MSTP Multiple Spanning Tree Protocol

Multiple Spanning Tree Protocol (MSTP) is a protocol used in computer networks to provide loop-free redundant paths while ensuring efficient resource utilization. It is an extension of the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP). MSTP is defined in the IEEE 802.1Q standard and is also known as IEEE 802.1s.

The main purpose of MSTP is to prevent loops in a network topology by creating a loop-free tree structure. This tree structure allows for redundant paths to be used in case of link or switch failures without causing loops. By enabling multiple spanning trees, MSTP enables load balancing across different paths, leading to improved network performance.

MSTP achieves its objectives by dividing the network into multiple logical regions called Multiple Spanning Tree Instances (MSTIs). Each MSTI is associated with a specific group of VLANs. The network administrator can define the number of MSTIs and the VLANs associated with each instance according to the network requirements.

MSTP operates by electing a root bridge for each MSTI within the network. The root bridge is responsible for maintaining the loop-free topology within its MSTI. The root bridge is determined based on a configurable bridge priority value, where the bridge with the lowest priority becomes the root bridge. The root bridge is responsible for forwarding the traffic towards the destination.

After the root bridge is elected for each MSTI, the next step is to determine the forwarding path from each bridge to the root bridge. MSTP accomplishes this by exchanging Bridge Protocol Data Units (BPDUs) between switches. BPDUs contain information about the bridge's identity, its priorities, and the connectivity of the links.

Each bridge in the network selects its own root port, which is the port that provides the shortest path to the root bridge. It also selects the designated ports, which are the ports that connect to the non-root bridges in the same MSTI. The designated ports forward traffic toward the root bridge.

The port roles in MSTP include root port, designated port, alternate port, and backup port. The root port is the port that has the shortest path to the root bridge. The designated port is the port that forwards traffic towards the root bridge. The alternate port is a backup port that can be activated in case the root port fails. The backup port is another backup port that provides an alternate path to the designated port.

MSTP allows for load balancing across multiple paths by grouping VLANs into MSTIs. Each MSTI can have a different spanning tree, allowing traffic from different VLANs to take different paths. This improves network performance by distributing the traffic across the available links.

MSTP also supports VLAN mapping, which allows the same VLAN to be mapped to different MSTIs on different switches. This feature is useful when multiple switches need to share the same VLAN but belong to different MSTIs.

To ensure backward compatibility, MSTP is backward compatible with RSTP and STP. This means that MSTP bridges can interoperate with RSTP and STP bridges in the same network. MSTP can detect the presence of STP and RSTP bridges and adjust its behavior accordingly.

In terms of convergence time, MSTP provides faster convergence compared to STP. This is achieved by using the Multiple Spanning Tree Instances, which allows for parallel calculations of the spanning trees for each MSTI. The parallel calculations reduce the overall convergence time by avoiding unnecessary blocking of ports.

MSTP also supports Rapid Per-VLAN Spanning Tree (RPVST), which extends the fast convergence benefits of RSTP to individual VLANs within an MSTI. RPVST maintains a separate spanning tree for each VLAN, allowing for faster convergence in case of VLAN-specific topology changes.

In conclusion, Multiple Span Spanning Tree Protocol (MSTP) is a powerful extension of the Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP) that provides loop-free redundancy and efficient resource utilization in computer networks. By dividing the network into Multiple Spanning Tree Instances (MSTIs), MSTP enables load balancing, faster convergence, and improved network performance.

One of the key features of MSTP is its ability to create multiple logical regions within a network called MSTIs. Each MSTI is associated with a specific group of VLANs, allowing for better control and segmentation of the network. The network administrator has the flexibility to define the number of MSTIs and the VLANs associated with each instance based on the network requirements.