Gratuitous ARP

Posted 2020-10-03

Gratuitous ARP 也叫做免费ARP，与普通ARP不一样，普通ARP是请求对方IP地址对应的MAC地址，而免费ARP是当主机或者路由器端口UP的时候所发出的，请求的是自己IP对应的MAC地址。

 免费ARP的作用，主机可以用它来确定另一台主机是否设置了同样的IP地址。如果免费ARP请求收到了恢复说明广播域内存储IP地址冲突。

如下图：

当AR1的GE0/0/1口和AR3的GE0/0/1口都配置成相同IP地址13.13.13.1/24是的时候会提示IP地址冲突

免费ARP收到了MAC地址为MAC=00e0-fc4f-4c0c的回复。  

[RA-GigabitEthernet0/0/1] un shu
[RA-GigabitEthernet0/0/1]
Aug 17 2017 10:18:58-08:00 RA %%01IFPDT/4/IF_STATE(l)[6]:Interface GigabitEthernet0/0/1 has turned into UP state.
[RA-GigabitEthernet0/0/1]
Aug 17 2017 10:18:58-08:00 RA %%01IFNET/4/LINK_STATE(l)[7]:The line protocol IP on the interface GigabitEthernet0/0/1 has entered the UP state.
[RA-GigabitEthernet0/0/1]
Aug 17 2017 10:18:58-08:00 RA ARP/4/ARP_IPCONFLICT_TRAP:OID 16777216.50331648.100663296.16777216.67108864.16777216.3674669056.83886080.419430400.2063597568.33554432.100663296 ARP detects IP conflict. (IP address=1.13.13.13, Local interface=GigabitEthernet0/0/1, Local MAC=00e0-fc53-1cb8, Local vlan=0, Local CE vlan=0, Receive interface=GigabitEthernet0/0/1, Receive MAC=00e0-fc4f-4c0c, Receive vlan=0, Receive CE vlan=0, IP conflict type=Local IP conflict).
[RA-GigabitEthernet0/0/1]
Aug 17 2017 10:18:58-08:00 RA %%01ARP/4/ARP_DUPLICATE_IPADDR(l)[8]:Received an ARP packet with a duplicate IP address from the interface. (IpAddress=1.13.13.13, InterfaceName=GigabitEthernet0/0/1, MacAddress=00e0-fc4f-4c0c)

下面是抓包情况：