BAD : The country shares boundaries with Ruanda and Burundi.
GOOD : The country shares borders with Ruanda and Burundi.
Use boundary when you are talking about an area of land within a country: 'The Mississippi River forms a natural boundary between Tennessee and Arkansas.' 'Their farm is just inside the boundary of the National Park.'
The place where two countries meet is the border : 'We're about to cross the border between Austria and Switzerland.'