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Chapter 9 Port Loopback Detection Function
9.1 Introduction to Port Loopback Detection Function
With the development of Chassis Switches, more and more users begin to access the network through
Ethernet switches. In enterprise network, users access the network through layer-2 s witches, which means
urgent demands for bot h internet and the int ernal layer 2 Interworking. When layer 2 Int erworking is required,
the messages will be forwarded through MAC addressing the accuracy of which is the key to a correct
Interworking bet ween users. In layer 2 switching, the messages are forwarded through MAC addressing.
Layer 2 devices learn MAC addresses via learning source MA C 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 MA C is already learnt by the lay er 2 device, only with a different source port, the original
source port will be modified to the new one, whic h 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 net work 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
whet her it has gone, the devic es supporting port control (such as port isolation and port MAC address
learning control) can maintain t hat automatically, which will not only reduce the burden of net work managers
but also response time, minimizing the effect caused loopbacks to the network.
9.2 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 )
Configuration
9-3
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Troubleshooting
