Manual

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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