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Configuration Guide - Ethernet Switching

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, MUX VLAN, 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 Background

STP 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 9-1 Typical LAN networking

On the network shown in Figure 9-1, 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 9-1, STP is not enabled on the switches. If ServerA 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 9-1. ServerA sends a unicast packet to ServerB. If ServerB is temporarily removed from the network at this time, the MAC address entry for ServerB will be deleted on S1 and S2. The unicast packet sent by ServerA to ServerB 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 ServerA to ServerB, 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 ServerA. 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: 2019-05-08

Document ID: EDOC1100004351

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