ARP is a standard part of the IP protocol suite. The behaviour of the router will not be that much different for whatever IOS release.
I have two remarks regarding your observations:
- When the router sees a packet from the PC (or other node) it examines the source-adress and updates the cache with it. The ARP cache is updated when a PC sends an IP packet to a destination beyond the routers LAN interface.
This does not need to be an ARP request. It can be any IP packet. In practice the first packet mostly is an ARP-request because the ARP max-age on a PC is much shorter than on the router.
-Two PC's communicating on the same subnet does not update the routers ARPcache.
I disagree. I was under the impression that when a device receives an ARP request for an IP address that is already in it's ARP cache, the device will update it's ARP cache with the sender's MAC address. This is known as a 'gratuitous ARP' and happens on routers. If you change the MAC address on a Cisco router, the router sends a gratuitous ARP so that the end devices update their ARP cahce with the new MAC address.
This must therefore mean a device receiving an ARP request, would update its cache with the senders IP and MAC??
I believe Darren's post reflects the standard behavior for ARP implementations these days. I've run into situations where a device didn't seem to be behaving properly in this regard (e.g., ignoring gratuitous ARPs), but for the most part I believe you'll find that devices update their ARP caches as per above.
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