Overview of MSTP

Generally, redundant links are used on an Ethernet switching network to provide link backup and enhance network reliability. The use of redundant links, however, may produce loops, causing broadcast storms and rendering the MAC address table unstable. As a result, the communication quality deteriorates, and the communication service may even be interrupted. The Spanning Tree Protocol (STP) is introduced to solve this problem.

STP refers to STP defined in IEEE 802.1D, the Rapid Spanning Tree Protocol (RSTP) defined in IEEE 802.1w, and the Multiple Spanning Tree Protocol (MSTP) defined in IEEE 802.1s.

MSTP is compatible with RSTP and STP, and RSTP is compatible with STP.

Purpose

After a spanning tree protocol is configured on an Ethernet switching network, it calculates the network topology and implements the following functions to remove network loops:

• Loop cut-off: The potential loops on the network are cut off by blocking redundant links.

• Link redundancy: When an active path becomes faulty, a redundant link can be activated to ensure network connectivity.

Basic Concepts of MSTP

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 regions 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 Figures 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 Figures 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 ports.

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

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.

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

For a vector, the smaller the priority value, the higher the priority. The vector comparison principle is as follows:

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 a 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 uses 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.

Introduction to RRPP

Definition

The Rapid Ring Protection Protocol (RRPP) is a link layer protocol used to prevent loops on an Ethernet ring network.

Once a network is established, RRPP-enabled devices discover and eliminate loops on the network by blocking certain interfaces. If a network fault occurs, RRPP-enabled devices unblock blocked interfaces and switch data services to a functioning link.

Purpose

The ring network topology is applied to metropolitan area networks (MANs) and enterprise intranets to improve network reliability. If a fault occurs on a node or on a link between nodes, data services are switched to the backup link to ensure service. However, broadcast storms may occur on ring networks.

Many protocols can prevent broadcast storms on ring networks. However, if a fault occurs on a ring network, most protocols are slow to switch data services to the backup link. The network convergence is slow, causing service interruptions.

To shorten the convergence time and eliminate the impact of network scale on convergence time, Huawei developed RRPP. Compared with other Ethernet ring protocols, RRPP has the following advantages:

• RRPP can be applied to large networks because the convergence time is not affected by the number of nodes on the ring network.

• RRPP prevents broadcast storms caused by data loops when an Ethernet ring is complete.

• If a fault occurs on an Ethernet ring network, the backup link rapidly restores the communication among the Ethernet ring network nodes.

Basic RRPP Concepts

After an RRPP domain and ring are created, RRPP specifies devices on the ring network as nodes in different roles. Nodes on the ring network detect the ring network status and transmit topology changes by sending, receiving, and processing RRPP packets through primary and secondary interfaces. Nodes on the ring network block or unblock the interfaces based on the ring network status. RRPP can prevent loops when the ring is complete, and rapidly switch service data to the backup link if a device or link fails, ensuring nonstop service transmission.

RRPP Entities

A group of interconnected switches configured with the same domain ID and control VLAN constitute an RRPP domain.

Figure 1 illustrates the entities in an RRPP domain.

Figure 1 RRPP network

RRPP Domain ID

An RRPP domain ID distinguishes an RRPP domain.

RRPP Ring

A physical RRPP ring uses an Ethernet ring topology. An RRPP domain comprises a single ring or multiple interconnected rings. When multiple interconnected rings exist, one ring is the major ring and the others are sub-rings.

An RRPP domain may have multiple sub-rings but only one major ring. The RRPP domain in Figure 1 consists of a major ring and a sub-ring.

RRPP is applied to the networking of a single ring, intersecting rings, and tangent rings. For details about different ring types, see Common RRPP Rings.

Control VLAN and Data VLAN

In an RRPP domain, a control VLAN is used to transmit only RRPP packets, while a data VLAN is used to transmit data packets. The control VLAN is relative to the data VLAN.

When an RRPP domain consists of a major ring and sub-rings, the RRPP domain is configured with two control VLANs: a major control VLAN and a sub-control VLAN. A major control VLAN belongs to the major ring, while a sub-control VLAN belongs to a sub-ring. You only need to specify the major control VLAN. The VLAN whose ID is one greater than the ID of the major control VLAN automatically becomes the sub-control VLAN.

