MSTP Background

RSTP is an enhancement to STP, implementing faster convergence of the network topology. However, RSTP and STP have a common problem: All VLANs on a LAN use one spanning tree, and VLAN-based load balancing cannot be performed. Once a link is blocked, it will no longer transmit traffic, wasting bandwidth and causing the forwarding of packets from certain VLANs to fail.

Figure 10-1 STP/RSTP defect

On the LAN shown in Figure 10-1, STP or RSTP is enabled. The broken line represents the spanning tree. S6 is the root bridge. The links between S1 and S4 and between S2 and S5 are blocked. VLAN packets are transmitted using the corresponding links marked with "VLAN 2" or "VLAN 3."

Server A and Server B belong to VLAN 2 but they cannot communicate with each other because the link between S2 and S5 is blocked and the link between S3 and S6 denies packets from VLAN 2.

To overcome this issue that is present in STP and RSTP, the IEEE released 802.1s in 2002, defining the Multiple Spanning Tree Protocol (MSTP). In addition to implementing fast convergence, MSTP also provides multiple paths to load balance VLAN traffic.

MSTP divides a switching network into multiple regions, known as Multiple Spanning Tree (MST) regions. Each MST region has multiple spanning trees, known as Multiple Spanning Tree Instances (MSTIs), that are independent of each other.

An instance is a collection of VLANs. Binding multiple VLANs to an instance saves communication cost and reduces resource usage. The topology of each MSTI is calculated independently from other MSTIs, and traffic can be balanced among MSTIs. Multiple VLANs that have the same topology can be mapped to one instance. Whether a port forwards packets from a VLAN depends on the port status in the MSTI.

Figure 10-2 Multiple spanning trees in an MST region

On the network shown in Figure 10-2, MSTP maps VLANs to MSTIs in the VLAN mapping table. Each VLAN can be mapped to only one MSTI. This means that traffic of a VLAN can be transmitted in only one MSTI. An MSTI, however, can correspond to multiple VLANs.

In this manner, devices within the same VLAN can communicate with each other; packets of different VLANs are load balanced along different paths.