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52 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Configuring basic port parameters
2
Port loop detection
This feature allows the Dell PowerConnect device to disable a port that is on the receiving end of a
loop by sending test packets. You can configure the time period during which test packets are sent.
Strict mode and loose mode
There are two types of loop detection; Strict Mode and Loose Mode. In Strict Mode, a port is
disabled only if a packet is looped back to that same port. Strict Mode overcomes specific
hardware issues where packets are echoed back to the input port. In Strict Mode, loop detection
must be configured on the physical port.
In Loose Mode, loop detection is configured on the VLAN of the receiving port. Loose Mode
disables the receiving port if packets originate from any port or VLAN on the same device. The VLAN
of the receiving port must be configured for loop detection in order to disable the port.
Recovering disabled ports
Once a loop is detected on a port, it is placed in Err-Disable state. The port will remain disabled
until one of the following occurs:
• You manually disable and enable the port at the Interface Level of the CLI.
• You enter the command clear loop-detection. This command clears loop detection statistics
and enables all Err-Disabled ports.
• The device automatically re-enables the port. To set your device to automatically re-enable
Err-Disabled ports, refer to “Configuring the device to automatically re-enable ports” on
page 53.
Configuration notes
• Loopback detection packets are sent and received on both tagged and untagged ports.
Therefore, this feature cannot be used to detect a loop across separate devices.
The following information applies to Loose Mode loop detection:
• With Loose Mode, two ports of a loop are disabled.
• Different VLANs may disable different ports. A disabled port affects every VLAN using it.
• Loose Mode floods test packets to the entire VLAN. This can impact system performance if too
many VLANs are configured for Loose Mode loop detection.
NOTE
Dell recommends that you limit the use of Loose Mode. If you have a large number of VLANS,
configuring loop detection on all of them can significantly affect system performance because of the
flooding of test packets to all configured VLANs. An alternative to configuring loop detection in a
VLAN-group of many VLANs is to configure a separate VLAN with the same tagged port and
configuration, and enable loop detection on this VLAN only.
NOTE
When loop detection is used with L2 loop prevention protocols, such as spanning tree (STP), the L2
protocol takes higher priority. Loop detection cannot send or receive probe packets if ports are
blocked by L2 protocols, so it does not detect L2 loops when STP is running because loops within a
VLAN have been prevented by STP. Loop detection running in Loose Mode can detect and break L3