Protocol packets on the major ring are transmitted in the major control VLAN, and RRPP packets on the sub-rings are transmitted in the sub-control VLAN. Protocol packets on the sub-rings are transmitted as data packets on the major ring. For example, in Figure 1, when the secondary interface of the master node on the major ring is blocked, both data packets and protocol packets on the sub-ring must be blocked. When the secondary interface is unblocked, both data packets and protocol packets on the sub-ring are forwarded. Protocol packets on the sub-ring are transmitted as data packets on the major ring, and protocol packets on the major ring are only transmitted on the major ring.

Node

Each device on an RRPP ring is a node. Nodes on the RRPP ring are classified into the following types:

• Master node

The master node determines how to handle topology changes. Each RRPP ring must have only one master node.

Any device on an Ethernet ring can serve as the master node.

The master node can be in either a Complete or Failed state. The master node status indicates the RRPP ring status.

• Transit node

On an RRPP ring, all nodes except the master node are transit nodes. A transit node monitors the status of its directly-connected links and notifies the master node of link changes.

A transit node can be in LinkUp, LinkDown, or Preforwarding state.

§  When the primary and secondary interfaces of a transit node are Up, the transit node is in LinkUp state. The transit node can receive and forward data packets and RRPP packets.

§  When the primary or secondary interface of a transit node is Down, the transit node is in LinkDown state.

§  When the primary or secondary interface of a transit node is Blocked, the transit node is in Preforwarding state and can receive and forward only RRPP packets.

• Edge node and assistant edge node

A switch functions as an edge node or an assistant edge node on a sub-ring, and functions as a transit node on the major ring.

On the link where the major ring and sub-ring overlap, if the switch on one intersection point is an edge node, the switch on the other intersection point is an assistant edge node.

A sub-ring has only one edge node and one assistant edge node.

Edge nodes and assistant edge nodes are special transit nodes. They support the same states as transit nodes but differ in the following situations:

§  If an edge interface is Up, the edge node or assistant edge node is in LinkUp state and can receive and forward data packets and RRPP packets.

§  If an edge interface is Down, the edge node or assistant edge node is in LinkDown state.

§  If an edge interface is blocked, the edge node or assistant edge node is in Preforwarding state and can receive and forward only RRPP packets.

If the changes of the link status on the interface of an edge node or assistant edge node causes the state transition, only the edge interface status changes.

Note

The status of the RRPP ring on a node is the status of the node.

Interfaces

Interfaces are classified into the following types:

• Primary interface and secondary interface

On both the master node and transit node, one of the two interfaces connected to an Ethernet ring is the primary interface, and the other is the secondary interface. The interface roles depend on the configuration.

The primary and secondary interfaces on the master node provide different functions:

§  The master node sends Hello packets from its primary interface and receives Hello packets on its secondary interface.

§  Based on the network status, the master node blocks the secondary interface to prevent loops or unblocks the secondary interface to ensure communication among all the nodes on the ring.

The primary and secondary interfaces on a transit node provide the same function.

• Common interface and edge interface

On an edge node or an assistant edge node, an interface shared by the major ring and a sub-ring is called the common interface. An interface used only by a sub-ring is called the edge interface.

The common interface is considered an interface on the major ring and belongs to both the major control VLAN and sub-control VLAN. The edge interface belongs only to the sub-control VLAN.

Common RRPP Rings

RRPP can be applied to networks consisting of a single ring, intersecting rings, or tangent rings. Different networks require different RRPP domain configurations:

• All the devices on a single ring must be configured in the same RRPP domain.

• All the devices on intersecting rings must be configured in the same RRPP domain.

• Devices on two tangent rings must be configured in different RRPP domains. The tangent rings are equal to two single rings and must be configured in two RRPP domains. Each RRPP domain has only one ring.

Single Ring

When only a single ring exists in the network topology, you can define one RRPP domain and one RRPP ring. This topology is applicable to simple ring networks and features a quick response to topology changes and short convergence time.

Figure 2 Single ring

Intersecting Rings

When two or more rings exist in the network topology, and multiple common nodes exist between two neighboring rings, they are considered intersecting rings and you need to define only one RRPP domain. Configure one ring as the major ring and the remaining rings as sub-rings. This topology is applicable when the master node on a sub-ring needs to be dual-homed to the major ring through the edge node and assistant edge node to provide uplink backup.

