Spanning-tree Protocol

STP（spanning-tree protocol，生成树协议），STP的出现是为了解决因冗余链路而形成的环路问题，逻辑上阻塞某些端口的数据转发。

冗余链路可以解决单点链路故障问题，但冗余又会造成环路

环路带来的问题：
1.广播风暴
2.Mac地址表不稳定
3.重复帧的转发

STP的出现刚好能解决以上问题

---

二层的帧没有防环field；
三层IPv4 header TTL；
routing有防环机制；
so 二层通过生成树防环

在STP中的一种报文——BPDU，可以决定选举：

1. 皇帝（根交换机）
2. 钦差，传递御旨（指定端口，传递最优的BPDU，用于转发数据）
3. 太守，接收，执行皇帝命令（根端口，用于接收最优的BPDU，用于转发业务数据）
4. 庶民（非指定端口，在STP的逻辑里是不转发数据的，被阻塞）

STP收敛过程

STP收敛时间

生成树交换机上默认开启，根据它的选举原则，所选的根交换机不一定是我们所希望的那个，所以就需要修改它的优先值来选举根

SW2 # sh span vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778   ------优先级
             Address     aabb.cc00.0100    ------根桥的MAC地址
             This bridge is the root      ------这个是根桥
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     aabb.cc00.0100
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/1               Desg FWD 100       128.2    Shr 
Et0/2               Desg FWD 100       128.3    Shr 

SW1#sh span vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778    ------根桥优先级
             Address     aabb.cc00.0100 ------根桥MAC
             Cost        100   ------到达根桥Cost值
             Port        1 (Ethernet0/0)  ------与根桥相连的端口
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     aabb.cc00.0900
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Root FWD 100       128.1    Shr 
Et1/3               Desg FWD 100       128.8    Shr 

SW3#sh span vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778
             Address     aabb.cc00.0100
             Cost        100
             Port        4 (Ethernet0/3)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     aabb.cc00.0700
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/3               Root LIS 100       128.4    Shr 
Et1/0               Desg LIS 100       128.5    Shr 

SW4#sh span vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    32778
             Address     aabb.cc00.0100
             Cost        200
             Port        6 (Ethernet1/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32778  (priority 32768 sys-id-ext 10)
             Address     aabb.cc00.0a00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et1/1               Root FWD 100       128.6    Shr 
Et1/2               Altn BLK 100       128.7    Shr 

选根桥的时候，优先值默认为32768，再比较MAC地址，地址小的被选为根交换机，MAC越小，说明机器越老，性能越低，所以这时候我们需要人工修改优先级来选举根桥。
如我们想让SW1成为根桥
修改命令如下：

SW1(config)#spanning-tree vlan 10 priority 0 
SW1(config)#do sh span vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    10  ----这里的优先级是vlan-id+优先级
             Address     aabb.cc00.0900
             This bridge is the root  ----SW1已成为root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    10     (priority 0 sys-id-ext 10)
             Address     aabb.cc00.0900
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  15  sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0               Desg FWD 100       128.1    Shr 
Et1/3               Desg FWD 100       128.8    Shr