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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
MSTP Principles

MSTP Principles

This section describes the principles of MSTP.

MSTP Background

RSTP, an enhancement to STP, implements fast convergence of the network topology. There is a defect for both RSTP and STP: 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 failure in forwarding certain VLAN packets.

Figure 7-42  STP/RSTP defect

On the network shown in Figure 7-42, STP or RSTP is enabled. The broken line shows the spanning tree. S6 is the root switching device. The links between S1 and S4 and between S2 and S5 are blocked. VLAN packets are transmitted by using the corresponding links marked with "VLAN2" or "VLAN3."

Server A and Host 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 fix the defect of STP and RSTP, the IEEE released 802.1s in 2002, defining the Multiple Spanning Tree Protocol (MSTP). MSTP implements fast convergence and provides multiple paths to load balance VLAN traffic.

MSTP divides a switching network into multiple regions, each of which has multiple spanning trees that are independent of each other. Each spanning tree is called a Multiple Spanning Tree Instance (MSTI) and each region is call a Multiple Spanning Tree (MST) region.

NOTE:

An instance is a collection of VLANs. Binding multiple VLANs to an instance saves communication costs and reduces resource usage. The topology of each MSTI is calculated independent of one another, and traffic can be balanced among MSTIs. Multiple VLANs that have the same topology can be mapped to one instance. The forwarding status of the VLANs for a port is determined by the port status in the MSTI.

Figure 7-43  Multiple spanning trees in an MST region

As shown in Figure 7-43, 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.

Two spanning trees are calculated:
  • MSTI 1 uses S4 as the root switching device to forward packets of VLAN 2.
  • MSTI 2 uses S6 as the root switching device to forward packets of VLAN 3.

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

Basic MSTP Concepts

MSTP Network Hierarchy

As shown in Figure 7-44, 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 7-44   MSTP network hierarchy

MST Region
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 7-45, the MST region D0 contains the switching devices S1, S2, S3, and S4, and has three MSTIs.

Figure 7-45  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 7-45, 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 7-47, 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 7-46, each MSTI has its own regional root.

Figure 7-46   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 7-45.

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

CIST Root
Figure 7-47  MSTP network

On the network shown in Figure 7-47, 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 7-47, 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 7-47, 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 7-47, 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 7-47, 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 7-32 lists all port roles in MSTP.

NOTE:

Except edge ports, all ports participate in MSTP calculation.

A port can play different roles in different spanning tree instances.

Table 7-32  Port roles

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 7-48, 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 7-48, 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 7-48, 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 7-48, 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 7-49, 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 7-49, 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.

Figure 7-48  Root port, designated port, alternate port, and backup port

Figure 7-49  Master port and regional edge port

MSTP Port Status

Table 7-33 lists the MSTP port status, which is the same as the RSTP port status.

Table 7-33  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 7-34 lists the relationships between port roles and port status.

Table 7-34  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

NOTE:

Yes: The port supports this status.

No: The port does not support this status.

MST BPDUs

MSTP calculates spanning trees on the basis of Multiple Spanning Tree Bridge Protocol Data Units (MST BPDUs). By transmitting MST BPDUs, spanning tree topologies are computed, network topologies are maintained, and topology changes are conveyed.

Table 7-35 shows differences between TCN BPDUs, configuration BPDUs defined by STP, RST BPDUs defined by RSTP, and MST BPDUs defined by MSTP.

Table 7-35  Differences between BPDUs

Version

Type

Name

0

0x00

Configuration BPDU

0

0x80

TCN BPDU

2

0x02

RST BPDU

3

0x02

MST BPDU

MST BPDU Format

Figure 7-50 shows the MST BPDU format.

Figure 7-50  MST BPDU format

The first 36 bytes of an intra-region or inter-region MST BPDU are the same as those of an RST BPDU.

Fields from the 37th byte of an MST BPDU are MSTP-specific. The field MSTI Configuration Messages consists of configuration messages of multiple MSTIs.

Table 7-36 lists the major information carried in an MST BPDU.

Table 7-36  Major information carried in an MST BPDU

Field

Byte

Description

Protocol Identifier

2

Indicates the protocol identifier.

Protocol Version Identifier

1

Indicates the protocol version identifier. 0 indicates STP; 2 indicates RSTP; 3 indicates MSTP.

BPDU Type

1

Indicates the BPDU type:
  • 0x00: Configuration BPDU for STP

  • 0x80: TCN BPDU for STP

  • 0x02: RST BPDU or MST BPDU

CIST Flags

1

Indicates the CIST flags.

CIST Root Identifier

8

Indicates the CIST root switching device ID.

