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Cisco MDS9509 switch, no ISL's, no WAN extenstions, all local within the same datacenter ports.  The port utilization is low (for it and others within the same group).  I'm getting BB credit zero's counts within WLA on ports that are going to my DMX (1x gets a fairly constant 400-600 for hours, others are spikey with nothing but then get a 2000+ spike).  I've heard two differnt things on this, yes it is always an issue because you are holding back an I/O and no it only becomes a big issue on ISL and extended fibre ports.  I'm thinking that it most likely is an issue, but not sure why I'm getting them when I'm pushing less than a 100MB (total send + receive) on a 24 port blade with 4GB connections (so I'm not overloading asic group) and I'm not counting any errors on the port.
BB Credit Zero on local ports