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Fabric Congestion on a stacked Switch

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TitleFabric Congestion on a stacked Switch
Symptoms
  • Running the command "debug hal show Congestion" shows Fabric Congestion.
  • Voice/IPTV or normal traffic issues
Environment
  • EXOS
  • Summit Stack
  • Black Diamond
Cause
Lets take a moment to learn what a fabric is so we understand where Fabric Congestion comes from.  A switch fabric is the internal or external links that connect network nodes so they can work together.  In EXOS this can be stacking links, and in a Black Diamond X8 or 8800 it links the cards together.  On some standalone switches there are two internal chips that have fabric links connecting them together.

Where there is a fabric link it's possible there can be fabric congestion.  Fabric congestion can be caused by the below reasons.
  • Over subscription of fabric links. -- Traffic is dropped on egress so the card showing fabric congestion is receiving too much traffic, and cant forward it all on the fabric links.
  • CRC errors on front facing ports can cause the Fabric congestion counter to increase.
  • Fabric link issues
  • On stacked switches excessive broadcast, multicast, or unknown unicast can lead to fabric congestion due to the traffic being unable to transit the blocked stack port.
Resolution
Over subscription of Fabric links:
  • You can move some traffic from one card to another to prevent oversubscription of fabric links.
  • With Stacking you can upgrade stacking technology if possible.
  • If you have one MSM on a Black Diamond 8800 adding another MSM adds more fabric bandwidth.
  • Adding another FM module to a Black Diamond X8 adds more fabric bandwidth.
CRC errors on front facing ports: Fabric link issues:
Note: Make sure to disable sys-health-check when done testing
Additional notes

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