WS-C3750G-48TS-S - It necessary to perform a local ping to put up an arp cache entry.

[Erico Verissimo]

Hello, 

I have a WS-C3750G-48TS-S in stack mode and I am experiencing the following situation:

When is put a new device like a printer in the 3750, for example, in the same network is able to reach the device through ping but out of the local network it is not work. The routing system is working i. e. from other network I can reach the default gateway from the printer but dont the printer. For what the communication works it is necessary perform a ping from the 3750 to a printer, after this I can see the arp cache entry.

 It is looking like that 3750 is not making the arp cache entry when a device request arp system and it only work when I perform a ping from 3750 CLI. Someone could help me with this issue? Thanks!

[Mark Malone]

setup a static arp and map it to the the mac address of the printer so its always there and wont time out after 4hrs

example

arp 1.1.1.1 0001.0002.0003
 

[Erico Verissimo]

Mark, thank you for the response.

I belive that static arp could resolve the problem, but tha arp cache has about five hundred entries. This device is experiencing high cpu utilization sometimes and the newest devices put on the 3750 is having this behavior.

[Erico Verissimo]

Mark,

I would like understand why when I make a ping the arp cache entry work .. I expected that when a ARP request was requested the 3750 dynamically make the association, in generally it make that but lately the 3750 is presenting this behavior.

[Mark Malone · Accepted solution]

ARP is dynamic so when you initiate a ping it maps the mac to ip address and puts it in the table , every further traffic to that ip is seen by the switch and the arp table resets the timer back to 0 for that particular ip, I'f you want to find out exactly what's the cause setup a span session and capture the traffic between the device and switch and you will see if it's the device not initiating the arp correctly or its the switch not responding or debug arp on the router , wireshark may be easier to interpret though

If your having high cpu you need to run the show proc cpu sorted when it's happening see what process is running it up , if your not available to check it live setup a cpu eem script to store the high cpu to flash when it's occurring  In a .txt file 

[Erico Verissimo]

Mark,

Thank you again!!