Port Loop Detection

Loop detection can be used in an MCT topology to detect Layer 2 loops that occur due to misconfigurations, for example, on the client side when MCT links are not configured as trunk links on the MCT-unaware client.

In MCT, ICL links must be up at all times to prevent the cluster from going down. These links must not be shut down when a loop is detected in a network. Instead, other available ports (CCEPs) must be shut down. If loop detection BDPUs are received on the ICL port, instead of shutting down the ICL links, all CCEPs are error-disabled, and the user is notified with the following log message.

Loop-detection: Packet received on ICL port <port_number> for vlan <vlan_id>. Errdisable CCEPs.

Strict mode loop detection can be enabled on ICL ports. 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. This process assists in detecting hardware faults on ICL ports.

Loop detection can be enabled on MCT and non-MCT VLANs simultaneously. There is no change in loop detection behavior when it is enabled on non-MCT VLANs.

The following example shows how to configure loop detection on MCT and non-MCT VLANS.

device(config)# vlan 1905 
device(config-vlan-1905)# loop-detection
device(config-vlan-1905)# end