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Understanding of MSTP - Basic MSTP Concepts

Latest reply: May 24, 2021 19:11:47 389 57 29 0 4

OBJECTIVE


The purpose of this post is to present an introduction to the MSTP basics concets.


MSTP Network Hierarchy


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.


01

Figure 1 - MSTP network hierarchy


MST Region


An MST region contains multiple switching devices and network segments between them. Theswitching 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.


02

Figure 2 - MST region


VLAN Mapping Table


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 Root


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.


03

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.


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 2.


If the CIST root is in an MST region, the CIST root is the master bridge of the region.


CIST Root


04

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.


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 using STP or RSTP based on all the nodes.


As shown in Figure 4, the MST regions are connected to form a CST.


IST


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.


CIST


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.


SST


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.


Port Role


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.


i_f42.gifExcept 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.

Designated port

The designated port on a switching device forwards BPDUs to the downstream switching device.


As shown in Figure 5, AP2 and AP3 are designated ports on S1; CP2 is a designated port on S3.

Alternate port
  • From the perspective of sending BPDUs, an alternate port is blocked after a BPDU sent by another bridge is received.


  • From the perspective of user traffic, an alternate port provides an alternate path to the root bridge. This path is different than using the root port.


As shown in Figure 5, BP2 is an alternate port.

Backup port
  • From the perspective of sending BPDUs, a backup port is blocked after a BPDU sent by itself is received.


  • From the perspective of user traffic, a backup port provides a backup/redundant path to a segment where a designated port already connects.


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 6, 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 6, 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.

Table 1 - Port Roles


07

Figure 5 - Root port, designated port, alternate port, and backup port


08

Figure 6 - Master port and regional edge port


MSTP Port Status


Table 2 lists the MSTP port status, which is the same as the RSTP port status.


Port Status Description
ForwardingA 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.


Table 2 - Port Status


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 
ForwardingYesYesYesNoNo
LearningYesYesYesNoNo
Discarding YesYesYesYesYes

Table 3 - Relationships between port roles and port status.


i_f42.gifYes: The port supports this status. No: The port does not support this status.


--- End




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simchamnan
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hemin88
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andersoncf1
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Vlada85
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