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CCNP Enterprise Core (350-401) Cisco Datalink Layer

Network Loops and Spanning Tree Protocol

In a layer 2 topology, the network packets do not decrement their time-to-live counters as they would in a layer 3 network. To put it simply, packets on the data-link layer can forward continuously until the network is overwhelmed.

Symptoms of a forwarding loop can include high processor utilisation and memory usage, slow connectivity and timeouts. The switch struggles to keep up with the network changes as it needs to re-write to its CAM table that it the same MAC address on another port. The switch can pick this up if it happens continuously by displaying the following log message:

%SW_MATM-4-MACFLAP_NOTIF: Host 0000.0c63.f1a6 in vlan 1 is flapping between port Gi1/0/1 and port Gi1/0/2

A few examples of the cause of a network loop:

  • End users using a network switch or hub that does not support or understand spanning tree protocol.
  • A misconfigured network device that has bridged two ports together
  • Spanning tree protocol is disabled on the switch
  • A misconfigured load balance that transmits traffic out multiple ports with the same MAC address
  • A network switch plugged into itself

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