单项选择题 Acknowldgement, Sequencing, and Flow control are characteristics of which OSI layer?（）

单项选择题 The administrator is unable to establish connectivity between two Cisco routers. Upon reviewing the command output of both routers, what is the most likely caust of the problem?（）

单项选择题 What is valid reason for a switch to deny port access to new devices when port security is enabled?（）