[Poweredgec-tools] shared mode BMC on c6220

Adam Thompson athompso at athompso.net
Mon Nov 24 12:02:32 CST 2014

On 14-11-24 11:26 AM, Andrew Cooper wrote:
> Hello,
> We recently tested going from dedicated mode to shared mode for bmc nic_mode on our C6220 nodes.  We assumed that the shared mode meant an active-backup “bond” between the two embedded ethernet ports (gb1, gb2, 1GB on-motherboard).  We also set the bmc to use a vlan id.
> With both nics up, things function as expected, we can access the bmc web interface/console without issue.   When dropping the first nic, we are no longer able to access the web interface/console.   We are using dhcp for the bmc address and can see that it is trying to request an address, but based on switch mac tables and dhcp server logs, the request never makes it out of the second nic.  The operating system is configured with an active-backup bond and works as expected on the second nic.
> Is shared mode supposed to act like a bond, or is it only functional on the first embedded nic?  Does the inclusion of a vlan change this?
> Thanks,
> -Andrew

The BMC can only be wired to a single PHY at a time, and Dell always 
connects it to the first LOM port.
So, your initial assumption was incorrect.  The BMC is only connected to 
one of the two ports.

I don't know if Dell's documentation addresses this, but IIRC the 
"Overview" powerpoint presentation on poweredgec.com does.  So does most 
Intel and Broadcom documentation, as every BMC works the same way in 
this regard.

What most of that documentation says is, albeit using different words: 
"shared mode is incompatible with bonding".  I remember both Broadcom 
BASP and Intel NIC drivers under Windows pop up a warning when you 
enable an aggregated virtual interface; if you didn't see the warning, I 
assume you're not using Windows.

Note that if you had turned on LACP instead of active/backup, you would 
have seen a (statistically, given enough samples) 50% failure rate 
because of the way packets are hashed.

Setting a VLAN tag doesn't change any of this.

-Adam Thompson
  athompso at athompso.net
  Cell: +1 204 291-7950
  Fax: +1 204 489-6515

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