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ARP timeout and mac address timeout on Catalyst 6500 with sup720 and MSFC

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Jun 8th, 2006
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I have a question regarding changing the router ARP timeout from the default of 4 hours to something close to the switch mac address timeout to minimize the unicast flooding that can occur with asymmetric routing topologies. Are there any negatives to lowering the ARP timeout to something around 5 minutes?


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Roberto Salazar Thu, 06/08/2006 - 09:38
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ARP flooding every 5 minutes? Why not adjusting the cam entry closer to ARP timeout instead?


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tdrais Thu, 06/08/2006 - 10:08
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I would increase the mac timeout somewhat and descrease the arp to match.


One issue that will mess this up is topology change notification.


If you do not have your edge ports set to portfast whenever a user up/downs his port a TCN will be sent to the root bridge. The root bridge will send out a topology change to all the switchs which will change the age out about 35 seconds. You now have your arp

issue back again.


Of course if you set the mac age out too high and your users move around from switch to switch you will get complaints

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