Subject: Re: Gratuitous ARP

From: "M. T. Hollinger" <[email-masked]>
Date: Tue, 2 Jun 1998 21:35:58 -0400

>Gratutious ARP is supported on Windows NT, not 9X.  I am not real sure
about
>which Unix versions support it.

Sorry, but I think you are mistaken.  It works fine on Windows 95, and all
the other legacy systems I have tried.

I just did a quick test, using a Windows 95 system and two OpenVMS
systems.  The OpenVMS systems send out unsolicited ARP broadcasts every
time you create a new interface (or add an IP address to an existing
interface).  I watched the output from "arp -a" in an MS-DOS window on the
Windows 95 system as I created interfaces on the two OpenVMS systems, in
turn, using the same IP address.  The ARP cache on the Windows 95 system
was immediately updated as I did this, repeated several times.

In fact, the particular software product I work on (UCX) relies on other
hosts updating their ARP caches upon receipt of such unsolicited
broadcasts in order to support our cluster alias failover feature.
Although our product has been on the market 10 years now, and has
developed a large installed base, I have heard no complaints about cluster
alias not working from any particular vendor's client hosts.

For this reason, I have repeatedly opposed the use of a shared MAC address
by VRRP.  It confuses bridges and hubs, complicates troubleshooting, and
makes VRRP harder to implement.  I'll say it again, since the topic has
arisen: there is no reason to require a shared MAC address.  At most, this
should be an optional provision in VRRP for those vendors who
ill-advisedly wish to implement it.

            - Mark