From: Faisal Saleem (faisal2heaven@live.com)
Date: Mon Aug 25 2008 - 10:55:00 ART
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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This archive was generated by hypermail 2.1.4 : Mon Sep 01 2008 - 08:15:32 ART