CIST External Path Cost

4

Indicates the total path costs from the MST region where the switching device resides to the MST region where the CIST root switching device resides. This value is calculated based on link bandwidth.

CIST Regional Root Identifier

8

Indicates the ID of the regional root switching device on the CIST, that is, the IST master ID. If the root is in this region, the CIST Regional Root Identifier is the same as the CIST Root Identifier.

CIST Port Identifier

2

Indicates the ID of the designated port in the IST.

Message Age

2

Indicates the lifecycle of the BPDU.

Max Age

2

Indicates the maximum lifecycle of the BPDU. If the Max Age timer expires, it is considered that the link to the root fails.

Hello Time

2

Indicates the Hello timer value. The default value is 2 seconds.

Forward Delay

2

Indicates the forwarding delay timer. The default value is 15 seconds.

Version 1 Length

1

Indicates the BPDUv1 length, which has a fixed value of 0.

Version 3 Length

2

Indicates the BPDUv3 length.

MST Configuration Identifier

51

Indicates the MST configuration identifier, which has four fields.

CIST Internal Root Path Cost

4

Indicates the total path costs from the local port to the IST master. This value is calculated based on link bandwidth.

CIST Bridge Identifier

8

Indicates the ID of the designated switching device on the CIST.

CIST Remaining Hops

1

Indicates the remaining hops of the BPDU in the CIST.

MSTI Configuration Messages(may be absent)

16

Indicates an MSTI configuration message. Each MSTI configuration message occupies 16 bytes. If there are n MSTIs, MSTI configuration messages are of nx16 bytes.

Configurable MST BPDU Format

Currently, there are two MST BPDU formats:

  • dot1s: BPDU format defined in IEEE 802.1s.

  • legacy: private BPDU format.

If a port transmits either dot1s or legacy BPDUs by default, the user needs to identify the format of BPDUs sent by the peer, and then runs a command to configure the port to support the peer BPDU format. Once the configuration is incorrect, a loop probably occurs due to incorrect MSTP calculation.

By using the stp compliance command, you can configure a port on a Huawei datacom device to automatically adjust the MST BPDU format. With this function, the port automatically adopts the peer BPDU format. The following MST BPDU formats are supported by Huawei datacom devices:

  • auto

  • dot1s

  • legacy

In addition to dot1s and legacy formats, the auto mode allows a port to automatically switch to the BPDU format used by the peer based on BPDUs received from the peer. In this manner, the two ports use the same BPDU format. In auto mode, a port uses the dot1s BPDU format by default, and keeps pace with the peer after receiving BPDUs from the peer.

Configurable Maximum Number of BPDUs Sent by a Port at a Hello Interval

BPDUs are sent at Hello intervals to maintain the spanning tree. If a switching device does not receive any BPDU during a certain period of time, the spanning tree will be re-calculated.

After a switching device becomes the root, it sends BPDUs at Hello intervals. Non-root switching devices adopt the Hello Time value set for the root.

Huawei datacom devices allow the maximum number of BPDUs sent by a port at a Hello interval to be configured as needed.

The greater the Hello Time value, the more BPDUs sent at a Hello interval. Setting the Hello Time to a proper value limits the number of BPDUs sent by a port at a Hello interval. This helps prevent network topology flapping and avoid excessive use of bandwidth resources by BPDUs.

MSTP Topology Calculation

MSTP Principle

In MSTP, the entire Layer 2 network is divided into multiple MST regions, which are interconnected by a single CST. In an MST region, multiple spanning trees are calculated, each of which is called an MSTI. Among these MSTIs, MSTI 0 is also known as the internal spanning tree (IST). Like STP, MSTP uses configuration messages to calculate spanning trees, but the configuration messages are MSTP-specific.

Vectors

Both MSTIs and the CIST are calculated based on vectors, which are carried in MST BPDUs. Therefore, switching devices exchange MST BPDUs to calculate MSTIs and the CIST.

  • Vectors are described as follows:

    • The following vectors participate in the CIST calculation:

      { root ID, external root path cost, region root ID, internal root path cost, designated switching device ID, designated port ID, receiving port ID }

    • The following vectors participate in the MSTI calculation:

      { regional root ID, internal root path cost, designated switching device ID, designated port ID, receiving port ID }

    The priorities of vectors in braces are in descending order from left to right.

    Table 7-37 describes the vectors.

    Table 7-37  Vector description

    Vector Name

    Description

    Root ID

    Identifies the root switching device for the CIST. The root identifier consists of the priority value (16 bits) and MAC address (48 bits).

    The priority value is the priority of MSTI 0.

    External root path cost (ERPC)

    Indicates the path cost from a CIST regional root to the root. ERPCs saved on all switching devices in an MST region are the same. If the CIST root is in an MST region, ERPCs saved on all switching devices in the MST region are 0s.

