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.
_______________________________________________________________________
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kke...@sandiego.edu
Maher Hall, 192 |5998 Alcalá Park | San Diego, CA 92110-2492 | 619.260.6859
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On Wed, May 30, 2018 at 3:46 PM, Daniel Hilst Selli <
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>
> > Reply-To: xCAT Users Mailing list <xcat-user@lists.sourceforge.net>
> > To: 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,
> >
> > ---------------------------------------------------------------------
> > ---------
> > Check out the vibrant tech community on one of the world's most
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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,
>
>
> ------------------------------------------------------------
> ------------------
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
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>
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