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ME60 V800R010C10SPC500 Configuration Guide - LAN Access and MAN Access 01

This is ME60 V800R010C10SPC500 Configuration Guide - LAN Access and MAN Access
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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).
Overview of STP/RSTP

Overview of STP/RSTP

STP/Rapid Spanning Tree Protocol (RSTP) is used to block redundant links on the Layer 2 network and trim a network into a loop-free tree topology.

Introduction

On a complex network, loops are inevitable. With the requirement for network redundancy backup, network designers tend to deploy multiple physical links between two devices, one of which is the master and the others are the backup. Loops are likely or bound to occur in such a situation.

Loops will cause broadcast storms, thereby exhausting network resources and paralyzing the network. Loops also cause flapping of MAC address tables and thus damages MAC address entries.

The devices running STP discover loops on the network by exchanging information with each other and trim the ring topology into a loop-free tree topology by blocking a certain interface. In this manner, replication and circular propagation of packets are prevented on the network. In addition, it is prevented that the processing performance of devices is degraded when continuously processing repeated packets.

STP, however, converges the network topology slowly. In 2001, the IEEE published document 802.1w to introduce an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP). RSTP is developed based on STP but outperforms STP.

One Root Bridge

A tree topology must have a root. Therefore, the root bridge is introduced by STP.

There is only one root bridge on the entire STP-enabled network. The root bridge is the logical center of but is not necessarily at the physical center of the entire network. The root bridge changes dynamically with the network topology.

After the network converges, the root bridge generates and sends out configuration BPDUs at specific intervals. Other routers forward the BPDUs, ensuring that the network topology is stable.

Two Types of Measurements

The spanning tree is calculated based on two types of measurements: ID and path cost.

  • ID

    Two types of IDs are available: Bridge IDs (BIDs) and Port IDs (PIDs).

    • BID

      IEEE 802.1D defines that a BID is composed of a 16-bit bridge priority and a bridge MAC address. The bridge priority occupies the left most 16 bits and the MAC address occupies the rightmost 48 bits.

      On an STP-enabled network, the router with the smallest BID is selected to be the root bridge.

    • PID

      The PID is composed of a 4-bit port priority and a 12-bit port number. The port priority occupies the left most 4 bits and the port number occupies remaining bits on the right.

      The PID is used to select the designated port.

      NOTE:

      The port priority affects the role of a port in a specified spanning tree instance. For details, see STP Topology Calculation.

  • Path cost

    The path cost is a port variable and is used to select a link. STP calculates the path cost to select a robust link and blocks redundant links to trim the network into a loop-free tree topology.

    On an STP-enabled network, the accumulative cost of the path from a certain port to the root bridge is the sum of the costs of all the segment paths into which the path is separated by the ports on the transit bridges.

    Table 13-1 shows the path costs defined in IEEE 802.1t. Different router manufacturers use different path cost standards.

    Table 13-1 List of path costs

    Port Speed

    Port Mode

    STP Path Cost (Recommended Value)

    802.1D-1998

    802.1T

    legacy

    0

    -

    65535

    200000000

    200,000

    10 Mbps

    Half-Duplex

    100

    2000000

    2,000

    Full-Duplex

    99

    1999999

    1,999

    Aggregated Link 2 Ports

    95

    1000000

    1800

    Aggregated Link 3 Ports

    95

    666666

    1600

    Aggregated Link 4 Ports

    95

    500000

    1400

    100 Mbps

    Half-Duplex

    19

    200000

    200

    Full-Duplex

    18

    199999

    199

    Aggregated Link 2 Ports

    15

    100000

    180

    Aggregated Link 3 Ports

    15

    66666

    160

    Aggregated Link 4 Ports

    15

    50000

    140

    1000 Mbps

    Full-Duplex

    4

    20000

    20

    Aggregated Link 2 Ports

    3

    10000

    18

    Aggregated Link 3 Ports

    3

    6666

    16

    Aggregated Link 4 Ports

    3

    5000

    14

    10 Gbps

    Full-Duplex

    2

    2000

    2

    Aggregated Link 2 Ports

    1

    1000

    1

    Aggregated Link 3 Ports

    1

    666

    1

    Aggregated Link 4 Ports

    1

    500

    1

    NOTE:

    The rate of an aggregated link is the sum of the rates of all Up member links in the aggregated group.

