Subject: RE: Virtual MAC Addresses in VRRP

From: Mike Shand <[email-masked]>
Date: Mon, 10 Nov 1997 10:04:50 -0000

On Monday, November 10, 1997 8:43 AM, Danny J. Mitzel [SMTP:[email-masked]] 
wrote:
>
> my personal take is that in the face of these incompatible options there
> is probably more interest/benefit in accommodating the broken end-host
> implementations than dealing with LANE limitations.

I would tend to agree. When we first started working on this stuff about 3 
years ago there were certainly a fair number of hosts that didn't accept the 
ARP, despite what the RFC says. Things may have changed in the intervening 
years and I would certainly hope that new versions of implementations 'do the 
right thing', but there will still be a fair number of the broken ones out 
there. In many ways this is the worst possible scenario, since unless we could 
characterize ALL versions of ALL implementations we would never be sure it was 
going to work in any given environment.

There is a second, perhaps less significant, issue with the ARP solution. 
Correct operation, even with correctly implemented hosts, relies on the 
successful reception of the ARP message. There are many reasons why a single 
ARP message may not be received by all hosts. Unlike (say) ICMP redirects which 
are merely optimizations (i.e. failure to receive them doesn't result in 
service failure), and which will auto-repeat (failure to receive will result in 
the next packet being sent to the original router and ANOTHER redirect will be 
generated), the ARP message is critical to the continued correct operation, and 
there is no mechanism to generate another one if the first one fails. Clearly 
an approximate solution can be devised where each ARP is sent (say) 3 times, in 
the hope that at least one will 'take', but there is always the possibility 
(for example a serious burst of overload related to the failure of the original 
router) that ALL the ARPs be lost. Continuously repeating the ARP (say) every 
10 seconds *might* be a solution, but it is rather expensive, and inelegant! 
There is no simple way to know when it is safe to stop (if we propose listening 
for packets sent to the 'wrong' MAC address, we might as well be doing the 
virtual MAC solution!).

	Mike