IP NAT INSIDE SOURCE with HSRP Redundancy?

I found a previous thread on this here:
http://www.groupstudy.com/archives/ccielab/200711/msg00420.html
The question never seems to get answered and I am stuck trying to explain it.
What does the REDUNDANCY <hsrpgroup> switch do on the IP NAT INSIDE SOURCE command?
The only explanations I can find are that it prevents unpredictable NATting by allowing only the active HSRP router to perform the NAT (by answering ARP with it's BIA MAC). Surely any packet to be NATted will only ever arrive at the active router and will use the virtual MAC of the HSRP address?

Cisco site says "Enables the router to respond to ARP queries using BIA MAC, if HSRP is configured on the NAT inside interface."

What ARP queries are we talking about? I can't see how it's relevant for a static source translation. Arp for HSRP def gate give virtual mac, translated, arp for dest ip, then arp for HSRP def gate in reverse direction gives another virtual mac, then arp for original source.

Can anyone help me out here? What am I missing - apart from my entire weekend?
Thanks!

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Received on Sun Oct 25 2009 - 11:41:23 ART