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How to troubleshoot CRC errors or general link issues on a fiber link

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TitleHow to troubleshoot CRC errors or general link issues on a fiber link
Objective
How to troubleshoot CRC errors or general link issues on a fiber link
Environment
  • S-Series
  • SecureStack
  • All firmware versions
Procedure
Link issues on a fiber link are typically associated with CRC errors.  You can check to see if CRC errors are incrementing by looking at a "show rmon stats [port-string]"
S4 Chassis(su)->show rmon stats ge.3.4

Port: ge.3.4
--------------------
Index          = 3004
Owner          = monitor
Data Source    = ifIndex.32004
Direction      = rx+tx

Drop Events    = 0              Packets            = 21990372
Collisions     = 0              Octets             = 2611662092
Jabbers        = 0              0    -   64 Octets = 1544283
Broadcast Pkts = 1187146        65   -  127 Octets = 14082607
Multicast Pkts = 2530684        128  -  255 Octets = 6314203
CRC Errors     = 4763           256  -  511 Octets = 44377
Undersize Pkts = 0              512  - 1023 Octets = 4741
Oversize Pkts  = 0              1024 - 1518 Octets = 161
Fragments      = 0
These errors can also be seen in a "show port counters [port-string]"
S4 Chassis(su)->show port counters ge.3.4

Port: ge.3.4   MIB2 Interface: 32004   Bridge Port: 644
No counter discontinuity time
--------------------------------------------------------------------

MIB2 Interface Counters
-----------------------
In Octets                     1415524687
In Unicast Pkts                  9714955
In Multicast Pkts                1441643
In Broadcast Pkts                1187106
In Discards                            0
In Errors                           4763
In Unknown Protocol                    0
Out Octets                    1196166449
Out Unicast Pkts                 8557733
Out Multicast Pkts               1089169
Out Broadcast Pkts                    81
Out Discards                           0
Out Errors                             0
Out Queue Length                       0

802.1Q Switch Counters
----------------------
Frames Received                 12343704
Frames Transmitted               9646983
Frames Filtered                   291051
You may also see these in the "Out Errors" row.

In either case, if these counters are actively climbing, this means we have a physical link issue.  One of the components of the fiber link is faulty and needs to be replaced.  There are generally 7 components to a fiber link:
  • Local SFP port
  • Local GBIC
  • Local patch cable
  • Fiber run
  • Remote patch cable
  • Remote GBIC
  • Remote SFP port
Each of these components should be swapped out individually until the CRC errors stop incrementing.  The identified faulty hardware should be replaced.  Typically the GBIC should be the first thing to try, as that has the most potential for failure.  Sometimes a fiber run cannot be changed, and in this case it is usually adequate to test the fiber; both pairs, if applicable.
Additional notes

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