Re: Migrating from one server to another

• Previous message: Holger.VanKoll_at_SWISSCOM.COM: "Re: Migrating from one server to another"
• Maybe in reply to: Adam Hanel: "Migrating from one server to another"
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Date:         Mon, 28 Jun 2004 08:10:06 -0700
To: aix-l@Princeton.EDU

Oh heck no. M$ clients and HA boxen on the same subnet? I cringe to
even consider that...

No, this is for the clients on the other side of a router - I believe
that the router just passes the ARP request on, but the client rejects
it (or at least that's how it sounded from what the instructors were
saying). I don't pretend to actually understand it (until I have time
to research it better), but two different instructors have said the same
thing, so there must be a grain of truth in there somewhere ;)

--rm

-----Original Message-----
From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU]On Behalf Of
Holger.VanKoll@SWISSCOM.COM
Sent: Monday, June 28, 2004 8:03 AM
To: aix-l@Princeton.EDU
Subject: Re: Migrating from one server to another

so you have windows-clients in the same subnet as the servers running
hacmp?!?

if you do so (and cannot move them out), you could add them to the
pinglist

check with a simple ping before to see if they will refresh the
arp-cache

-----Original Message-----
From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU] On Behalf Of
Robert Miller
Sent: Monday, June 28, 2004 4:49 PM
To: aix-l@Princeton.EDU
Subject: Re: Migrating from one server to another

Fresh from a set of HACMP classes, I have a bit more to offer on this,
at least according to the instructors (I haven't dug into the networking
stuff to figure out where gratuitous ARP is yet...).

According to the instructors, the problem lies with the *windows*
clients (imagine that...), and that they are the ones that will not
accept the gratuitous ARP requests to refresh their cache. Not sure yet
where to look for this setting, perhaps the M$ support site will shed
some more light on the subject...

--rm

-----Original Message-----
From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU]On Behalf Of
Coonfield, Ben
Sent: Friday, June 18, 2004 12:39 PM
To: aix-l@Princeton.EDU
Subject: Re: Migrating from one server to another

I'm using the same function. The latest versions of HACMP not only
tolerate but use IP aliases, that is, HACMP automatically moves an IP
alias from one box to another automatically.

The switch issue is because using this function requires a function
called "Gratuitous ARP". Basically, this means that when you add an IP
alias, the target broadcasts an ARP reply packet to tell the routers
where the IP address is now located. It is exactly like a regular ARP
reply, it is called gratuitous because there was no arp request, just
the reply. I believe that most network hardware handles this with no
problem, but some switches might need to be configured to ensure that
they will permit this.

-----Original Message-----
From: IBM AIX Discussion List [mailto:aix-l@Princeton.EDU]On Behalf Of
Green, Simon
Sent: Friday, June 18, 2004 2:08 PM
To: aix-l@Princeton.EDU
Subject: Re: Migrating from one server to another

We had some problems at the start with the network switches, because
they weren't updating their arp cache when the alias got moved to a
different box: took thirty minutes to refresh, or taking the interface
down and up again. That was sorted out by one of my colleagues, but I
forget how. Since I'm going home in a minute, I can't be bothered to
look it up, either. :-)

We've also had a problem on some servers where there are lots of aliases
- >10? - where one of the aliases seems to become the "primary" address
- top of the list in netstat -i. When you unconfigure that alias to
move it somewhere, it takes the default gateway with it and also messes
up the routing table in some as-yet vague and undetermined way! (I
think that it retain an entry for that interface with the alias address,
instead of having the correct non-alias address.) I've only seen this
once for sure, but I think it's actually happened twice. We haven't
been able to reproduce the problem, though.

We had a problem once when we added an application alias that was in a
different subnet to the primary. Subsequently, connections to other
nodes in that second subnet failed, because IP thought they were local,
and didn't send them to the gateway. That's not really a problem with
the concept, though, just an administrative error.

There may be some issues with older versions of HACMP, but I think
current ones should cope with it.

rsh, ftp etc work fine to the aliases. We don't do NFS mounts using the
aliases, though. (I think that could cause some confusion with exports,
although I don't think it would be difficult to sort out. But we intend
the aliases to be used by *users*, not for actually configuring stuff on
the servers themselves.)

Aliases are added through a script we set up, with a centrally
maintained list of aliases. We do not have them in the ODM. That makes
it easier to move them around without having to remove the ODM
entry: just change the master config file and push it out with rdist.

We're very happy with the way it all works.

--
Simon Green
Altria ITSC Europe Ltd
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> -----Original Message-----
> From: Bill Verzal [mailto:BVerzal@KOMATSUNA.COM]
> Sent: 18 June 2004 19:44
> To: aix-l@Princeton.EDU
> Subject: Re: Migrating from one server to another
>
>
> Nice concept.  I think I'll store that in the back of my head.  Have
> you come across any issues with that procedure ?
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• Previous message: Holger.VanKoll_at_SWISSCOM.COM: "Re: Migrating from one server to another"
• Maybe in reply to: Adam Hanel: "Migrating from one server to another"
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