPDU) going through the network and being compared by the switches in the network and the switch with the lowest bridge ID in the network is chosen as the Root Switch. Less is better in the STP root selection process!

After the root switch is identified by all other switches the switches then adhere to these rules:

STP Rule 1: All the ports of the root switch must be in forwarding mode.

STP Rule 2: The root port must be set to forwarding mode.

STP Rule 3: In a single LAN segment the port of the designated switch that connects to the        LAN segment must be placed in forwarding mode.

STP Rule 4: All the other ports in all the VLAN-specific switches must be placed in blocking mode.

NOTE: The addition or removal of VLAN’s in the STP per-VLAN spanning tree mode triggers spanning tree recalculation at that moment and only the traffic on that VLAN is disrupted.

Credits:

Network 101: Understanding Spanning Tree – Enterprise Networking Planet
Spanning Tree Protocol – Techtarget Network
Spanning Tree Protocol – Wikipedia
Radia Perlman – Wikipedia
Radia Perlman Thesis – Network Layer Protocols with Byzantine Robustness (1988)
Spanning Tree Protocol – Cisco
Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches (17 August 2006) – Cisco