Detailed Explanation of IPv6 MSTP

Background of MSTP

RSTP is an improvement on STP, which enables fast convergence of network topology. However, RSTP and STP still have the same defect: since all VLANs in a LAN share a spanning tree, it is impossible to achieve load balancing of data traffic between VLANs. When a link is blocked, it will not carry any traffic, resulting in bandwidth waste and the inability to forward messages from some VLANs.

Basic Concepts of MSTP

As shown in Figure 1, an MSTP network contains one or more MST regions, and each MST region contains one or more MSTIs. MSTIs are composed of switching devices running STP/RSTP/MSTP. MSTIs are tree-like networks formed after all switching devices running STP/RSTP/MSTP are calculated by the MSTP protocol.

Figure 1 MSTP network hierarchy diagram

MST Region

An MST region is a Multiple Spanning Tree Region, which is composed of multiple switching devices in a switching network and the network segments between them. Devices in the same MST region have the following characteristics:

• MSTP is enabled on both networks.
• Have the same domain name.
• Have the same VLAN to spanning tree instance mapping configuration.
• Have the same MSTP revision level configuration.

A LAN can have multiple MST regions, which are physically connected directly or indirectly. Users can use MSTP configuration commands to divide multiple switching devices into the same MST region. As shown in Figure 2, MST Region D0 consists of switching devices S1, S2, S3, and S4, and there are three MSTIs in the region.

Figure 2 Schematic diagram of the basic concept of MST Region

VLAN Mapping Table

The VLAN mapping table is an attribute of the MST region and describes the mapping relationship between VLANs and MSTIs.

As shown in Figure 2, the VLAN mapping table of MST region D0 is:

• VLAN1 is mapped to MSTI1
• VLAN2 and VLAN3 are mapped to MSTI2
• The remaining VLANs are mapped to MSTI0

Domain root

Regional roots are divided into IST (Internal Spanning Tree) regional roots and MSTI regional roots.

The IST regional root is shown in Figure 4. In B0, C0, and D0, the switching device closest to the total root (CIST Root) in the IST spanning tree is the IST regional root. Multiple spanning trees can be generated in an MST region, and each spanning tree is called an MSTI. The MSTI regional root is the root of each multiple spanning tree instance. As shown in Figure 3, different MSTIs in the region have their own regional roots.

Figure 3 Schematic diagram of the basic concept of MSTI

MSTIs are independent of each other and can correspond to one or more VLANs, but a VLAN can only correspond to one MSTI.

Main Bridge

The Master Bridge is also known as the IST Master. It is the switching device closest to the root in the domain, such as S1 in Figure 2. If the root is in the MST region, the root is the master bridge of the region.

Total root

Figure 4 Schematic diagram of basic concepts of MSTP network

As shown in Figure 4, the root bridge of the CIST (Common and Internal Spanning Tree) is the root bridge of the CIST. The root bridge is a device in area A0.

CST

The Common Spanning Tree (CST) is a spanning tree that connects all MST regions in a switching network. If each MST region is considered a node, the CST is a spanning tree calculated by these nodes using the STP or RSTP protocol. As shown in Figure 4, the thicker lines connecting the regions form the CST.

IST

The internal spanning tree IST (Internal Spanning Tree) is a spanning tree within each MST region.

IST is a special MSTI with an MSTI ID of 0, usually referred to as MSTI0.

IST is a segment of CIST in the MST region.

As shown in FIG. 4 , thinner lines connect all switching devices in a domain to form an IST.

CIST

The public and internal spanning tree CIST is a single spanning tree that connects all switching devices in a switching network and is calculated and generated by the STP or RSTP protocol. As shown in Figure 4, the IST of all MST regions plus the CST constitute a complete spanning tree, namely the CIST.

SST

There are two situations to form a single spanning tree SST (Single Spanning Tree):

• A switching device running STP or RSTP can belong to only one spanning tree.
• There is only one switching device in the MST region, and this switching device forms a single spanning tree.

As shown in FIG4 , the switching device in B0 is a single spanning tree.