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Cisco Bug: CSCus39958 - Link remains up/up when mis-wired after splicing the GBIC

Last Modified

Feb 15, 2018

Products (1)

  • Cisco Catalyst 6000 Series Switches

Known Affected Releases

15.1(1)SY4 15.1(2)SY

Description (partial)

As shown in the above diagram, if we were to splice up the GBIC/SFP into TX/RX component and mis-wire the links such that 

1) only RX from one switch (1) connects to TX of the other switch(2) and RX of this switch (2) connects to TX of a third switch, we will see link go up/up on switch 1. 
2) only TX from one switch(1) connects to RX of the other switch(2) and the TX of this switch(2) connects to RX of a third switch(3), we will see link go up/up on switch 3.

> Ideally under such circumstances, the link should go down/down on all three switches. 
> SInce the link comes up on one end, this leads to STP issues and blackhole of traffic
> This behavior has been tested on 6500 sup 2t and sup 720 and is not dependent on a particular hardware/sup/line card and IOS version
> The same behavior is seen if the set up is done using two 6500 and one 4500/3k platform

Splicing up the GBIC/SFP into TX/RX and mis-wiring the links among three switches as shown in the diagram below :

      Link Down                                                      Link Down
      Protocol Down                                                Protocol Down 
      ===============                                ===============
     |                                 | TX---------------RX  |                                |
     |    SUP2t                   |                               |   Sup2t                    |
     |                                 | RX              --TX     |                                |
      ===============                |               ================
                                                        |          Link UP, protocol Up  
                                                       -RX  =================
                                                            |                                        |                                    
                                                            |           sup720                 |
                                                        TX|                                        |
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