Router A is unable to reach Router B.Both routers are running ios version 12.0.After reviewing the command output and graphic,what is the most likely cause of the problem?（）

单项选择题Which command enables IPv6 forwarding on a cisco router?（）

A. ipv6 host
B. ipv6 unicast-routing
C. ipv6 local
D. ipv6 neighbor

单项选择题In the implementation of VLSM techniques on a network using a single Class C IP address,which subnet mask is the most efficient for point-to-point serial links?（）

A. 255.255.255.240
B. 255.255.255.254
C. 255.255.255.252
D. 255.255.255.0
E. 255.255.255.248

单项选择题Which IPv6 address is valid?（）

A. 2031:0:130F::9C0:876A:130B
B. 2001:0DB8:0000:130F:0000:0000:08GC:140B
C. 2001:0DB8:0:130H::87C:140B
D. 2031::130F::9C0:876A:130B

单项选择题Which of the following protocols uses both TCP and UDP ports?（）

A. SMTP
B. Telnet
C. FTP
D. DNS

单项选择题Which command enhances the 802.1D convergence time on ports that are connected to hosts?（）

A. spanning-tree backbonefast
B. spanning-tree uplinkfast
C. spanning-tree portfast
D. spanning-tree cost512