What is STP? - Explain Advantages and Disadvantages

The Spanning Tree Protocol is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them.

STP is a protocol. It actively monitors all links of the network. To finds a redundant link, it uses an algorithm, known as the STA (spanning-tree algorithm). The STA algorithm first creates a topology database then it finds and disables the redundant links. Once redundant links are disabled, only the STP-chosen links remain active. If a new link is added or an existing link is removed, the STP re-runs the STA algorithm and re-adjusts all links to reflect the change.

STP (Spanning Tree Protocol) automatically removes layer 2 switching loops by shutting down the redundant links. A redundant link is an additional link between two switches. A redundant link is usually created for backup purposes.

Just like every coin has two sides, a redundant link, along with several advantages, has some disadvantages. The biggest disadvantage of a redundant link is that it creates a loop between switches. If a loop exists between two switches, they do not work properly.

Root Bridge (Election procedure):

A Root Bridge is the starting point of the STP network topology. To elect a Root Bridge from all switches of the network, STP uses two parameters; a variable known as bridge priority and the MAC addresses of participating switches.

A switch that has the lowest bridge priority value, is elected as the root bridge. If the bridge priority value is the same in all switches, the switch which has the lowest MAC address is elected as the Root Bridge.The selection process of the Root Bridge happens each time when a network change occurs like a new switch is added in the network topology, or an existing switch is removed or the current Root Bridge is failed.

Non-Root Bridge:

Except the Root Bridge, all remaining switches of the network are considered as the Non-Root Bridges. Non-Root Bridges receive updates from the Root Bridge and update their STP databases relatively.

How Spanning Tree Protocol works

The spanning tree algorithm determines the network (which computer hosts are in which segment) and this data is exchanged using BPDUs. It is broken down into two steps:

Step 1: The algorithm determines the best message a bridge can send by evaluating the configuration messages it has received and choosing the best option.

Step 2: Once it selects the top message for a particular bridge to send, it compares its choice with possible configuration messages from the non-root-connections it has. If the best option from step 1 isn't better than what it receives from the non-root-connections, it will prune that port.

STP Configuration:

To enable STP on a switch, an administrator connects to the switch by typing spanning-tree mode <selected mode> in global configuration mode. The network administrator then adjusts spanning tree operation to control its behavior by first electing a root bridge, which becomes the central reference point for STP. Next, the administrator will identify root ports and designated ports.

Advantages of STP:

• Easy to use.
• Proven technology.
• Offers wide support for bridges and switches.
• Provides link redundancy while simultaneously preventing undesirable loops.
• Offers various backups that become active if the main connection experiences hiccups.

Disadvantages of STP:

• As data centers increase use of virtualization technology, STP may not be able to handle increased input/output (I/O) demands.
• Full network capacity is not realized when using STP. Even when there are equal-cost multiple paths on a network, all traffic flows along a single path as defined by a spanning tree. This restriction of traffic means alternative, and perhaps more direct, paths are blocked.