Checking Whether a Network Loop Exists

Procedure

1. If a network loop occurs, the involved switch's MAC address entries frequently flap, and the accompanied broadcast storm causes many protocol packets to be flooded to the switch's CPU, raising its CPU usage.
   1. Problem Locating
      A network loop on a switch causes the following to occur:

      • The switch's CPU usage exceeds 80%.
      • Indicators of interfaces in the VLAN where a loop has occurred blink frequently.
      • The switch's MAC address entries frequently flap.
      • Management users cannot remotely log in to the switch, or after users log in to the switch using the console port, the switch does not respond to user operations.
      • A lot of ICMP packets are lost during ping tests.
      • The display interface command output shows that a large number of broadcast packets have been received on the switch's interface.
      • Loop alarms are generated after loop detection is enabled.
      • The PCs attached to the switch receive many broadcast packets or unknown unicast packets.
   2. Handling Suggestion
      1. Locate the interface where a broadcast storm has occurred based on indicator blinking and traffic volume.
      2. Locate the loop-generating switch hop by hop based on the link topology.
      3. Locate the interface where the loop has occurred and remove the loop.