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CLI-based Configuration Guide - Ethernet Switching

AR650, AR1600, and AR6100 V300R003

This document describes how to configure the components for LAN services, including link aggregation groups, VLANs, voice VLANs, MAC address tables, transparent bridging, as well as GVRP, STP/RSTP, and MSTP protocols.
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MAC Address Flapping

MAC Address Flapping

What Is MAC Address Flapping

MAC address flapping occurs when a MAC address is learned by two interfaces in the same VLAN and the MAC address entry learned later overrides the earlier one. Figure 1-4 shows how MAC address flapping occurs. In the MAC address entry with MAC address 0011-0022-0034 and VLAN 2, the outbound interface is changed from GE0/0/1 to GE0/0/2. MAC address flapping can cause an increase in the CPU usage on the device.

MAC address flapping does not occur frequently on a network unless a network loop occurs. If MAC address flapping frequently occurs on your network, you can quickly locate the fault and eliminate the loops according to alarms and MAC address flapping records.

Figure 1-4  MAC address flapping

MAC Address Flapping Detection

The device can detect MAC address flapping. When MAC address flapping occurs, the device can provide diagnosis information, including the flapping MAC address, interfaces between which the MAC address flaps, and VLAN that the interfaces belong to. A loop may exist on the interfaces between which the MAC address flaps. You will know how the loop is generated by checking interfaces where MAC addresses are flapping.

Figure 1-5  MAC address flapping detection

As shown in Figure 1-5, Switch B should not be connected to Switch C. When the two switches are connected, Router, Switch B, and Switch C form a loop. When Port1 of Router A receives a broadcast packet, Router A forwards the packet to Switch B. The packet is then sent to Port2 of Router A. Router A detects that the source MAC address of the packet flaps from Port1 to Port2. If the MAC address flaps between the two ports frequently, Router A considers that MAC address flapping occurs.

  • MAC address flapping detection allows a device to detect changes in traffic based on learned MAC addresses, but the device cannot obtain the entire network topology. It is recommended that this function be used on an interface when the interface connects to a user network where loops may occur.
Updated: 2019-04-12

Document ID: EDOC1100041791

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