As shown in Figure 1, the MSTP network consists of one or more MST regions. Each MST region contains one or more MSTIs. An MSTI is a tree network consisting of switching devices running STP, RSTP, or MSTP.
Figure 1 MSTP network hierarchy
An MST region contains multiple switching devices and network segments between them. The switching devices of one MST region have the following characteristics:
MSTP-enabled
Same region name
Same VLAN-MSTI mappings
Same MSTP revision level
A LAN can comprise several MST regions that are directly or indirectly connected. Multiple switching devices can be grouped into an MST region by using MSTP configuration commands.
As shown in Figure 2, the MST region D0 contains the switching devices S1, S2, S3, and S4, and has three MSTIs.
Figure 2 MST region
The VLAN mapping table is an attribute of the MST region. It describes mappings between VLANs and MSTIs.
As shown in Figure 2, the mappings in the VLAN mapping table of the MST region D0 are as follows:
VLAN 1 is mapped to MSTI 1.
VLAN 2 and VLAN 3 are mapped to MSTI 2.
Other VLANs are mapped to MSTI 0.
Regional roots are classified into Internal Spanning Tree (IST) and MSTI regional roots.
In the region B0, C0, and D0 on the network shown in Figure 4, the switching devices closest to the Common and Internal Spanning Tree (CIST) root are IST regional roots.
An MST region can contain multiple spanning trees, each called an MSTI. An MSTI regional root is the root of the MSTI. On the network shown in Figure 3, each MSTI has its own regional root.
Figure 3 MSTI
MSTIs are independent of each other. An MSTI can correspond to one or more VLANs, but a VLAN can be mapped to only one MSTI.
The master bridge is the IST master, which is the switching device closest to the CIST root in a region, for example, S1 shown in Figure 2.
If the CIST root is in an MST region, the CIST root is the master bridge of the region.
Figure 4 MSTP network
On the network shown in Figure 4, the CIST root is the root bridge of the CIST. The CIST root is a device in A0.
A Common Spanning Tree (CST) connects all the MST regions on a switching network.
If each MST region is considered a node, the CST is calculated by using STP or RSTP based on all the nodes.
As shown in Figure 4, the MST regions are connected to form a CST.
An IST resides within an MST region.
An IST is a special MSTI with the MSTI ID being 0, called MSTI 0.
An IST is a segment of the CIST in an MST region.
As shown in Figure 4, the switching devices in an MST region are connected to form an IST.
A CIST, calculated by using STP or RSTP, connects all the switching devices on a switching network.
As shown in Figure 4, the ISTs and the CST form a complete spanning tree, the CIST.
A Single Spanning Tree (SST) is formed in either of the following situations:
A switching device running STP or RSTP belongs to only one spanning tree.
An MST region has only one switching device.
As shown in Figure 4, the switching device in B0 forms an SST.
Based on RSTP, MSTP has two additional port types. MSTP ports can be root ports, designated ports, alternate ports, backup ports, edge ports, master ports, and regional edge port.
The functions of root ports, designated ports, alternate ports, and backup ports have been defined in RSTP. Table 1 lists all port roles in MSTP.
Except edge ports, all ports participate in MSTP calculation.
A port can play different roles in different spanning tree instances.
Port Role | Description |
Root port | A root port is the non-root bridge port closest to the root bridge. Root bridges do not have root ports. Root ports are responsible for sending data to root bridges. As shown in Figure 5, S1 is the root; CP1 is the root port on S3; BP1 is the root port on S2; DP1 is the root port on S4. |
Designated port | The designated port on a switching device forwards bridge protocol data units (BPDUs) to the downstream switching device. As shown in Figure 5, AP2 and AP3 are designated ports on S1; BP2 is a designated port on S2; CP2 is a designated port on S3. |
Alternate port |
As shown in Figure 5, DP4 and AP4 are alternate ports. |
Backup port |
As shown in Figure 5, CP3 is a backup port. |
Master port | A master port is on the shortest path connecting MST regions to the CIST root. BPDUs of an MST region are sent to the CIST root through the master port. Master ports are special regional edge ports, functioning as root ports on ISTs or CISTs and master ports in instances. As shown in Figure 5, S1, S2, S3, and S4 form an MST region. AP1 on S1, being the nearest port in the region to the CIST root, is the master port. |
Regional edge port | A regional edge port is located at the edge of an MST region and connects to another MST region or an SST. During MSTP calculation, the roles of a regional edge port in the MSTI and the CIST instance are the same. If the regional edge port is the master port in the CIST instance, it is the master port in all the MSTIs in the region. As shown in Figure 5, AP1, DP2, and DP3 in an MST region are directly connected to other regions, and therefore they are all regional edge ports of the MST region. As shown in Figure 5, AP1 is a regional edge port and also a master port in the CIST. Therefore, AP1 is the master port in every MSTI in the MST region. |
Edge port | An edge port is located at the edge of an MST region and does not connect to any switching device. Generally, edge ports are directly connected to terminals. After MSTP is enabled on a port, edge-port detecting is started automatically. If the port fails to receive BPDU packets within (2 x Forward Delay - 2) seconds, the port is set to an edge port. Otherwise, the port is set to a non-edge port. As shown in Figure 5, BP3 is an edge port. |
Figure 5 Port roles
Table 2 lists the MSTP port status, which is the same as the RSTP port status.
Port Status | Description |
Forwarding | A port in the Forwarding state can send and receive BPDUs as well as forward user traffic. |
Learning | A port in the Learning state learns MAC addresses from user traffic to construct a MAC address table. In the Learning state, the port can send and receive BPDUs, but not forward user traffic. |
Discarding | A port in the Discarding state can only receive BPDUs. |
There is no necessary link between the port status and the port role. Table 3 lists the relationships between port roles and port status.
Port Status | Root Port/Master Port | Designated Port | Regional Edge Port | Alternate Port | Backup Port |
Forwarding | Yes | Yes | Yes | No | No |
Learning | Yes | Yes | Yes | No | No |
Discarding | Yes | Yes | Yes | Yes | Yes |
Yes: The port supports this status.
No: The port does not support this status.
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