Device unreachable unless pinged by router first?

[techsupportALDL]

Hello all,

I have a two HVAC controller devices connected to 2960Xs which connect to a 4500X doing some basic stub routing.  The devices consistently "disappear" off the network and are unreachable by our facilities team.  They can't reach the web interface, pinging doesn't work, just gone. However, if I go to the 4500X and ping the IP addresses, they suddenly wake up and re-appear. Any thoughts on what could be causing this and how I can rectify it?   The devices are maintained by an outside service, but I provided addressing info, etc. Could something be misconfigured on their end? 

Thanks! 

[MHM Cisco World]

Can I see the config of port that connect controller to SW?
MHM

[M02@rt37]

Hello @techsupportALDL 

If the HVAC controllers become unreachable unless they are pinged by the router first, and this action seems to "wake them up," it's possible that there might be an issue related to ARP on the network. 

Best way is to examine the ARP table on the 4500X switch when the HVAC controllers are unreachable and compare it to the ARP table after you ping them from the router. Check if the MAC addresses associated with the HVAC controllers are present in the ARP table.

Also, verify the ARP aging time on the 4500X switch. If the ARP entries are expiring too quickly, it could result in communication issues. You can adjust the ARP aging time if needed.

You should consider configuring static ARP entries on the 4500X switch for the IP addresses of the HVAC controllers. This ensures that the ARP cache is pre-populated with the correct MAC addresses.

Do yo know if ARP inspection is enabled on the switch ? 

Other idea, verify that there are no duplicate IP addresses in the network, especially for the HVAC controllers. Duplicate IP addresses can lead to ARP conflicts.

Finnaly, examine the switch logs for any ARP-related messages or errors. Logs may provide insights into ARP failures or issues. Other debug way, use a network packet capture tool to capture ARP traffic during the period when the HVAC controllers are unreachable. Analyze the captured packets to identify any anomalies or issues with ARP resolution.

[MHM Cisco World]

if there is no issue but you still need ping to make SW learn MAC or controller then 
I think you need EEM to run auto ping when the show arp not list controller entry 

MHM