User's Manual

Configuration Guide of XGS-5240-Series
105
Chapter 11 Port Loopback Detection
Function Configuration
11.1 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.