本來是看一個題目覺得奇怪的,于是去查CISCO 文檔,結果發現那個題目就是CISCO 文檔上的東西.

只是這個文檔本來是發布IOS  bug的一個通告,下面是關鍵內容.

Loopback Test: Test for a spanning-tree BPDU error. A crossover cable should be connected between two unused aclearcase/" target="_blank" >ccess ports. In this case, FastEthernet0/1 and FastEthernet0/2 are configured as access ports and are connected by a crossover cable. See RJ-45 Cables for information on Ethernet RJ-45 crossover pinouts. The interface configuration is shown below:

Switch#show run interface FastEthernet0/1     

Building configuration...          

Current configuration : 72 bytes     

!     

interface FastEthernet0/1     

switchport mode access     

no ip address     

end          

Switch#show run interface FastEthernet0/2     

Building configuration...          

Current configuration : 72 bytes     

!     

interface FastEthernet0/2     

switchport mode access     

no ip address     

end

If, after 30 seconds, one of the port LEDs turns amber and the other stays green, the switch is working correctly, either because it does not have the problem or already has the Cisco IOS software upgrade that corrects the problem. If, after 30 seconds, both of the LEDs turn green and flash rapidly, the Cisco IOS software will need to be upgraded. This can also be verified by looking at the output from the following commands. If the switch is working correctly, the number of BPDUs received on the blocked port will be a non-zero number and will increase every two seconds, using the default timers

WORKING_Switch#show spanning-tree interface FastEthernet 0/1 detail     

Port 1 (FastEthernet0/1) of VLAN0001 is forwarding     

Port path cost 19, Port priority 128, Port Identifier 128.1.     

Designated root has priority 32769, address 000a.4107.7400     

Designated bridge has priority 32769, address 000a.4107.7400     

Designated port id is 128.1, designated path cost 0     

Timers: message age 0, forward delay 0, hold 0     

Number of transitions to forwarding state: 1     

BPDU: sent 237, received 1     

^^^^^^^^^^^^^^^^^^^^^^^^^^ Port is forwarding and sending BPDUs correctly. 

WORKING_Switch#show spanning-tree interface FastEthernet 0/2 detail