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Fabric Congestion on a stacked Switch

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TitleFabric Congestion on a stacked Switch
  • Running the command "debug hal show Congestion" shows Fabric Congestion.
  • Voice/IPTV or normal traffic issues
  • EXOS
  • Summit Stack
  • Black Diamond
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.
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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