If it was caused by some traffic, then it would have been a broadcast/multicast packet, as under normal operation two of the IDSM do not pass any traffic (as they are in the chassis in which FWSM in standby/secondary). This happened once is more than two years I think, so the chances of it happening again would be quite less. All came up after reboot, but the real worry is WHAT caused it? :)
To answer your question, yes all IDSM(s) share the same VLANs. Two are present on one chassis bridging the VLANS on the switches with the FWSM SVIs (Primary FWSM). The other two are on the second Core switch with the Secondary/Standby FWSM. There is ECLB (load balancing) for both pairs.
I have had this happen once before, with a single IDSM in each of two 6513's. It was a redundant switch fabric, and to be honest I just rebooted the IDSMs and didn't investigate it further. It never happened again, and that was on 5.x about a year ago. So it sounds like it might be the same thing. But who knows. In my situation they were both monitoring the same VLANs so that's why I was thinking some sort of anomalous broadcast traffic.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...