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From: Arthur Chance <freebsd@qeng-ho.org>
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Subject: igb interfaces and kern.ipc.nmbclusters
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man igb contains the section

kern.ipc.nmbclusters
         The maximum number of mbuf clusters allowed.  If the system has
         more than one igb card or jumbo frames are enabled, this value
         will need to be increased.

Given that I'm in the process of setting up a new server with two igb 
interfaces, is there a suggestion for what I should set it to?

My current value is

root@arthur:4# sysctl kern.ipc.nmbclusters
kern.ipc.nmbclusters: 2020126