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Hi, A few questions on Gratuitous ARP (GARP) (I am a network printer):

  1. Lets say I power up on the network:
    (a) Do I send out a GARP?
    (b) At what point do I do this in the power-up sequence?
    (c) Do I do this un-asked-for? Or should I wait for someone (a router, for example) to ask first?

  2. Your website says "the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff." However, all the example screen-shots on-line don't seem to do this... Any help in structuring a GARP?

  3. I am told that I should send out a GARP when I establish a link...
    (a) True?
    (b) if so, do I do this un-asked-for?
    Or should I wait for someone (a router, for example) to ask first?

asked 13 Apr '15, 08:05

JayPrab1's gravatar image

JayPrab1
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accept rate: 0%

edited 13 Apr '15, 12:25

Kurt%20Knochner's gravatar image

Kurt Knochner ♦
24.8k1039237

sounds like homework for me!

Please post your answers first and then we will help you to get it straight ....

BTW: Teachers of networking classes are known to monitor this Q&A site!

(13 Apr '15, 09:41) Kurt Knochner ♦

"Please post your answers first and then we will help you to get it straight ...." Not sure I understand this comment... New to this forum, so not familiar with the regs...

(13 Apr '15, 10:26) JayPrab1

Not sure I understand this comment... New to this forum, so not familiar with the regs...

It means: I (and probably others here) am willing to help you with your questions/homework, but I (and probably others here) am NOT willing to do the whole homework for you, by simply answering the questions you should have answered yourself

;-))

So, please show what you have done so far to find an answer and post your findings. I (and probably others here) will then comment these findings and your ideas.

(13 Apr '15, 11:54) Kurt Knochner ♦

Fair enough. There is precious little about GARP on-line. I know that something like a Router can broadcast an ARP request to which my NIC (printer) will respond. But, if a device BEYOND the router (not part of the printer's subnet) wants to access the printer, then the router needs to know the printer's MAC address. The question is this: Does the printer send out an unsolicited GARP braodcast? If so, at what point in time does it do so? If not, is it in response to a GARP request ONLY? This is a TIMING issue I am asking about after looking quite exhaustively... Not just laziness... Thanks for your help!

(13 Apr '15, 12:40) JayPrab1

Or, please give me a link to some source of this information, if you can. Thanks!

(13 Apr '15, 12:48) JayPrab1

Or, please give me a link to some source of this information, if you can. Thanks!

I suggest the following videos:

First ARP:

https://www.youtube.com/watch?v=OZi3tVrpI6U
https://www.youtube.com/watch?v=xTOyZ6TWQdM

and then Gratuitous ARP:

https://www.youtube.com/watch?v=JPLn6wfAmZY

After the last video ask yourself: When does a printer need to send a gratuitous ARP and does it really need it (if so: for what)?

Further information:

http://networkengineering.stackexchange.com/questions/7713/how-does-gratuitous-arp-work
https://devcentral.f5.com/questions/gratuitous-arp-how-its-working
http://www.rhyshaden.com/arp.htm

Regards
Kurt

permanent link

answered 13 Apr '15, 12:55

Kurt%20Knochner's gravatar image

Kurt Knochner ♦
24.8k1039237
accept rate: 15%

edited 15 Apr '15, 03:27

Thanks for the links, Kurt! Will go thru them.

(13 Apr '15, 13:14) JayPrab1
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question asked: 13 Apr '15, 08:05

question was seen: 4,450 times

last updated: 15 Apr '15, 03:28

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