学员提问：
我觉得A和C应该都选啊，为什么答案不是呢?
这个题也不懂
捷盈讲师及学员解答：
第一题是多选 除了选RP DP 还有block呢
第二题可以参考下面的实验
Rack30Sw1(config)#spanning-tree vlan 1 root primary
Rack30Sw1(config)#exit
Rack30Sw1#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0012.daa2.7400
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address 0012.daa2.7400
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Desg FWD 19 128.19 P2p
Fa0/20 Desg FWD 19 128.20 P2p
Fa0/21 Desg FWD 19 128.21 P2p
Fa0/22 Desg FWD 19 128.22 P2p
Fa0/23 Desg FWD 19 128.23 P2p
Fa0/24 Desg FWD 19 128.24 P2p
Rack30Sw2(config)#spanning-tree vlan 1 root secondary
Rack30Sw2(config)#exit
Rack30Sw2#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0012.daa2.7400
Cost 19
Port 25 (FastEthernet0/23)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28673 (priority 28672 sys-id-ext 1)
Address 0019.aac9.1b00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Desg FWD 19 128.21 P2p
Fa0/20 Desg FWD 19 128.22 P2p
Fa0/21 Desg FWD 19 128.23 P2p
Fa0/22 Desg FWD 19 128.24 P2p
Fa0/23 Root FWD 19 128.25 P2p
Fa0/24 Altn BLK 19 128.26 P2p
Rack30Sw3#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0012.daa2.7400
Cost 19
Port 19 (FastEthernet0/19)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0012.dad7.1c00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Root FWD 19 128.19 P2p
Fa0/20 Altn BLK 19 128.20 P2p
Fa0/21 Altn BLK 19 128.21 P2p
Fa0/22 Altn BLK 19 128.22 P2p
Rack30Sw4#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 0012.daa2.7400
Cost 19
Port 23 (FastEthernet0/21)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 001c.f903.ec80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/19 Altn BLK 19 128.21 P2p
Fa0/20 Altn BLK 19 128.22 P2p
Fa0/21 Root FWD 19 128.23 P2p
Fa0/22 Altn BLK 19 128.24 P2p
根桥要保证连到到其他桥的线路畅通而没有环路
附图
