Until a proper example or guide is available for IPv6 deployments, the description of underlay_ipv6_numbered could be improved to specify requirements around node variables uplink_ipv6_pool (or downlink_pools), vtep_loopback_ipv6_pool and maybe mlag_peer_l3_ipv6_pool for MLAG.

Without these, a "standard" underlay with IPv6 would fail. The error messages around the missing variables are super clear though.

Also, the description says "Currently sets both underlay and overlay, including MLAG, to use IPv6 addresses." but for MLAG, the peer-link VLAN will use IPv4 addressing by default unless mlag_peer_address_family: ipv6 is set under the node config.