单项选择题
A. for multiple ISDN connections
B. for modem concentrated dial-up connections
C. for a group of users in the same vicinity sharing a connection
D. for use by mobile users
多项选择题 Which two design concerns must be addressed when designing a multicast implementation?（）
单项选择题 Which routing protocol supports a flexible area structure using routing levels one and two?（）
单项选择题 Which site-to-site VPN solution allows Cisco routers, PIX Firewalls, and Cisco hardware clients to act as remote VPN clients in order to receive predefined security policies and configuration parameters from the VPN headend at the central site?（）