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Configuration Guide - Ethernet Switching

S7700 and S9700 V200R011C10

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). The switching 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 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 exhausted, and the network finally breaks down.

  • 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.

Updated: 2019-10-18

Document ID: EDOC1000178310

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