Can't ping Linux hosts on a certain subnet unless core router pings them first

Question

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?

Edit:

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.

Answer 1

CAM and ARP table are populeted while you ping from 4500. Is just aging faster from CAM than from ARP. If you would wait 4 hours it would time out as well from ARP. Is those 4500 clustered? If yes I would say that VLAN is not getting to Active node for mac address population. All the request are coming to passive node and they are dropped. Maybe STP topology for particular VLAN is blocking state to Active node. I would compare STP topology of working vlan with not working one. Double check uplinks from access layer for this VLAN. Could be as well that VSL link between 4500 is not allowing particualr vlan. You need as well make sure that both 4500 have ports in this vlan. As well check floodset between 4500:

#show platform hardware floodset vlan [VLANID]

If both 4500's having floodset on required interfaces.

Temporary fix would be increase vlan aging time for exmaple:

#mac address-table aging-time 14400 vlan [VLANID]

Answer 2

Issue seems to be unable to ping from different subnets host machine . But able to ping from core switch where subnets directly connected networks .

Your are preciouly able to ping from core switch because it is directly connected network.we can see ARP table in core switch maping IP address to directy connected subnets host IP address . Where as unable to ping from another subnet hosts because check for any access-list Configured on core switch for any restriction.