Nope the underlay can be v4-v4, just need to be able to carry the v4+v6 overlay to allow for migration of addresses.

From: Tyler Conrad <Tyler@tgconrad.com>
Date: Friday, July 10, 2020 at 12:39 PM
To: David Hubbard <dhubbard@dino.hostasaurus.com>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Subject: Re: IPv6 over vxlan+evpn Arista?

Do you need to carry the v6 af in the underlay? I’ve used 6pe/6vPE to carry v6 over v4 next-hops in the overlay without issue, but can’t say I’ve tested a dual-stack vtep. 

On Fri, Jul 10, 2020 at 08:07 David Hubbard <dhubbard@dino.hostasaurus.com> wrote:

Hi all, was curious if anyone is doing dual stack v4/v6 over Arista’s implementation of vxlan / evpn (the inter-data center transport would be v4)?  They have plenty of references for v4 deployments but had to check on v6 support, which can make one nervous; they did confirm it’s supported.  Looking to leverage it as a way to migrate a dual-stack data center without re-addressing.

Thanks,

David