Hello!
There are a few different ways to perform loopback testing, depending on what you're trying to do. Based upon your configuration, when you say "loopback testing", I'm assuming you mean a scenario where you are looping back cables between two different physical interfaces of the same switch (for example, Ethernet1/1 connects to Ethernet1/2). You are then assigning a unique VRF to each physical interface, then placing both physical interfaces in the same broadcast domain/subnet (for example, Ethernet1/1 in VRF Blue is assigned 192.0.2.1/24 and Ethernet1/2 in VRF Red is assigned 192.0.2.2/24). You are then pinging between the two interfaces (e.g. ping 192.0.2.2 vrf Blue and/or ping 192.0.2.1 vrf Red) to validate that the ports operate as expected.
If the above accurately describes the "loopback testing" you are performing, then on modern Nexus 9000 series switches, you will most likely need to configure a unique, non-default static MAC address on at least one of the two physical interfaces. On modern Nexus 9000 series switches, this is needed so that the switch does not send a packet with the same source and destination MAC address, both of which are assigned to the switch as BIA (Burned-In Addresses). To simplify a lot of complex technical details, the ASIC "doesn't like" receiving packets where the source and destination addresses are both BIA and tends to drop those packets. You can work around this by statically assigning a non-BIA MAC address (e.g. 0000.0000.aaaa) to one of the two physical interfaces, which should allow the ping to start working.
Just so that you're aware, if the intent of this test is to validate that all ports of a Nexus 9000 switch function as expected prior to deploying it into production, I've seen this type of testing called "snake testing", and there are other ways to perform it that may suit your needs. Check out this blog post, which does an excellent job of explaining how to perform this type of testing.
I hope this helps - thank you!
-Christopher
