Troubleshooting Stp Data Loops - Cisco Nexus 9000 Series Troubleshooting Manual

Troubleshooting STP Data Loops

Checklist
Use the show spanning-tree summary totals command to verify that the total number
of logical interfaces in the Active state are less than the maximum allowed. For information
on these limits, see the Cisco Nexus 9000 Series NX-OS Layer 2 Switching Configuration
Guide.
Verify the primary and secondary root bridge and any configured Cisco extensions.
Use the following commands to view STP configuration and operational details:
• show running-config spanning-tree
• show spanning-tree summary
• show spanning-tree detail
• show spanning-tree bridge
• show spanning-tree mst
• show spanning-tree mst configuration
• show spanning-tree interface interface-type slot/port [detail]
• show tech-support stp
• show spanning-tree vlan
Use the show spanning-tree blockedports command to display the ports that are blocked by STP.
Use the show mac address-table dynamic vlan command to determine if learning or aging occurs at each
node.

Troubleshooting STP Data Loops

Data loops are a common problem in STP networks. Some of the symptoms of a data loop are as follows:
• High link utilization, up to 100 percent
• High CPU and backplane traffic utilization
• Constant MAC address relearning and flapping
• Excessive output drops on an interface
When the l2fm logging level is greater than or equal to 4, the switch logs occurrences of host MAC address
flapping to help you locate STP data loops. If it detects a MAC address move within less than 1 second and
if 10 consecutive moves occur, the switch disables learning on the VLAN for one of the ports between which
the MAC address is moving. Learning is disabled for 120 seconds and reenabled automatically. Syslogs are
generated while learning is disabled and enabled. You can configure the logging level using the logging level
l2fm log-level command.
Cisco Nexus 9000 Series NX-OS Troubleshooting Guide, Release 7.x
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Troubleshooting STP
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