单项选择题 An OSPF non-backbone area allows external routes to be flooded within the area. These external routes are propagated into the backbone. However, external routes from other areas are not allowed to enter this area. Which type of OSPF area does this describe?（）

单项选择题 What is the purpose of the OSPF hello packet?（）

多项选择题 What information do OSPF hello packets contain?（）