Cisco Router And Switch Troubleshooting:
Detecting And Correcting Port Speed Mismatches
By Chris Bryant, CCIE #12933
STP is one of the most stable technologies you'll ever work with, but certain conditions can cause it to become unstable or inoperable. STP can be debugged, but before doing so, consider the following three situations that can cause STP issues.
A duplex mismatch occurs when the endpoints of a switch-switch connection are using different duplex settings. The problem, of course, is that a port placed into half-duplex will not be able to transmit a BPDU if it happens to be receiving one at the same time - hello, switching loop!
This scenario must be guarded against, since bridging loops can occur as a result. Luckily, if you make this error, the switch is going to let you know really fast! In the following config, Sw1 and Sw2 are trunking on fast 0/11. I changed Sw2's fast0/11 port to half-duplex and Sw1's to full. Quickly, I find out I shouldn't have done that:
SW2(config)#int fast 0/11
SW2(config-if)#speed 100
SW2(config-if)#duplex half
SW1(config)#int fast 0/11
SW1(config-if)#speed 100
SW1(config-if)#duplex full
SW1#
00:14:47: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/11
(not half duplex), with SW2 FastEthernet0/11 (half duplex).
00:14:47: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/11
(not half duplex), with SW2 FastEthernet0/11 (half duplex).
00:14:47: %CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet0/11
(not half duplex), with SW2 FastEthernet0/11 (half duplex).
The message is so important that the router gave it to us three times! Placing Sw1's fast 0/11 port into half-duplex stops the messages, and the trunk is up and operational
SW1(config)#int fast 0/11
SW1(config-if)#duplex half
SW1#show interface trunk
Port Mode Encapsulation Status Native vlan
Fa0/11 desirable 802.1q trunking 1
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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