When the systems are idle or powered down we are seeing heavy and constant traffic on the network on all machines. We ran a network trace of the traffic and have discovered that the IPMI BMCB modules are constantly sending out ARP requests to the network. We have verified that the BMCB modules have been programmed with their correct IP and MAC addresses.

The odd thing is that each BMCB module is sending out an ARP request for its own address and replies with its own address. Is this a bug in your IPMI firmware?

According to the IPMI specification, the BMC needs to let other workstations know of its existence, even no OS is running. But, only IPMI packets will be forwarded to BMC; no ARP request will be sent to BMC. Therefore, when OS is not running, or BMC uses a diferent IP from OS, then remote console is unable to find BMC (no ARP request will be replied).

IPMI is using Gratuitous ARP. A Gratuitous ARP is an ARP Response where the responder sends out the internet-to-physical mapping of its own IP Address. Since the sender’s internet-to-physical address mapping is part the request, receivers use that information to update their own caches with the sender’s address mapping. Thus, remote consoles do not need to send an ARP when they connect to BMC at their first time. Gratuitous ARP is also a standard, not a bug.