MAC addr takeover

Cary B. O'Brien cobrien at access.digex.net
Sat Oct 17 10:09:50 MDT 1998


> but on the other hand you have to update the MAC tables in your ethernet
> switch if you are using one. is this really easier?
> 
> David Lang
> 
> On Fri, 16 Oct 1998, Michael Rowan wrote:
> 
> > Date: Fri, 16 Oct 1998 16:33:57 -0400
> > From: Michael Rowan <mtr at cutaway.com>
> > To: Flavius Bindea <flav at club-internet.fr>
> > Cc: linux-ha at muc.de
> > Subject: Re: MAC addr takeover
> >
> > Flavius Bindea wrote:
> > >
> > > Hi,
> > >
> > > I'm new on this mail list and I don't have many knowledge about
> > > HA, but this is a very interesting feature.
> > >
> > > Why should MAC takeover be implemented. Is IP takeover not
> > > enough ?
> > >
> >
> > IP takeover suffers from ARP cache updates.  Client
> > machines, sometimes numbering in the hundreds or thousands,
> > all are communicating with the server.  This means they have
> > the MAC address for the desination IP address in their arp
> > cache.  When the IP takeover happens, all of these entries
> > must be purged, either by hand, through gratuitous arps, or
> > by timing out.  None of the easy solutions work for all
> > platforms, so its a pain in the ass.
> >
> > When you fail both the IP and MAC address over, its a win
> > since you don't care about the ARP cache entries.  They all
> > are still valid.
> >

(From a lurker)

Hold on folks.  I see some confusion here. The only hosts that use the
MAC address to get to the server are the ones on the same IP network.
I.E. things on the same physical lan (well, can broadcast to each other)
 that have IDENTICAL network numbers.

In most configurations, clients would go through at least one
router to get to the server.  I.E. they would be on different IP
networks.  Remember, the I in IP stands for InterNetwork.

It is unlikely that you would have thousands of devices using the 
same network.  You could, but it would probably be too busy and
get all clogged up.

-- cary



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