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S12700 V200R010C00 Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring link aggregation, VLANs, Voice VLAN, VLAN mapping, QinQ, GVRP, MAC table, STP/RSTP/MSTP, SEP, and so on.
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STP prevents loops on a local area network (LAN). Switching devices running STP exchange information with one another to discover loops on the network and then block certain ports to eliminate loops. As the scale of LANs continues to grow, STP has become an increasingly important protocol.

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. In Figure 14-1, STP is not enabled on the switching devices. 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 eventually exhausted, and the network breaks down.

  • MAC address flapping causes unstable MAC address entries.

    Even unicast packets can cause MAC address flapping on switching devices.

    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. When the unicast packet sent by HostA to HostB is received by port 1 on S1, no matching MAC address entry is found, so the unicast packet is forwarded to port 2.

    Port 2 on S2 receives the unicast packet from port 2 on S1 and sends it out through port 1. Port 1 on S2 also receives the unicast packet sent by HostA to HostB, and sends it out through port 2. These transmissions repeat and port 1 and port 2 on S1 and S2 continuously receive unicast packets from HostA. S1 and S2 modify their MAC address entries each time, causing the MAC address table to flap. As a result, MAC address entries are unstable.

Updated: 2019-08-21

Document ID: EDOC1000142081

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