Re: Gratuitous ARP Message??

From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Mon Aug 25 2008 - 15:33:53 ART

no ip gratuitous-arps

http://www.cisco.com/en/US/docs/ios/ipaddr/command/reference/iad_arp.html#wp1013090

"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. "

The command reference does not explicitly indicate that the command
stops: gratuituous ARPs being sent when an interface is initialised;
being sent when the router becomes a HSRP/VRRP active router; or causing
the ARP table to be modified when they are received.

Gratuitous Arps are required by the VRRP RFC and should be used by DHCP
clients for duplicate address detection according to the DHCP RFC.

I'm not sure which current RFC requires the use of gratuitous ARPs when
an interface initialises, but there is a recent proposed standard
(rfc5227) about the useage of them for detecting and reacting to
duplicate address conflicts.

The command seems to have a much more limited effect than it's semantics
suggest. Perhaps the behaviour has changed, but I think you have
spotted an error in the book.

Paul.

Faisal Saleem wrote:
> R1 (f0/0) <-------10.1.1.0/24------->(f0/0) R0 (f1/0)
> <--------11.1.1.0/24-------> (f0/0)R2
>
> R0(config)#interface f0/0
> R0(config-if)#ip address 10.1.1.2 255.255.255.0
> R0#debug arp
> ARP packet debugging is on
>
> similarly same ip address on R1 and as soon as i will no shut the interface
> then we will see the gratuitous arp messages on both routers however these are
> disable bydefault on both routers. So any idea why router is showing this
> message??
>
> R1(config)#interface f0/0
> R1(config-if)#ip address 10.1.1.2 255.255.255.0
> R1(config-if)#no shut
>
> *Jun 27 02:24:09.059: IP ARP: sent rep src 10.1.1.2 ca05.0f04.0000,
> dst 10.1.1.2 ffff.ffff.ffff FastEthernet0/0
> *Jun 27 02:24:09.063: IP ARP: sent rep src 10.1.1.2 ca05.0f04.0000,
> dst 10.1.1.2 ffff.ffff.ffff FastEthernet0/0
> *Jun 27 02:24:09.067: IP ARP: rcvd rep src 10.1.1.2 ca04.0f04.0000, dst
> 10.1.1.2
> FastEthernet0/0
> *Jun 27 02:24:09.067: %IP-4-DUPADDR: Duplicate address 10.1.1.2 on
> FastEthernet0
> /0, sourced by ca04.0f04.0000
> *Jun 27 02:24:09.067: IP ARP: Gratuitous ARP throttled.
> *Jun 27 02:24:09.071: IP ARP: 10.1.1.2 added to arp_defense_Q
> *Jun 27 02:24:09.071: IP ARP: rcvd rep src 10.1.1.2 ca04.0f04.0000, dst
> 10.1.1.2
> FastEthernet0/0
> *Jun 27 02:24:09.071: IP ARP: Gratuitous ARP throttled.
> *Jun 27 02:24:09.079: %SYS-5-CONFIG_I: Configured from console by console
> *Jun 27 02:24:09.119: IP ARP: 10.1.1.2 removed from arp_defense_Q
> *Jun 27 02:24:09.119: IP ARP: sent rep src 10.1.1.2 ca05.0f04.0000,
> dst 10.1.1.2 ca05.0f04.0000 FastEthernet0/0
> ==============================================================
>
> Below is the copy past from Jeff Doyle(TCP/IP V1)
>
> Gratuitous ARP
>
> A host might occasionally issue an ARP Request with its own IPv4 address as the
> target address. These ARP Requests, known as gratuitous ARPs, have several
> uses:
>
> A gratuitous ARP might be used for duplicate address checks. A device that
> issues an ARP Request with its own IPv4 address as the target and receives an
> ARP Reply from another device will know that the address is a duplicate.
>
> A gratuitous ARP might be used to advertise a new data-link identifier. This
> use takes advantage of the fact that when a device receives an ARP Request for
> an IPv4 address that is already in its ARP cache, the cache will be updated
> with the sender's new hardware address.
>
> A router running Hot Standby Router Protocol (HSRP) that has just taken over as
> the active router from another router on a subnet issues a gratuitous ARP to
> update the ARP caches of the subnet's hosts.
>
> Many IP implementations do not use gratuitous ARP, but you should be aware of
> its existence. It is disabled by default in IOS but can be enabled with the
> command ip gratuitous-arps."
>
>
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