====== Linux 網路不通先檢查 ARP ======

通常是將實體網路更換 switch PORT 或是更換 IP 後，發現網路不通的狀況..

  * 可以先用 arping <gateway ip> 讓 switch 更新 arp table
<code>
[root@pd920 ~]# arping 192.168.11.254
ARPING 192.168.11.254 from 220.130.131.239 eth0
Unicast reply from 192.168.11.254 [00:90:1A:82:83:FE]  41.405ms
Unicast reply from 192.168.11.254 [00:90:1A:82:83:FE]  41.848ms
Unicast reply from 192.168.11.254 [00:90:1A:82:83:FE]  41.642ms
Unicast reply from 192.168.11.254 [00:90:1A:82:83:FE]  38.575ms
Unicast reply from 192.168.11.254 [00:90:1A:82:83:FE]  39.111ms
Sent 5 probes (1 broadcast(s))
Received 5 response(s)
</code>

  * 用 arp -a 看看本身的 arp table 是否正確
<code>
[root@pd920 ~]# arp -a
220-130-131-254.HINET-IP.hinet.net (220.130.131.254) at 00:90:1A:82:83:FE [ether] on eth0
? (192.168.11.254) at 00:90:1A:82:83:FE [ether] on eth0
www.ichiayi.com (220.130.131.240) at 00:13:74:00:5C:xx [ether] on eth0
vm-mail.ichiayi.com (192.168.11.236) at 00:0C:29:48:E8:13 [ether] on eth1
</code>
<note>
如果發現本身的 arp table 錯亂 .. \\
Exp. 出現兩筆相同 ip 不同 mac  \\
就使用 **server network restart** 清除 arp table 重新自動建立 
</note>

{{tag>linux 網路 arp}}