[Poweredgec-tools] shared mode BMC on c6220

Michael_Stumpf at Dell.com Michael_Stumpf at Dell.com
Mon Nov 24 13:53:50 CST 2014


Dell - Internal Use - Confidential
Hi,

It's already been correctly stated but I wanted to weigh in to confirm this:


-          Shared mode is intended to be pertain to the first 1000Mb LOM (LAN-On-Motherboarded; embedded ethernet PHY).  There was a behavior in some earlier firmwares where failover would occur to the second LOM, but this was corrected (removed).  It was indeed a bonded link, and that did show through the BMC web GUI.  However, it is not typically desirable, because a momentary link drop can cause the BMC to failover which may be an extremely unexpected and dangerous result, if say LOM0 were intranet and LOM1 were public-internet facing.


-          Dedicated mode switches the BMC to only use the 10/100 embedded dedicated BMC-only PHY that is also present.

The intended use cases are:

-           "dedicated" as an optimal air gapped out of band mgmt solution--a second way in to the servers

-          "shared" as a way to save cable & switch ports for uses where this level of security isn't necessary or useful

If you're looking to maximize connectivity, you could connect both and then programatically switch shared/dedicated mode on the server itself.


Regards,

Michael Stumpf
Storage, Embedded Management
Dell | PowerEdge Portfolio Sys Engr


-----Original Message-----
From: poweredgec-tools-bounces On Behalf Of Adam Thompson
Sent: Monday, November 24, 2014 12:03 PM
To: poweredgec-tools
Subject: Re: [Poweredgec-tools] shared mode BMC on c6220

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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