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Community Member

Adjusting ARP timeout

Considering modifying the default ARP timeout of 4 hours to a greater value for reasons of Wake-On-LAN. Anyone know of any caveats in doing this?

2 REPLIES
Hall of Fame Super Gold

Re: Adjusting ARP timeout

Setting the ARP timeout is a compromise between having timely and accurate information about network components vs reducing the amount of work that the router has to do.

When you make the timer longer it reduces the amount of work that the router has to do (because it is less frequent that it must flush the ARP table and then repopulate the ARP table). But when you make the timer longer it will take longer to discover that some device is no longer on the network. So your information may have a little inaccuracy. In general I think that there is little risk in making the timer longer.

There is one caveat that I am aware of. If the router is connected to an Ethernet switch on its LAN interface there is probably a mismatch between the ARP timer on the router and the timeout for addresses in the layer 2 forwarding table (the mac-address-table or the CAM depending on the model of switch). If the switch forwarding table timeout is shorter (which it is by default) there is a possibility that some device will have its MAC address still in the ARP table but will have timed out in the switch table. The unknown MAC in the switch table may cause flooding of unicast traffic to all ports of the switch. Making the ARP timer longer could exagerate this effect.

Whether this is something to be concerned about in your environment is something that you will need to consider.

HTH

Rick

Community Member

Re: Adjusting ARP timeout

Thanks for the post. Well thought out and thorough.

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