Mac flapping

[simc92]

Hello,

Would you have an idea of what could cause MAC flapping in a loopless topology (the wiring has been checked).
A Cisco switch is connected to a router of another brand (our supplier's). There is a data vlan and a voice vlan, only the telephone exchange on the voice vlan causes MAC flapping (between the port connected to the router and the one on which the telephone exchange is connected).

We did not have MAC flapping with the old vendor's Cisco router. Could this be due to a misconfiguration on the new router?

Thank you.

[M02@rt37]

Hello @simc92,

MAC flapping typically occurs when there are multiple paths for traffic between network devices, resulting in confusion about the correct destination for a MAC address. However, in a loopless topology where the wiring has been verified, other factors can cause MAC flapping.

Spanning Tree Protocol (STP) Misconfiguration: STP is used to prevent loops in a network. If STP is misconfigured or disabled on any device in the network, it can result in MAC flapping. Ensure that STP is enabled and properly configured on all switches in the network, including the new route

Incompatible Spanning Tree Implementations: Different vendors may implement STP using different protocols, such as Cisco's Rapid Spanning Tree Protocol (RSTP) or IEEE's Spanning Tree Protocol (STP). If the new router is using a different STP implementation than the old Cisco router, it can lead to compatibility issues and MAC flapping. Check the STP settings on the new router and ensure they are compatible with the Cisco switch.

VLAN Configuration Mismatch: Verify that the VLAN configurations on both the Cisco switch and the new router are consistent. Ensure that the data VLAN and voice VLAN configurations match on both devices. Any inconsistencies in VLAN configuration can lead to MAC flapping.

To pinpoint the exact cause of MAC flapping in your scenario, it would be helpful to gather more information about the configuration of the new router, such as the STP settings, VLAN configurations, and MAC address learning mechanisms. Additionally, reviewing the logs and error messages on the affected devices may provide valuable insights into the root cause of the issue.

[simc92]

Thank you, we asked our operator for the configuration and the logs of the equipment.

[Richard Burts]

I would hope that the logs would have helpful information, especially the mac address and the interfaces between which it is flapping. Once we know what interfaces are involved it would be good if we could see the configuration of the affected devices.