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Redundant HMCS, flat open network?

  • 1.  Redundant HMCS, flat open network?

    Posted Wed February 24, 2021 09:12 AM
    I'm well aware that the best practices for two redundant HMCs is to give each HMC a public/open network connection, and a private VLAN for FSP communication for each HMC. Then each POWER system attaches their HMC1 and HMC2 ports to the respective private VLAN. That's 3 networks total: one open, two private.

    I'm in a situation where I've been asked to connect both two HMCs and the HMC ports on the FSP to the same open network. The customer doesn't want any extra VLANs at all. No problem assigning static IPs on the FSP ports, and I know a single HMC can manage FSPs on an open network on a single port.

    The question is will the FSP allow both HMC ports to be on the same subnet for redundant HMCs?

    I can't find this documented anywhere, and in my old notes I found a reference to B1A38B24 where the ASMI may reject IPs on the same subnet?

    Does anyone else know if this is allowed?

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    Russell Adams
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  • 2.  RE: Redundant HMCS, flat open network?

    Posted Wed February 24, 2021 10:53 AM
    To update, I found the error code where it says that the two FSP ports cannot be on the same subnet. This is for POWER8 though.

    https://www.ibm.com/support/knowledgecenter/POWER8_REF/p8ecr/fspsp65.htm

    "A configuration error occurred in the static IP address assignment mode. If the private LAN is set up for static IPs, the system administrator might have selected IPs that are on the same subnet."

    Was this changed in POWER9?

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    Russell Adams
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  • 3.  RE: Redundant HMCS, flat open network?

    Posted Thu February 25, 2021 12:41 PM
    Hi

    No change for Power9. AFAIK FSP will be configured with a private TCP network you can chose as options in the HMC config panel. Not quite sure FSP can be in a open network but i remember FSP port config can be modified using ASMI.

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    Cesar Daniel Delgado Ponce
    Systems Specialist
    Mercantil Bank Venezuela
    Caracas
    +584166119255
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  • 4.  RE: Redundant HMCS, flat open network?

    IBM Champion
    Posted Thu February 25, 2021 03:11 AM
    Even if you network guys don't like the answer, but you may have multiple subnets on the same VLAN. You can configure all ports into the same VLAN and use 3 different IP subnets for open and 2 private networks with static IP addresses on FSPs.

    If you get both FSP ports into the same IP subnet, you may get into problems. FSP is nothing more than a "usual" computer in this case. Try to configure 2 different network ports on your AIX or Linux box into the same IP network and watch how IP packets go out of the box.

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    Andrey Klyachkin

    https://www.power-devops.com
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  • 5.  RE: Redundant HMCS, flat open network?

    Posted Thu February 25, 2021 04:26 AM
    Yes, I could do subnets on the VLAN. Wouldn't make me popular!

    I do understand the multiple IP issue, but this is a special case. The key is the redundant HMCs. Other applications could allow multiple managers to connect to the same IP address. The HMC wants a one to one relationship between HMC and FSP port. If I could have one IP on one FSP port and have two HMCs connect, this wouldn't be an issue.

    I found it odd that I can't find the same documentation for POWER9.

    Right now I intend to move the second HMC to another subnet and VLAN, and put the second HMC port on that. That will require no additional cabling.

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    Russell Adams
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  • 6.  RE: Redundant HMCS, flat open network?

    Posted Thu February 25, 2021 02:14 PM
    Andrey is right, two addresses in one subnet (especially on two different MACs/interfaces) in one system usually means you're looking for trouble.
    Static IP A1 from pvt subnet A on HMC1 pvt interface.
    Static IP B1 from pvt subnet B on HMC2 pvt interface.
    Static IP A2 from pvt subnet A on FSP port1.
    Static IP B2 from pvt subnet B on FSP port2.
    Choose subnets A and B wisely, keep them small (/30 perhaps) to minimize probability of breaking needed traffic.
    Traffic stays within broadcast domain, no routing/gateways needed, network guys may not even notice until they really dig into this...

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    Lech Szychowski
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  • 7.  RE: Redundant HMCS, flat open network?

    Posted Fri February 26, 2021 08:56 AM
    I'm familiar with the problem of having a multihomed host on the same subnet, I encounter this problem all the time in the PowerHA world.

    Because the HMC and FSP communication is a specific application, I'm asking in that context. Would it be possible for two HMCs to speak to the same FSP port on a single IP?

    Much of this feels like an artificial limitation left over from the conversion from serial connections. So the key issue isn't will I cause problems with two IPs on the same subnet, it's can I use two HMCs on one subnet to talk to the FSP.

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    Russell Adams
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  • 8.  RE: Redundant HMCS, flat open network?

    Posted Mon March 01, 2021 10:02 AM
    To answer my own question for future searches, IBM has said that two HMCs on the same subnet may not manage an FSP, even on a single IP/port. They stated the HMCs will compete to manage the server, knocking the other HMC offline and causing "No connection" errors. Thus it is not a supported configuration.

    This is an application topology layer issue with the HMC having a 1:1 mapping between HMCs and FSP ports extended to networking. That's not a problem, it just needs to be documented somewhere that while it may look like it's working that it is not supported.

    Redundant HMCs must always have two networks for each HMC/FSP pair, open or private.

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    Russell Adams
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  • 9.  RE: Redundant HMCS, flat open network?

    Posted Tue March 02, 2021 03:37 AM
    Two networks ie two different subnets? OK, seems fair enough because from the FSP network stack perspective having two different interfaces in the same subnet makes it difficult to tell which one should be used to contact each HMC. But is it required to have a separate VLAN for each of this subnet?

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    Lech Szychowski
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  • 10.  RE: Redundant HMCS, flat open network?

    Posted Tue March 02, 2021 04:59 AM
    On Tue, Mar 02, 2021 at 08:37:02AM +0000, Lech Szychowski via IBM Community wrote:
    > Two networks ie two different subnets?

    Two different subnets yes. The HMC needs a 1:1 relationship between
    the HMC and the FSP port (IP), and having two IPs on the same subnet
    is a common problem in networking.

    > But is it required to have a separate VLAN for each of this
    > subnet?

    No. VLANs aren't needed in this instance, just subnets. We are using
    the open network with static IPs, so that's ok. The static IPs could
    share a VLAN.

    VLANs are mandatory if you are using a private interface with DHCP
    enabled, because you are going to conflict with other DHCP services on
    that VLAN.

    ------------------------------------------------------------------
    Russell Adams Russell.Adams@AdamsSystems.nl
    Principal Consultant Adams Systems Consultancy
    http://adamssystems.nl/




  • 11.  RE: Redundant HMCS, flat open network?

    Posted Tue March 02, 2021 05:11 AM
    > VLANs are mandatory if you are using a private interface with DHCP enabled

    Yeah, separate broadcast domains required in this case.
    Good to know that solution I proposed in post #6 will actually work and be a supported one even if implemented within just one VLAN.


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    Lech Szychowski
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