Hi,
I guess you are referring to 55xx series switches. I got confused when I saw you mentioning 5900.
It's not supported on 55xx nexus switches (including 5596UP). It's supported only on 56xx based switches like 5672, 56128 etc.
It's not supported on Nexus 600x series as well. Thats why, you will always find VxLAN config guides for 56xx and not for other switches. This is due to h/w ASIC difference which enables this support.
I believe this is captured in some of the document but i'm unable to pull the same.
Regards,
Sunil.