ip arp gratuitous and ip gratuitous-arp

[mjoannic1]

Hi everyone,

I am a bit confused with those two commands: ip arp gratuitous and ip gratuitous-arp

What are each command doing and what would be a use case of such commands?

Thanks in advance for your help.

Regards,

Matthieu

[Reza Sharifi]

Hi,

Have a look at these 2 links, one related to each command:

https://supportforums.cisco.com/discussion/12257536/what-gratuitous-arp

A Cisco router will send out a gratuitous ARP message out of all interfaces when a client connects and negotiates an address over a PPP connection. However, by default, gratuitous ARP messages are not sent out when the client receives the address from the local address pool. The ip gratuitous-arps non-local command option is the default form and is not saved in the running configuration.

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr/command/ipaddr-cr-book/ipaddr-i3.html

HTH

[mjoannic1]

Hi Reza,

Thanks for your answer.

So as far as I understand:

• ip arp gratuitous: disable the ability for an SVI or router interface to send gratuitous ARP is that correct? If so, am I correct in assuming disabling gratuitous ARP using "no ip arp gratuitous" will impact the functionality of protocols such as HSRP/VRRP?

• ip gratuitous-arp: this is specific to PPP connections. Disabling this using "no ip gratuitous-arp" will NOT impact the functionality of protocols such as HSRP/VRRP?

Regards,

Matthieu 

[Austin Sabio]

If I may to add, I would say they are the same just syntax variations across different codes/platforms.

[mjoannic1]

From my understanding (see previous post) they are quite different or maybe I'm missing something?

[Austin Sabio]

Unless there's a cisco documentation shows "ip arp gratuitous" and "ip gratuitous-arp" syntax's are different. The concept is one -gratuitous arp-, different syntax's. But I agree with you if you are referring to "no ip gratuitous-arp" as a syntax is specific to PPP config. Now how does disabling gratuitous arp play with HSRP/VRRP and PPP is a different story and you got it right. I hope this helps. Thanks!

[Reza Sharifi · Accepted solution]

Hi,

• ip arp gratuitous: disable the ability for an SVI or router interface to send gratuitous ARP is that correct? If so, am I correct in assuming disabling gratuitous ARP using "no ip arp gratuitous" will impact the functionality of protocols such as HSRP/VRRP?

I have never done it but I think it will impact the functionally of the protocol since it will disable sending arp packets.

BTW, the command to disable it for HSRP is "no standby arp gratuitous"

I also noticed that this command is not available on all platforms. 

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp_fhrp/configuration/15-sy/fhp-15-sy-book/HSRP-Gratutious-ARP.html

• ip gratuitous-arp: this is specific to PPP connections. Disabling this using "no ip gratuitous-arp" will NOT impact the functionality of protocols such as HSRP/VRRP?

That is correct. 

HTH

[mjoannic1]

Thanks Reza for clarifying this.

Regards,

Matthieu