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Prevent packet drops

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TitlePrevent packet drops
Symptoms
Customer sees dropped packets
Environment
EXOS switch
Cause
Congestion seen on ports on the path traffic is showing drops
show port congestion
Resolution
Congestion cannot be solved by just adding more buffer, the buffers are small and constant congestion cannot be solved with just adding buffers.
The folllowing actions can reduce port congestion;
  • Increase port speed or add more ports to a LAG (port sharing), this is the ultimate solution although not always possible.
  • remove unneeded QosProfiles (every created qosprofile takes buffer away from the shared buffer )
  • Configure the congested port to be able to use more shared-packet-buffer (in below example the port can use 100 % of the shared-packet-buffer instead of the default 20 %)
    • configure port <PORT> shared-packet-buffer 100
  • Reduce the amount of shared-packet-buffer the other ports can use  (decrease from the default 20 %)
  • If there are more ports that show congestion try to move ports to different asics, shared-packet-buffer is shared among ports on the same chip, if the slot or unit has more asics moving a congested port to another asic could make more buffer available to that port.
​Usefull commands to see buffers:
show port <PORT-LIST> buffer

Example output from an X460 with some QOS profiles configured:
X460.1 # show ports 1 buffer 
Packet Buffer Allocation for ports in range 1-24
 Total Packet Buffer Size: 3145728 bytes, Not Overcommitted
 Total Shared Buffer Size: 2828672
  Port 1  Max Shared Buffer Usage: 565632 bytes (20%)
   QP1: Reserved Buffer: 1536 bytes
   QP4: Reserved Buffer: 1536 bytes
   QP5: Reserved Buffer: 1536 bytes
   QP6: Reserved Buffer: 1536 bytes
   QP8: Reserved Buffer: 1536 bytes

In this command output you can see which other ports share the same buffer/asic in the first line.
If above command is not available (older EXOS versions) there is a debug command:
debug hal show platform packet-buffer ports <PORT-LIST>


Implementing QOS will not prevent packet drops, but will prevent higher priority traffic to be dropped because of low priority traffic.
Additional notes

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