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?（）

A. Authentication needs to be changed to PAP for both routers.
B. Serial IP addresses of routers are not on the same subnet.
C. Username/password is incorrectly configured.
D. Router names are incorrectly configured.

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

A. The denied MAC addresses have already been learned or confgured on another secure interface in the same VLAN.
B. The denied MAC address are statically configured on the port.
C. The minimum MAC threshold has been reached.
D. The absolute aging times for the denied MAC addresses have expired.

单项选择题What is the purpose using the traceroute command?（）

A. to map all the devices on a network.
B. to display the current TCP/IP configuration values.
C. to see how a device MAC address is mapped to its IP address.
D. to see the path a packet will take when traveling to a specified destination.
E. to display the MTU values for each router in a specified network path from source to a destination.

单项选择题Which type of attack is characterized by flood of packet that requesing a TCP connection to a server?（）

A. denial of service
B. brute force
C. reconnaissance
D. Trojan horse

单项选择题What is the default routing update period for RIPv2?（）

A. 15 seconds
B. 30 seconds
C. 180 seconds
D. 240 seconds