    Regional root ID

    Identifies the MSTI regional root. The regional root ID consists of the priority value (16 bits) and MAC address (48 bits).

    The priority value is the priority of MSTI 0.

    Internal root path cost (IRPC)

    Indicates the path cost from the local bridge to the regional root. The IRPC saved on a regional edge port is greater than the IRPC saved on a non-regional edge port.

    Designated switching device ID

    Identifies the nearest upstream bridge on the path from the local bridge to the regional root. If the local bridge is the root or the regional root, this ID is the local bridge ID.

    Designated port ID

    Identifies the port on the designated switching device connected to the root port on the local bridge. The port ID consists of the priority value (4 bits) and port number (12 bits). The priority value must be a multiple of 16.

    Receiving port ID

    Identifies the port receiving the BPDU. The port ID consists of the priority value (4 bits) and port number (12 bits). The priority value must be a multiple of 16.

  • The vector comparison principle is as follows:

    For a vector, the smaller the priority value, the higher the priority.

    Vectors are compared based on the following rules:

    1. Compare the IDs of the roots.

    2. If the IDs of the roots are the same, compare ERPCs.

    3. If ERPCs are the same, compare the IDs of regional roots.

    4. If the IDs of regional roots are the same, compare IRPCs.

    5. If IRPCs are the same, compare the IDs of designated switching devices.

    6. If the IDs of designated switching devices are the same, compare the IDs of designated ports.

    7. If the IDs of designated ports are the same, compare the IDs of receiving ports.

    If the priority of a vector carried in the configuration message of a BPDU received by a port is higher than the priority of the vector in the configuration message saved on the port, the port replaces the saved configuration message with the received one. In addition, the port updates the global configuration message saved on the device. If the priority of a vector carried in the configuration message of a BPDU received on a port is equal to or lower than the priority of the vector in the configuration message saved on the port, the port discards the BPDU.

CIST Calculation

After completing the configuration message comparison, the switching device with the highest priority on the entire network is selected as the CIST root. MSTP calculates an IST for each MST region, and computes a CST to interconnect MST regions. On the CST, each MST region is considered a switching device. The CST and ISTs constitute a CIST for the entire network.

MSTI Calculation

In an MST region, MSTP calculates an MSTI for each VLAN based on mappings between VLANs and MSTIs. Each MSTI is calculated independently. The calculation process is similar to the process for STP to calculate a spanning tree. For details, see STP Topology Calculation.

MSTIs have the following characteristics:
  • The spanning tree is calculated independently for each MSTI, and spanning trees of MSTIs are independent of each other.

  • MSTP calculates the spanning tree for an MSTI in the manner similar to STP.

  • Spanning trees of MSTIs can have different roots and topologies.

  • Each MSTI sends BPDUs in its spanning tree.

  • The topology of each MSTI is configured by using commands.

  • A port can be configured with different parameters for different MSTIs.

  • A port can play different roles or have different status in different MSTIs.

On an MSTP-aware network, a VLAN packet is forwarded along the following paths:
  • MSTI in an MST region
  • CST among MST regions
MSTP Responding to Topology Changes

MSTP topology changes are processed in the manner similar to that in RSTP. For details about how RSTP processes topology changes, see Details About RSTP.

MSTP Fast Convergence

MSTP supports both ordinary and enhanced Proposal/Agreement (P/A) mechanisms:
  • Ordinary P/A

    The ordinary P/A mechanism supported by MSTP is implemented in the same manner as that supported by RSTP. For details about the P/A mechanism supported by RSTP, see Details About RSTP.

  • Enhanced P/A

    Figure 7-51  Enhanced P/A mechanism

    As shown in Figure 7-51, in MSTP, the P/A mechanism works as follows:

    1. The upstream device sends a proposal to the downstream device, indicating that the port connecting to the downstream device wants to enter the Forwarding state as soon as possible. After receiving this BPDU, the downstream device sets its port connecting to the upstream device to the root port, and blocks all non-edge ports.

    2. The upstream device continues to send an agreement. After receiving this BPDU, the root port enters the Forwarding state.

    3. The downstream device replies with an agreement. After receiving this BPDU, the upstream device sets its port connecting to the downstream device to the designated port, and the port enters the Forwarding state.

By default, Huawei datacom devices use the fast transition mechanism in enhanced mode. To enable a Huawei datacom device to communicate with a third-party device that use the fast transition mechanism in common mode, configure the Proposal/Agreement mechanism on the Huawei datacom device so that the Huawei datacom device works in common mode.

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Updated: 2019-01-11

Document ID: EDOC1000176006

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