Logical port mirroring lets you replicate and redirect all of the traffic coming in or out of a logical switch port attached to a VM VIF port. The mirrored traffic is sent encapsulated within a Generic Routing Encapsulation (GRE) tunnel to a collector so that all of the original packet information is preserved while traversing the network to a remote destination.
Compared to the physical port mirroring, logical port mirroring ensures that all of the VM network traffic is captured. If you implement port mirroring only in the physical network, some of the VM network traffic fails to be mirrored. This happens because communication between VMs residing on the same host never enters the physical network and therefore does not get mirrored. With logical port mirroring you can continue to mirror VM traffic even when that VM is migrated to another host.
The port mirroring process is similar for both VM ports in the NSX-T Data Center domain and ports of physical applications. You can forward the traffic captured by a workload connected to a logical network and mirror that traffic to a collector. The IP address should be reachable from the guest IP address on which the VM is hosted. This process is also true for physical applications connected to gateway nodes.