A Windows server has 10 IP addresses.
Intermittently, all IP addresses except for the primary address become inaccessible.
This may well be a problem on the server itself, but in the course of looking everywhere, I noticed something on a core switch that I don't understand.
The switch is a 6509, sup720/msfc3, IOS Enterprise Services, v12.2(18)SXE5.
When all is working as it should, "sh arp" on the switch has entries for all 10 of the server's IP addresses.
However, as time passes, the Age in Minutes value for the server's primary IP address restarts, like this:
sh arp 1: Age = 5
sh arp 2: Age = 0
sh arp 3: Age = 3
sh arp 4: Age = 2
(Sh arp commands having been done at irregular intervals.)
For the other 9 IP addresses on the box, the Age value continues to increment until we have a crisis.
From this it would seem that the switch re-arps for the primary address, even though it has an entry for that IP in its table.
Why would it re-arp for an IP that it has in its table?
And, why re-arp for some addresses, but not for others?
(The age values for IP addresses on all our vlans are all over the map, anything from 0 up in to the 200s, so the phenomenon doesn't seem to be confined to this one device.)
Any thoughts/knowledge much appreciated...
That is an interesting behavior and there may be several things that could explain it. I think perhaps most likely is that the server periodically might be sending a gratuitous ARP (announcing itself). If the switch receives an ARP response from the switch (perhaps responding to some other device) it would refresh the entry in the ARP table.
I agree that the problem about the other addresses would seem to be a problem at the server. If you want to investigate either of these things further I would suggest that debug arp on the sup might be helpful.