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Fibre Channel (SAN)

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Regular Visitor
Posts: 1
Registered: ‎05-04-2018

Buffer Credit over allocation

Hi All,

 

I am trying to understand the consequence of allocating too many buffer credits to a E_PORT over a long distance link.

 

The long distance link in question is 4Gbps and roughly 100 Km (reported by the brocade switch via LS mode) distance is roughly 85km reported by the dark file supplier.

 

Calculating the number of buffer credits can be done in a number of ways.

 

Either using portbuffercalc <port> -distance 100 -speed 4 -framesize 2112

 

or manually via assuming a theoretical throughput of 1km link with full frame size would be 2 buffer credits for a 4Gbps port. 

 

frame size can be retrieved via the portbuffershow show command. Does anyone know the length of time the Avg Buffer Usage and FrameSize metric is calculated on?

 

These calculations come out to roughly 200 buffer credits required for the link. 

 

I understand that allocating to little buffer credits will cause the link to be under utilised or large gaps between frames traveling across the link.. What is the consequence of allocating too many buffer credits? say 440 to the port? Will it try to flood the link with to many frames causing frames to drop?

 

 

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