Figure 3 Intersecting rings

Tangent Rings

When two or more rings exist in the network topology and only one common node exists between two neighboring rings, they are considered to be tangent rings, and you need to configure the rings to belong to different RRPP domains. This topology is applicable to large-scale networks that require domain-based management.

Figure 4 Tangent rings

Introduction to SEP

Definition

The Smart Ethernet Protection (SEP) protocol is a ring network protocol specially used for the Ethernet link layer. A SEP segment consists of interconnected Layer 2 switching devices configured with the same SEP segment ID and control VLAN ID. A SEP segment is the basic unit of SEP.

Purpose

SEP blocks redundant links to prevent logical loops on a ring network. Redundant links are used on an Ethernet switching network to provide link backup and enhance network reliability. However, the use of redundant links may produce loops, causing broadcast storms and rendering the MAC address table unstable. As a result, communication quality deteriorates, and services may even be interrupted. Huawei switches support the following ring network protocols:

STP/RSTP/MSTP

STP, RSTP, and MSTP are standard protocols for breaking loops on Ethernet networks. Networks running these protocols converge slowly, failing to meet the transmission requirements of some real-time services. The convergence time is affected by the network topology. Huawei devices running STP, RSTP, or MSTP can communicate with non-Huawei devices.

RRPP

RRPP is a fast convergence Huawei proprietary protocol. RRPP requires a physical topology to be divided into logical topologies so that major rings and sub-rings can be differentiated. Therefore, RRPP does not apply to complex networks. A Huawei device running RRPP cannot communicate with any non-Huawei device.

Huawei developed SEP to overcome the disadvantages of the preceding ring network protocols. SEP has the following advantages:

• Works on diverse complex networks and supports all topologies and network topology queries. A network running SEP can connect to a network running STP, RSTP, MSTP, or RRPP.

Helps quickly locate blocked interfaces through network topology display. When a fault occurs, SEP can quickly locate the fault, improving network maintainability.

• Implements traffic load balancing by selectively blocking interfaces.

• Improves network stability by preventing traffic from switching back after link recovery.

Principles of SEP

SEP is a ring network protocol dedicated to the Ethernet link layer. Only two interfaces on a switching device can be added to the same SEP segment (a basic unit for SEP).

To prevent loops in a SEP segment, a ring protection mechanism selectively blocks interfaces to eliminate redundant Ethernet links. When a link on a ring network fails, the device running SEP immediately unblocks the interface and performs link switching to restore communication between nodes.

Figure 1 shows a typical SEP application. CE1 is connected to Network Provider Edges (NPEs) through a semi-ring formed by switches. A Virtual Router Redundancy Protocol (VRRP) group is deployed on the NPEs. NPE1 initially serves as the master and NPE2 as the backup. When the link between NPE1 and LSW5 or a node on the link becomes faulty, NPE1 becomes the backup. NPE2 then becomes the master. The following situations occur depending on whether SEP is deployed. The following assumes that the link between LSW1 and LSW5 becomes faulty.

• If SEP is not deployed on the semi-ring, CE1 traffic is still transmitted along the original path, but NPE1 does not forward traffic, causing traffic interruption.

• If SEP is deployed on the semi-ring, the blocked interface on LSW5 becomes unblocked, enters the Forwarding state, and sends link state advertisements (LSAs) to instruct other nodes on the SEP segment to update their LSA databases. CE1 traffic is transmitted along backup link LSW5 -> LSW2 -> LSW4 -> NPE2, ensuring uninterrupted traffic transmission.

Figure 1 SEP network

In typical SEP networking, a physical ring can be configured with only one SEP segment in which only one interface can be blocked. If an interface in a complete SEP segment is blocked, all service data is transmitted only along the path where the primary edge interface is located. The path where the secondary edge interface is located remains idle, wasting bandwidth.

SEP multi-instance is used to improve bandwidth efficiency and implement traffic load balancing. SEP multi-instance allows two SEP segments to be configured on a physical ring. Each SEP segment independently detects the completeness of a physical ring and blocks or unblocks interfaces without affecting the other.