PLANET WGSW-50040 - Manual (Page 71)

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Chapter 7 Port Loopback Detection Function
Configuration
7.1 Introduction to Port Loopback Detection Function
With the development of switches, more and more users begin to access the network through Ethernet
switches. In enterprise network, users access the network through layer-2 switches, which means urgent
demands for both internet and the internal layer 2 Interworking. When layer 2 Interworking is required, the
messages will be forwarded through MAC addressing the accuracy of which is the key to a correct
Interworking between users. In layer 2 switching, the messages are forwarded through MAC addressing.
Layer 2 devices learn MAC addresses via learning source MAC address, that is, when the port receives a
message from an unknown source MAC address, it will add this MAC to the receive port, so that the following
messages with a destination of this MAC can be forwarded directly, which also means learn the MAC address
once and for all to forward messages.
When a new source MAC is already learnt by the layer 2 device, only with a different source port, the original
source port will be modified to the new one, which means to correspond the original MAC address with the
new port. As a result, if there is any loopback existing in the link, all MAC addresses within the whole layer 2
network will be corresponded with the port where the loopback appears (usually the MAC address will be
frequently shifted from one port to another ), causing the layer 2 network collapsed. That is why it is a
necessity to check port loopbacks in the network. When a loopback is detected, the detecting device should
send alarms to the network management system, ensuring the network manager is able to discover, locate
and solve the problem in the network and protect users from a long-lasting disconnected network.
Since detecting loopbacks can make dynamic judgment of the existence of loopbacks in the link and tell
whether it has gone, the devices supporting port control (such as port isolation and port MAC address
learning control) can maintain that automatically, which will not only reduce the burden of network managers
but also response time, minimizing the effect caused loopbacks to the network.
7.2 Port Loopback Detection Function Configuration Task List
1. Configure the time interval of loopback detection
2. Enable the function of port loopback detection
3. Configure the control method of port loopback detection
4. Display and debug the relevant information of port loopback detection
5. Configure the loopback-detection control mode (automatic recovery enabled or not)
1． Configure the time interval of loopback detection
Command Explanation
Global Mode