1. A virtual link is established between area 1 of ar7 and ar2 to allow communication between area 3 and other areas.
2. Note: the virtual link must be the ospf rout-id of the opposite end, which is the ospf rout-id of ar7 and ar2 , not the IP of area 1 of the peer interface of ar7 and ar2.
3. After setting up the virtual link, you cannot set up a stub area in area 1 between ar7 and ar2
4. After area2 is set to nssa no-summary, ar6 cannot learn the route of area 0 1 3, and needs to enter a default route manually.