ARP常見問題：為什麼在各自的ARP超時過期後，ARP表中仍存在一些動態ARP條目？

簡介

本檔案介紹動態位址解析通訊協定(ARP)專案老化問題。

為什麼在各自的ARP超時過期後，ARP表中仍存在一些動態ARP條目？

在Cisco IOS^®軟體中，ARP快取逾時預設設定為四小時（240分鐘），但可以在介面組態模式下修改。

輸入show interfaces命令以顯示ARP快取超時：

ASR1k#show interfaces gi0/0/2 | include ARP
  Encapsulation ARPA, loopback not set
  ARP type: ARPA, ARP Timeout 04:00:00

ARP條目實際上儲存在ARP快取中，即使在超時過期之後。在本示例中，IP地址10.2.2.2的動態ARP條目已在ARP快取中存在253分鐘：

ASR1k#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.2.2.1                -   30e4.dbb7.7e02  ARPA   GigabitEthernet0/0/2
Internet  10.2.2.2              253   0004.c01d.7c1a  ARPA   GigabitEthernet0/0/2

額外時間是在每個動態ARP條目建立時新增到其中的抖動。向ARP快取超時新增了隨機抖動，以避免ARP條目的同步過期，這可能會觸發ARP風暴。抖動應該是一個介於0秒和30分鐘之間的隨機數，最大抖動為30分鐘。

以下過程介紹了如何確認抖動是隨機的：

1. 輸入show arp IP address detail命令以檢查ARP專案詳細資訊：
   
   

   ASR1k#show arp 10.2.2.2 detail
   ARP entry for 10.2.2.2, link type IP.
     Dynamic, via GigabitEthernet0/0/2, last updated 253 minutes ago.
     Encap type is ARPA, hardware address is 0004.c01d.7c1a, 6 bytes long.
     ARP subblocks:
     * Dynamic ARP Subblock
       Entry will be refreshed in 9 minutes and 4 seconds.
       It has 2 chances to be refreshed before it is purged.
       Entry is complete.
     * ARP HA
       ARP entry is a new entry and has not been synchronized to standby RP.
     * IP ARP Adjacency
       Adjacency (for 10.2.2.2 on GigabitEthernet0/0/2) was installed.
       Connection ID: 0

2. 再次清除ARP專案，並從show arp IP address detail 指令擷取輸出：
   
   

   ASR1k#clear arp 10.2.2.2
   ASR1k#show arp 10.2.2.2 detail
   ARP entry for 10.2.2.2, link type IP.
     Dynamic, via GigabitEthernet0/0/2, last updated 0 minute ago.
     Encap type is ARPA, hardware address is 0004.c01d.7c1a, 6 bytes long.
     ARP subblocks:
     * Dynamic ARP Subblock
       Entry will be refreshed in 261 minutes and 42 seconds.
       It has 2 chances to be refreshed before it is purged.
       Entry is complete.

   注意計時器已重置。
   
   
3. 重複步驟2，注意結果不同：
   
   

   ASR1k #clear arp 10.2.2.2
   ASR1k #show arp 10.2.2.2 det
   ARP entry for 10.2.2.2, link type IP.
     Dynamic, via GigabitEthernet0/0/2, last updated 0 minute ago.
     Encap type is ARPA, hardware address is 0004.c01d.7c1a, 6 bytes long.
     ARP subblocks:
     * Dynamic ARP Subblock
       Entry will be refreshed in 263 minutes and 58 seconds.
       It has 2 chances to be refreshed before it is purged.
       Entry is complete.

相關資訊

• Cisco IOS IP Addressing Services命令參考中的show arp命令
• 技術支援與文件 - Cisco Systems