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.