What is the default protocol that switches run?

The default protocol that switches run typically refers to the Spanning Tree Protocol (STP) and its variants. In many switch models, the default protocol is actually Rapid Spanning Tree Protocol (RSTP), which is an enhanced version of the original STP, designed to provide faster convergence times and improved network stability.

MSTP (Multiple Spanning Tree Protocol) is not the default protocol in most cases; it allows for multiple spanning trees over the same network, which is more complex and generally requires manual configuration of instances. While MSTP is useful for more complex network topologies, it is not the default for standard switch operations.

VLAN (Virtual Local Area Network) is not a spanning tree protocol; rather, it's a method to segment networks at Layer 2. VLANs facilitate the logical partitioning of a physical network and do not inherently provide loop prevention mechanisms that STP variants offer.

STP (Spanning Tree Protocol) is the original protocol used for loop prevention in Ethernet networks. Many switches default to using STP when they are first configured. While RSTP is a basically improved version of STP, it’s primarily the one that is widely referenced in discussions about modern switch configurations as the default protocol for efficient loop prevention.