Path: csiph.com!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Lynn Wheeler <lynn@garlic.com>
Newsgroups: alt.folklore.computers
Subject: Re: What happens to old MAC assignments?
Date: Wed, 24 Jul 2024 14:07:17 -1000
Organization: Wheeler&Wheeler
Lines: 27
Message-ID: <87plr2z64q.fsf@localhost>
References: <v7bd85$14q$1@reader1.panix.com> <v7bsvo$2ighc$5@dont-email.me> <v7cm85$dcb$2@tncsrv09.home.tnetconsulting.net> <v7ds1n$30nsb$1@dont-email.me> <v7f0hv$3b4$2@tncsrv09.home.tnetconsulting.net> <v7o21r$15v3i$1@dont-email.me> <v7o6kr$h2ts$1@paganini.bofh.team> <v7phef$1dsq8$7@dont-email.me> <v7qhuj$rr17$1@paganini.bofh.team> <v7r6ia$1ptki$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain
Injection-Date: Thu, 25 Jul 2024 02:07:20 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="4c2eab2f936b07e77a3f117f9c0a9b5e"; logging-data="2062996"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18FTHu23scDYRdbqaFDliR7rEcD/LvE7t0="
User-Agent: Gnus/5.13 (Gnus v5.13)
Cancel-Lock: sha1:I0PzqXB9g6NMCVXTAxgy0ZqvheA= sha1:Hl6zXsfYXOMPEmpItd4DYeljBXM=
Xref: csiph.com alt.folklore.computers:225952

Lars Poulsen <lars@beagle-ears.com> writes:
> If the two machines have the same IP address but different MAC
> addresses, the new machine cannot take over so long as the ARP entry
> survives. Old implementations would keep the ARP table entry as long
> as it was used. Newer implementations re-arp after half the entry's
> lifetime has expired. Been there, done that as the implementor of an
> embedded IP stack.
>
> If the new machine has the same MAC address as the old one, old MAC
> level routes may survive in one or more switching hubs. Best cure for
> that is sending a broadcast from the new location. (Seen as an issue
> in a wireless network when a mobile end node moves between access
> points.)

circa 1990, when we were doing IBM's HA/CMP ... and doing IP-address
take-over (as part of failure handling) ... while (BSD) Reno/Tahoe 4.3
TCP/IP stack implementations had ARP-cache time-out, it had special code
that would save the previous MAC&IP address pair (to avoid calling
ARP-cache routine, that special save never timed-out) ... in various
client/server scenarios, some (Reno/Tahoe based TCP/IP) client use was
the same server address and therefor there was (almost) never a call to
the ARP-cache routine. We had to do special hack to do pings to clients
from different IP-address (to force a call to the ARP-cache routine)
... when take-over was involved.

-- 
virtualization experience starting Jan1968, online at home since Mar1970