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S12700 V200R012C00 Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, VLAN aggregation, MUX VLAN, VLAN termination, Voice VLAN, VLAN mapping, QinQ, GVRP, VCMP, STP/RSTP/MSTP, VBST, SEP, RRPP, ERPS, LBDT, and Layer 2 protocol transparent transmission.
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STP/RSTP Background

STP/RSTP Background

STP prevents loops on a local area network (LAN). Devices running STP exchange information with one another to discover loops on the network, and block certain ports to eliminate loops. With the growth in scale of LANs, STP has become an important protocol for a LAN.

Figure 14-1  Typical LAN networking

On the network shown in Figure 14-1, the following situations may occur:
  • Broadcast storms cause a breakdown of the network.

    If a loop exists on the network, broadcast storms may occur, leading to a breakdown of the network. In Figure 14-1, STP is not enabled on the switches. If Host A sends a broadcast request, both S1 and S2 receive the request on port 1 and forward the request through their port 2. Then, S1 and S2 receive the request forwarded by each other on port 2 and forward the request through port 1. As this process repeats, resources on the entire network are exhausted, and the network finally breaks down.

  • MAC address table flapping causes unstable MAC address entries.

    Assume that no broadcast storm has occurred on the network shown in Figure 14-1. HostA sends a unicast packet to HostB. If HostB is temporarily removed from the network at this time, the MAC address entry for HostB will be deleted on S1 and S2. The unicast packet sent by HostA to HostB is received by port 1 on S1. S1 has no matching MAC address entry, so the unicast packet is forwarded to port 2. Then port 2 on S2 receives the unicast packet from port 2 on S1 and sends it out through port 1. In addition, port 1 on S2 also receives the unicast packet sent by HostA to HostB, and sends it out through port 2. As such transmissions repeat, port 1 and port 2 on S1 and S2 continuously receive unicast packets from HostA. S1 and S2 modify the MAC address entries continuously, causing the MAC address table to flap. As a result, MAC address entries are damaged.

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Updated: 2019-01-18

Document ID: EDOC1100038103

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