it is the expected behavior. if your adapters are on active/standby mode. cisco switches will only detect MAC of active adapters.

Hi

Is MAC flapping on Cisco switches considered expected behavior?
How can it be avoided?
Should it be modified to active/active with ip hash  or active/standby with port id ?

depends on what you want basically and what type of load balancing you are trying to design. if you want to use both nic then set to active/active. then cisco switches will discover both MAC.

@olaq Are you using Etherchannel on Cisco Switches ? IP Hash load balancing policy should be used only when Physical switches are configured in Etherchannel. 

Deafult load balancing policy is Route based on Originating Virtual Port ID.

I'm really puzzled because out of three ESXi hosts with the same configuration, one host doesn't experience MAC flapping regardless of being set to active/active or active/standby

Hi,

@DeviVmwarehe's right, without "some form" of LAG configured on the physical switches using "IP hash" as a load balancing method, in practical terms, is of no use, certainly not with an active/standby policy. and @Bivas he's right as well given the case as presented.

Regards,
Ferdinando

@olaq i would recommend you to modify the load balancing policy. also, check your switch security policies. 

Hi @DeviVmware 

I will follow your recommend
Thank you