gratuitous ARP

Unanswered Question
Jan 15th, 2010
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Hello,


Will cisco switches/routers learn and add entry in ARP cache using GARP packet??...

i.e when a switch/router receives an GARP,will the source learning take place?

GARP--gratuitous ARP


Thanks


Stephen

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Ganesh Hariharan Fri, 01/15/2010 - 04:36
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Hi Stephen,


As the defination about the Gratutous ARP says it is a feature of the Address Resolution Protocol wherein the host broadcasts an ARP request for its own hardware address.A gratuitous ARP could be a request or a reply. A gratuitous ARP Request is the one where the Source and Destination IP address will be of the host itself which issues the request and is broadcast to the subnet and normally there will not be a reply.

This concept is used to avoid ip confilct in the network by single device in the network.


A Gratuitious ARP reply is again the one where there was no request.

The switch to which the device is connected can update its MAC Address table against the switch port.


Hope that clear your query !!


Regards

Ganesh.H

Jon Marshall Fri, 01/15/2010 - 05:13
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stephen.stack wrote:


Hello,


Will cisco switches/routers learn and add entry in ARP cache using GARP packet??...

i.e when a switch/router receives an GARP,will the source learning take place?

GARP--gratuitous ARP


Thanks


Stephen


Stephen


Basically yes, a switch/router will update it's arp cache when it sees a gratuituos arp. As Ganesh said it is used to avoid address conflicts but the other major use i have seen is in clustering solutions where a machine fails over to another and the new active machine sends out a gratuitous arp to update the switch mac-address tables.


Jon

Giuseppe Larosa Fri, 01/15/2010 - 05:20
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Hello Stephen,

generally speaking with default settings the answer is yes.


For example, HSRP uses a gratuoitus ARP when a change of state happens with the objective to update Layer2 CAM table.


To be noted the real objective of gratuituos ARP is to update the ARP table changing an IP address to MAC entry, updating a L2 CAM table is a side effect but it is used too.


so even if VIP IP address and VIP MAC address doesn't change the information about what port the VIP MAC is learned is updated.


There are security features that disable the learning from gratuitous ARP


there is a global command


no ip gratuitous-arps

see
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/autosec.html#wp1093336

Hope to help
Giuseppe

stephen.stack Fri, 01/15/2010 - 06:58
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Guys


Thanks for all your comments. I think that nails/confirms it for me.


Regards


Stephen

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