We are making some changes to our network and after changing which router we use as our core router, there is a strange problem.
The problem is that there are two subnets (out of a total of more than 10) where hosts on those subnets can't be reached by hosts on other subnets, but as soon as I log on to the core switch/router (Catalyst 4507E+) and ping one of those hosts, then everyone can ping the host in question. Then about ten minutes after I've done the ping from the core router, the host that I enabled becomes unreachable from other subnets again.
The subnets are directly connected to the core.
So far my only guess is that there is some ARP problem. Pinging from the core helps these nodes successfully ARP and then the ARP cache entries time out and they are unreachable again, perhaps?
I've been unwittingly monitoring Windows hosts on the subnet with the main problem all along and they have never been unavailable, so it only affects Linux hosts on that subnet. That makes me think maybe this is more a question for ServerFault, since the problem is OS-specific.