Chapter 5 Port Loopback Detection Function Configuration; Introduction To Port Loopback Detection Function - Edge-Core ES4624-SFP Basic Management Manual

Chapter 5 Port Loopback Detection
Function Configuration

5.1 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.
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