In short the BMC *might* DHCP successfully, or not, but that has no influence
on the system and ultimately bmcsetup will assign a static address to the BMC
and the DHCP transaction (if any) will ultimately have no result of consequence
(unless your dynamic ip range is so small it can't spare an extra address for a
brief period, which is hopefully unlikely).
If you have no dynamic range, then the DHCP of the BMC (if any) would just
fail, and it's the same.
Serving from both nics if they are on the same network segment or vlan is
generally just a bad idea, it can work, but it's going to act weird and
confusing.
Note that confluent can now do discovery without PXE booting a system for
certain equipment, but even then a candidate BMC can fail DHCP and it can still
work in most cases. It can also collect PXE mac addresses without a viable
DHCPOFFER, so it can do PXE discovery without any dynamic range.
________________________________
From: Kevin Keane [kke...@sandiego.edu]
Sent: Wednesday, May 30, 2018 7:39 PM
To: xCAT Users Mailing list
Subject: Re: [xcat-user] [External] How to run discovery when using shared
interface for IPMI and host network?
It knows by the MAC address, as well as by the timing. You need to understand
the DHCP protocol for that to work.
The BMC is the DHCP *client* here. DHCP involves (primarily) four messages:
Discover, Offer, Request, Acknowledge.
Initially, the DHCP client sends a DHCP Discover message, addressed to the
broadcast address 0.0.0.0, and the corresponding broadcast MAC address (I
believe ff:ff:ff:ff:ff:ff). It puts its own MAC address into the frame as well.
Any DHCP server on the network (as long as it has IP addresses available) will
respond with a DHCP offer message. In IPv4 terms, this is a fairly normal
packet, with the sender being the DHCP server. Crucially, the destination of
the layer 2 frame is the MAC address of whatever sent the DHCP Discover
message. Which is the BMC.
The BMC would then respond with a DHCP Request message (it requests to actually
lease the IP address the DHCP server offered), and eventually the DHCP server
will acknowledge the request.
BTW - this only happens once, when you first turn on the system. Once the DHCP
client knows which DHCP server is available, for renewals, it will skip the
discover and offer, and only send a request and wait for the acknowledge.
Later, when the host uses DHCP, the same process happens, but with the host's
MAC address. Remember I said that the BMC and the host shouldn't have the same
MAC address? That's one of the reasons why!
The BMC has a second way to sort out whether the DHCP offer is for itself or
the host: usually, by the time the host sends the DHCP Discover message, the
BMC isn't waiting for a DHCP Offer, so it could ignore it. But that wouldn't
really help if both share a MAC address, because the DHCP server would happily
offer the same IP address to the host that the BMC is already using.
_______________________________________________________________________
Kevin Keane | Systems Architect | University of San Diego ITS |
kke...@sandiego.edu<mailto:kke...@sandiego.edu>
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On Wed, May 30, 2018 at 3:46 PM, Daniel Hilst Selli
<dan...@versatushpc.com.br<mailto:dan...@versatushpc.com.br>> wrote:
On Tue, 2018-05-29 at 20:31 +0000, Jarrod Johnson wrote:
> Note that dhcpinterfaces is from the perspective of the server, not the
> client.
>
> I am curious if you would have different interfaces to talk to host
> versus bmc even in shared scenario.
>
> -----Original Message-----
> From: Daniel Hilst Selli
> <dan...@versatushpc.com.br<mailto:dan...@versatushpc.com.br>>
> Reply-To: xCAT Users Mailing list
> <xcat-user@lists.sourceforge.net<mailto:xcat-user@lists.sourceforge.net>>
> To: xcat-user@lists.sourceforge.net<mailto:xcat-user@lists.sourceforge.net>
> Subject: [External] [xcat-user] How to run discovery when using shared
> interface for IPMI and host network?
> Date: Tue, 29 May 2018 16:16:16 -0300
>
> Hi everybody.
>
> I was following the sequential discovery in the xCAT docs. At some
> point it
> says:
>
> Set the correct NIC from which DHCP server provide service:
>
> chdef -t site dhcpinterfaces=eth1,eth2
>
>
> My question is, if I'm using shared interface for IPMI and host
> network, how to
> make sequential discovery work, so that IPMI and host get in the right
> network?
>
> Regards,
>
> ---------------------------------------------------------------------
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Thanks for the answers guys!!! I talked to my team, they are not using discovery
in this cases.. so I'm okay by now,
I didn't understand how setting up DHCP on only one network may help since DHCP
Request are broadcasted, so how would BMC know if the request is for it or for
the host!?!?!
Regards,
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