Three Elements

There are generally three elements used when a ring topology is to be trimmed into a tree topology: root bridge, root port, and designated port. Figure 13-1 shows the three elements.

Figure 13-1 STP network architecture

  • Root bridge

    The root bridge is the bridge with the smallest BID. The smallest BID is determined by exchanging configuration BPDUs.

  • Root port

    The root port is a port that has the fewest path cost to the root bridge. To be specific, the root port is determined based on the path cost. Among all STP-enabled ports on a network bridge, the port with the smallest root path cost is the root port. There is only one root port on an STP-enabled router, but there is no root port on the root bridge.

  • Designated port

    For description of a designated bridge and designated port, see Table 13-2.

    Table 13-2 Description of the designated bridge and designated port

    Object

    Designated Bridge

    Designated Port

    Device

    Device that forwards configuration BPDUs to a directly connected router

    Designated bridge port that forwards configuration BPDUs to a router

    LAN

    Device that forwards configuration BPDUs to a network segment

    Designated bridge port that forwards configuration BPDUs to a network segment.

    As shown in Figure 13-2, AP1 and AP2 reside on Device A; BP1 and BP2 reside on Device B; CP1 and CP2 reside on Device C.

    • Device A sends configuration BPDUs to Device B through AP1. Device A is the designated bridge of Device B, and AP1 on Device A is the designated port.

    • Two routers, Device B and Device C, are connected to the LAN. If Device B is responsible for forwarding configuration BPDUs to the LAN, Device B is the designated bridge of the LAN and BP2 on Device B is the designated port.

    Figure 13-2 Networking diagram of the designated bridge and designated port

After the root bridge, root port, and designated port are selected successfully, the entire tree topology is set up. When the topology is stable, only the root port and the designated port forward traffic. All the other ports are in the Blocking state and receive only STP protocol packets instead of forwarding user traffic.

Four Comparison Principles

STP has four comparison principles that form a BPDU priority vector {root BID, total path costs, sender BID, port ID}.

Table 13-3 shows the information that is carried in the configuration BPDUs.

Table 13-3 Four important fields

Field

Brief Description

Root BID

Each STP-enabled network has only one root bridge.

Root path cost

Cost of the path from the port sending configuration BPDUs to the root bridge.

Sender BID

BID of the router sending configuration BPDUs.

Port ID

PID of the port sending configuration BPDUs.

After a router on the STP-enabled network receives configuration BPDUs, it compares the fields shown in Table 13-3 with that of the configuration BPDUs. The four comparison principles are as follows:

NOTE:

During the STP calculation, the smaller the value, the higher the priority.

  • Smallest BID: used to select the root bridge. Devices running STP select the smallest BID as the root BID shown in Table 13-3.
  • Smallest root path cost: used to select the root port on a non-root bridge. On the root bridge, the path cost of each port is 0.
  • Smallest sender BID: used to select the root port when a router running STP selects the root port between two ports that have the same path cost. The port with a smaller BID is selected as the root port in STP calculation. Assume that the BID of Device B is smaller than that of Device C in Figure 1. If the path costs in the BPDUs received by port A and port B on Device D are the same, port B becomes the root port.
  • Smallest PID: used to block the port with a greater PID but not the port with a smaller PID when the ports have the same path cost. The PIDs are compared in the scenario shown in Figure 13-3. The PID of port A on Device A is smaller than that of port B. In the BPDUs that are received on port A and port B, the path costs and BIDs of the sending routers are the same. Therefore, port B with a greater PID is blocked to cut off loops.
    Figure 13-3 Topology to which PID comparison is applied

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Updated: 2019-01-04

Document ID: EDOC1100059440

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