Basic Concepts of MSTP
MSTP Network Hierarchy
As shown in Figure 10-3, the MSTP network consists of one or more MST regions, each of which contains one or more MSTIs. An MSTI is a tree network consisting of switching devices running STP, RSTP, or MSTP.
MST Region
- 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.
On the network shown in Figure 10-4, the MST region D0 contains the switching devices S1, S2, S3, and S4, and has three MSTIs.
VLAN Mapping Table
The VLAN mapping table is an attribute of the MST region. It describes mappings between VLANs and MSTIs.
- 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 Root
Regional roots are classified into Internal Spanning Tree (IST) and MSTI regional roots.
In the regions B0, C0, and D0 on the network shown in Figure 10-6, 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 10-5, each MSTI has its own regional root.
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.
Master Bridge
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 10-4.
If an MST region contains the CIST root, the CIST root is the master bridge of the region.
CIST Root
On the network shown in Figure 10-6, the CIST root is the root bridge of the CIST. The CIST root is a device in A0.
CST
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 STP or RSTP based on all the nodes.
On the network shown in Figure 10-6, the MST regions are connected to form a CST.
IST
An IST resides within an MST region and has the MSTI ID of 0. An IST is a segment of the CIST in an MST region.
On the network shown in Figure 10-6, the switching devices in an MST region are connected to form an IST.
CIST
A CIST, calculated by STP or RSTP, connects all the switching devices on a switching network.
On the network shown in Figure 10-6, the ISTs and the CST form a complete spanning tree, the CIST.
SST
- A switching device running STP or RSTP belongs to only one spanning tree.
- An MST region has only one switching device.
On the network shown in Figure 10-6, the switching device in B0 forms an SST.
Port Role
In addition to the port types in RSTP (root ports, designated ports, alternate ports, backup ports, and edge ports), MSTP has two other port types: master ports and regional edge ports.
The functions of root ports, designated ports, alternate ports, backup ports, and edge ports have been defined in RSTP. Table 10-2 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. On the network shown in Figure 10-7, S1 is the root; CP1 is the root port on S3; BP1 is the root port on S2. |
Designated port |
The designated port on a switching device forwards BPDUs to the downstream switching device. On the network shown in Figure 10-7, AP2 and AP3 are designated ports on S1; CP2 is a designated port on S3. |
Alternate port |
An alternate port serves the following functions:
On the network shown in Figure 10-7, BP2 is an alternate port. |
Backup port |
A backup port serves the following functions:
On the network shown in Figure 10-7, CP3 is a backup port. |
Master port |
A master port provides the shortest path from an MST region 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 functioning as master ports in MSTIs. On the network shown in Figure 10-8, 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. As such, 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. On the network shown in Figure 10-8, AP1, DP1, and DP2 in an MST region are directly connected to other regions, and therefore they are all regional edge ports of the MST region. AP1 is 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 detection is started automatically. If the port fails to receive BPDU packets within (2 x Hello Timer + 1) seconds, the port is set to an edge port. Otherwise, the port is set to a non-edge port. |
MSTP Port States
Table 10-3 lists the MSTP port states, which are the same as the RSTP port states.
Port State |
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 build 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 direct link between the port state and the port role. Table 10-4 lists the supported port states for each port role.
Port State |
Root Port/Master Port |
Designated Port |
Regional Edge Port |
Alternate Port |
Backup Port |
|---|---|---|---|---|---|
Forwarding |
Supported |
Supported |
Supported |
Not supported |
Not supported |
Learning |
Supported |
Supported |
Supported |
Not supported |
Not supported |
Discarding |
Supported |
Supported |
Supported |
Supported |
Supported |