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Spanning Tree Protocol for Bridges

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Spanning Tree Protocol, also known as STP, was produced in order to obtain a methodology where bridges can obtain layer 2 routing with redundant and loop-free operation. Spanning tree protocol is defined as a protocol where two bridges interconnect two computer network segments and allow the bridges to exchange information. This way only one of the bridges will be responsible for handling a message that is being sent between both of the computers within that same network. Removing STP and connecting switches in a loop will result in the duplication of the broadcasted packet because there is nothing at layer 2 that will work to prevent a loop. STP exchanges BPDU messages with other switches to detect loops. BPDU is only one of the necessary components that STP uses to carry out its job. STP uses other components as well that include, but are not limited to, root port, block port, and designated port.

Bridge Protocol Data Unit (BPDU) is a data message that is exchanged between switches transmitted by frames across a LAN. BDPU is used to detect loops in network topologies. BPDU frames contain information that involves switches ID, addresses, MAC address, originating switch ports, and switch port priority. The information contained in BPDU is necessary to configure a STP topology as well as maintain it. BPDU informs other switches of port changes. BPDU is actively used in STP when using the spanning tree algorithm (STA). What happens is when the BPDUs are received, the switch uses a math formula, the spanning tree algorithm. This is used to know when there is a layer 2 switch loop in the network. This later determines which of the ports will need to eventually be shut off and shut down. There are actually three types of BPDUs. These are Configuration BPDU (CBPDU), Topology Change Notification (TCN), and Topology Change Notification Acknowledgement (TCA). The CBPDU is a packet that is sent between bridges in a network to configure the devices interfaces. This is done either in a forwarding or blocking state. The TCN is used by the bridge when the bridge needs to signal a topology change. TCN is sent on the on the bridge’s root port. TCA is used to acknowledge a receipt of a configuration message. The main use and goal of implementing the Bridge Protocol Data Units with the spanning tree algorithm is in order to avoid layer 2 switching loops and also something known as broadcast storms. Using the units made up in BPDU this goal can be accomplished.

A bridge contains two or possibly more ports. The two that are often confused and are definitely located on the bridge are the root and the designated ports. The port that is connected on the same side that the STP root can be located is the root port. The Root Port is the port on the bridge, or switch, that has the least spanning tree path cost stretching from the root bridge all the way to the switch. There is and can only be one root port on a bridge. When a bridge has more than one path to choose from in order to reach the root bridge, then the bridge chooses one path, the shortest one usually, to be designated as the root port. A root port can also be identified as being a port that is facing “upstream” and is always facing, or pointing, towards the root bridge.

A designated port is the port on a local area network (LAN) segment that has the least path cost to get to the root bridge. This sounds very similar to what the definition of a root port is. But, a root port can not ever be a designated port. Because, the root port is the port on the switch with the least cost from the switch to the root bridge. Designated port is a port on the LAN with the least cost to the root bridge. Their locations dictate their differences. It is very easy to mix up the definitions of these two specific ports. The designated port is the port that is connecting the bridge to the network segment, as a favor for the segment. Designated ports are chosen between switches that share a network segment based on the switches cost to get to the root. The designated port is in a forwarding state. The port is used to forward traffic onto the segment.

Another important component used by STP is the blocking ports for blocking loops. Blocking ports are ports that are were made in order to not allow traffic to be accepted or even to be sent through the port that they are located. It is called blocking port because it does exactly that. The blocking port blocks traffic. The blocking ports are selected by STP to be put into a blocking state. These ports are chosen because they have no previous role to play in the STP process. So, they block traffic. The frames that are sent over the link that contains the blocking port will be blocked, or in other words, the frames will be dropped from that link because the blocking port did not allow that specific traffic through. This is done so that any port that form or create loops are blocked as necessary. Blocking ports are of course in a blocking mode. If there is a case where there are multiple paths between switches or bridges, one of those paths will be chosen to go into blocking mode to further make sure of loop prevention. BPDU, root port, designated port, and blocking port all play a major role and work together with one another in attempt for the efficient success for the STP protocol.

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Spanning Tree Protocol for Bridges. (2019, February 27). GradesFixer. Retrieved November 27, 2021, from
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