Brocade FastIron Ethernet Switch Administration Guide - Manual (Page 81)

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 82.
Port loopback detection 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
Brocade 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 Layer 2 loop prevention protocols, such as spanning tree (STP), the
Layer 2 protocol takes higher priority. Loop detection cannot send or receive probe packets if ports are
blocked by Layer 2 protocols, so it does not detect Layer 2 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 Layer 3 loops because STP cannot prevent loops across different VLANs. In these instances, the
ports are not blocked and loop detection is able to send out probe packets in one VLAN and receive
packets in another VLAN. In this way, loop detection running in Loose Mode disables both ingress and
egress ports.
Enabling loop detection
Use the loop-detection command to enable loop detection on a physical port (Strict Mode) or a VLAN
(Loose Mode). Loop detection is disabled by default. The following example shows a Strict Mode
configuration.
device(config)# interface ethernet 1/1
device(config-if-e1000-1/1)# loop-detection
The following example shows a Loose Mode configuration.
device(config)# vlan20
device(config-vlan-20)# loop-detection
Recovering disabled ports
FastIron Ethernet Switch Administration Guide 81
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