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Does the ARP timeout issue an ARP request before it expires to retain the entry in the ARP cache?

dpavun
Cisco Employee
Cisco Employee

Hi,

the default ARP timeout is 4 hours in the cisco routers. When an ARP entry is going to expire, will the device send an ARP request to retain it's entry?

for example,

if the default timeout is 4 hours, at 3 hour 59 minutes, is there any ARP request will be sent to retain its entry?

If the ARP request is sent before certain period(here 1 minute), is this time period is fixed or it can be configured any value? If configurable, what is the command to configure on cisco routers?

If there is any document reference,  please share..

Regards,

Dhamodiran

3 Replies 3

Mark Malone
Mentor
Mentor

Hi no unless traffic is initiated between switch and device it just times out and removes it from the table its up to the device the switch just keeps it in the table while its active , once a device sends some form of traffic you will see in the arp table the timer is reset for that device ip and it starts to countdown again

If you dont want the device to time out increase the arp timers or use a static arp entry in the switch

http://www.cisco.com/c/en/us/td/docs/app_ntwk_services/data_center_app_services/css11500series/v7-30/configuration/routing/guide/rtbrdggd/ARP.html

Thanks for the reply markmalone :)

 

But I have come across a post on this same forum with different answer, please refer the below link,

https://supportforums.cisco.com/discussion/11416946/arp-cache-timeout-cisco-routers

 

in which they mentioned "a unicast ARP request 60 seconds before the entry expires so that the entry can be updated"

Please clarify me on this.

 

I have one more doubt that. Switch maintains MAC table and Router maintains ARP table, I understand that MAC table entry is added from the data frame switch receives and the ARP table entry is added by the router based on the ARP request or ARP reply to the respective router.  But, when will be the switch ARP table and router mac table is updated and use of the both?

 

thats interesting if thas the case they seem a bit unsure in the doc provided still though , i did not know the switch sends a unicast arp to the end device to keep it in the table , i have read cisco pdfs before that state they just time out unless there some form of communication between the 2 , i suppose the best way to confirm this is span a port with an end pc device and run a debug at the same reduce the time right down for arp globally on the switch and then see exactly do you see a unicast arp at this stage to your device from the switch

The 2nd part i would say depends on the switch itself if its purely layer 2 then no it does not have an arp table the router on the network would be responsible for the arp table but if the switch is layer 2/3 then it would contain its own arp table , a true router like 3845 does not contain a mac table it only has arp table

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