如下图所示的网络,交换机配置信息如下,据此网络和配置,下列说法正确的是? (多选) Router B interface Serial1/0/0 link-protocol ppp ip address 10.0.21.1 255.255.255.0 # ospf 1 router-id 10.0.21.1 area 0.0.0.0 network 10.0.21.1 0.0.0.0 # Router A interface Serial1/0/0 link-protocol ppp ip address 10.0.12.1 255.255.255.0 # ospf 1 router-id 10.0.12.1 area 0.0.0.0 network 10.0.12.1 0.0.0.0 #