STP手动计算生成树！

如图找出根桥,根端口,指定端口.阻断端口




所有交换机的优先级都是用默认的值:32768,MAC地址大小关系 A<E<D<C<B
一 根桥
优先级一样大,那么就会查找MAC地址小的那个交换机为根桥. 所以: A为根桥
二 根端口
B: F0/15为根端口(RP) 理由=>直接连到根桥链路所在的端口.
C:F0/15为RP 理由=>同上.
D:F0/15为RP 理由=>到根桥路径最短的端口.(A->B->D).
E:F0/13为RP 理由=>因有两条链路连接到根桥的路径相同且比另一条到根桥的路径短(A->B->E , A->C->E)，那么这时就使用桥ID小的(即是从C发来的BPDU的桥ID小)。
三 指定端口 (路径最短的端口为指定端口,每个网段只有一个指定端口).
A-B段: A的F0/15到根桥路径最短(0),那么A的F0/15为指定端口(DP).===>根桥的每个端口都是指定端口
A-C段:A的F0/15为DP.
B-C段:C的F0/15为DP,B的F0/14即不是根端口,也不是指定端口,所以为阻断端口 理由=>B的F0/14与C的F0/14中的BPDU路径一样,那么就要比较下一个条件,发送桥ID小的端口为指定端口, 从C发送到B,C的桥ID小,那么C的F0/15为DP.
B-D段:B的F0/12为DP
B-E段:B的F0/13为DP ,E的F0/14为阻断端口
C-E段:C的F0/13为DP
D-E段:E的F0/15为DP,D的F0/14为阻断端口 同上(BC段)
启用生成树之后如下图.




验证:
A:
A#show spanning-tree brief
VLAN1

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address cc00.0508.0000

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768

Address cc00.0508.0000

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300
Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet0/13 128.14 128 19 FWD 0 32768 cc00.0508.0000 128.14 //这里还接了一个设备.

FastEthernet0/14 128.15 128 19 FWD 0 32768 cc00.0508.0000 128.15

FastEthernet0/15 128.16 128 19 FWD 0 32768 cc00.0508.0000 128.16
A#show cdp nei

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID

B Fas 0/15 148 R S I 3640 Fas 0/15

C Fas 0/14 142 R S I 3640 Fas 0/15

A#

B:
B#show spanning-tree brief
VLAN1

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address cc00.0508.0000

Cost 19

Port 16 (FastEthernet0/15)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768

Address cc00.0dac.0000

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300
Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet0/12 128.13 128 19 FWD 19 32768 cc00.0dac.0000 128.13

FastEthernet0/13 128.14 128 19 FWD 19 32768 cc00.0dac.0000 128.14

FastEthernet0/14 128.15 128 19 BLK 19 32768 cc00.092c.0000 128.15 //这里为阻断

FastEthernet0/15 128.16 128 19 FWD 0 32768 cc00.0508.0000 128.16
B#show cdp nei

B#show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID

A Fas 0/15 151 R S I 3640 Fas 0/15

C Fas 0/14 135 R S I 3640 Fas 0/14

D Fas 0/12 131 R S I 3640 Fas 0/15

E Fas 0/13 157 R S I 3640 Fas 0/14

B#

c:
C#show spanning-tree brief
VLAN1

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address cc00.0508.0000

Cost 19

Port 16 (FastEthernet0/15)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768

Address cc00.092c.0000

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300
Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet0/13 128.14 128 19 FWD 19 32768 cc00.092c.0000 128.14

FastEthernet0/14 128.15 128 19 FWD 19 32768 cc00.092c.0000 128.15

FastEthernet0/15 128.16 128 19 FWD 0 32768 cc00.0508.0000 128.15
C#show cdp nei

C#show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID

A Fas 0/15 154 R S I 3640 Fas 0/14

B Fas 0/14 143 R S I 3640 Fas 0/14

E Fas 0/13 159 R S I 3640 Fas 0/13

C#

D:
D#show spanning-tree brief
VLAN1

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address cc00.0508.0000

Cost 38

Port 16 (FastEthernet0/15)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768

Address cc00.0870.0000

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300
Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet0/14 128.15 128 19 BLK 38 32768 cc00.0658.0000 128.16 //阻断

FastEthernet0/15 128.16 128 19 FWD 19 32768 cc00.0dac.0000 128.13
D#show cdp nei

D#show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID

B Fas 0/15 165 R S I 3640 Fas 0/12

E Fas 0/14 121 R S I 3640 Fas 0/15

D#

E:
E#show spanning-tree brief
VLAN1

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address cc00.0508.0000

Cost 38

Port 14 (FastEthernet0/13)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32768

Address cc00.0658.0000

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300
Interface Designated

Name Port ID Prio Cost Sts Cost Bridge ID Port ID

-------------------- ------- ---- ----- --- ----- -------------------- -------

FastEthernet0/13 128.14 128 19 FWD 19 32768 cc00.092c.0000 128.14

FastEthernet0/14 128.15 128 19 BLK 19 32768 cc00.0dac.0000 128.14 //阻断

FastEthernet0/15 128.16 128 19 FWD 38 32768 cc00.0658.0000 128.16
E#show cdp nei

E#show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID

B Fas 0/14 178 R S I 3640 Fas 0/13

C Fas 0/13 172 R S I 3640 Fas 0/13

D Fas 0/15 168 R S I 3640 Fas 0/14

E#
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