The common border is the Tumen River. There is only a rail crossing.
Russia and China know that the Kim family is unstable and unlikely to continue in power.
They are not interested in a ground war with S Korea or the US. Both want to occupy as much of N Korea as possible after the Kim’s are deposed.
They want to direct the next NORK government.
China particularly is interested in occupation because they have never been happy about ancient treaties blocking their access to the Sea of Japan. Korea formerly paid tribute to China and is still considered a part of greater China just like Viet Nam.
North Korea restricts access to Tumen River at border
April 19, 2017 at 12:59 PM
"Russian border guards handing over a poor North Korean refugee back to their North Korean colleagues"
Many more pics, and info here: http://vienna-pyongyang.blogspot.com/2008_09_19_archive.html