所有的交换机启用STP之后，都会认为自己是根桥，BID默认32768
修改SW1的优先级为0，使SW1 为根桥
[SW1]stp priority 0 //修改命令
[SW2-GigabitEthernet0/0/1]stp cost 15000 //接口下可以修改RPC
SW2 G1/SW3 G2根端口RP
SW4 G1为根端口RP //RPC15000比G2小
SW1 G1/SW1 G2为指定端口DP
在链路3上选举指定端口，链路上只要SW2 G2端口没有角色 ，SW2 G2为指定端口
在链路4上选举指定端口，SW3 G1优先级高于SW4 G2 SW3 G1为指定端口
剩下的SW4为阻塞端口AP
[SW1]dis stp brief
MSTID Port Role STP State Protection
0 GigabitEthernet0/0/1 DESI FORWARDING NONE
0 GigabitEthernet0/0/2 DESI FORWARDING NONE
[SW2]dis stp brief
MSTID Port Role STP State Protection
0 GigabitEthernet0/0/1 ROOT FORWARDING NONE
0 GigabitEthernet0/0/2 DESI FORWARDING NONE
[SW3]dis stp brief
MSTID Port Role STP State Protection
0 GigabitEthernet0/0/1 DESI FORWARDING NONE
0 GigabitEthernet0/0/2 ROOT FORWARDING NONE
[SW4]dis stp brief
MSTID Port Role STP State Protection
0 GigabitEthernet0/0/1 ROOT FORWARDING NONE
0 GigabitEthernet0/0/2 ALTE DISCARDING NONE