I do not understand the role of the command " no ip gratuitous-arp"

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Aug 20th, 2007

I do not understand the role of the command " no ip gratuitous-arp" on a CAT6500.

What does it do when the ip gratuitous-arp is on; thanks

I have this problem too.
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Jon Marshall Tue, 08/21/2007 - 05:56

Hi

Gratuitous arp is used when a host wants to inform the switch that the mac-address has changed eg.

You have a cluster which has redundant connections and an IP to mac-address mapping. If the active NIC fails the IP address is moved across to the standby NIC but the standby NIC has a different mac-address. So the cluster sends out a gratuitous arp which informs the switch of the new IP to mac-address mapping.

The reason you might not want to allow gratuitous arp is that you might not want your switch updating it's arp table based on annoucements from devices on the LAN as you could very easily spoof mac-addresses and corrupt the arp cache.

HTH

Jon

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