Impact of Short ARP Timeout

[S Kumar]

Here is the brief setup:

1. We have a device configured with one public IP address.
2. This device has 2 modules and each module has its own mac-address and Ethernet interface.
3. Both modules work as active standby units.
4. If active module fails then standby module takes over automatically.

During the fail-over process, standby module takes over and becomes active but ARP cache at the router still has the old MAC address  (of module which is now standby module) so all the requests goes to the standby module instead of active.

This happens rarely but whenever it happens, we have to manually clear the arp-cache at the router. Currently, the arp cache timeout at the router is confiured as "ARP type: ARPA, ARP Timeout 04:00:00".  I am thinking to change the arp timeout from hrs to lets say 4 minutes. What is the impact of short ARP timeout?

Thanks in advance.

[Milos Megis]

Hi,
it will be more ARP request-response traffic on your network - more broadcasts.

It shouldn´t have significant impact. Maybe if you have configured stormcontrol you will need change configuration. Or if you have some devices sensitive on broadcasts.

Everything depends on count of network devices.

However if you configure shorter timeout then you will have outage too, but instead of 4 hours you will have 4 minutes (in your proposed case).

I don´t know what is that device which you mentioned, but try configure protocols intended for that purpose (HSRP, VRRP, GLBP) if they are supported. And it will work without outage.

[Richard Burts]

An IOS router maintains its ARP cache using a timeout value which by default is 4 hours. When an entry in the arp cache reaches that age then the router removes the entry from arp cache and sends an arp request to the host. If the host is still active on the network then it responds to the router and a new entry is placed in the arp cache and the aging timer is started for it. If you change the timeout from 4 hours to 4 minutes the router will go through this process much more frequently. So in addition to some increase in network traffic there is additional CPU processing on the router to refresh the arp entry more frequently. How much impact this would have will depend on how many entries are in the arp cache and on what other things are generating CPU load. But for most routers the impact would not be significant.

HTH

Rick