ARP常见问题：为什么在各自的ARP超时过期后，ARP表中仍然存在一些动态ARP条目？

简介

本文档介绍动态地址解析协议(ARP)条目老化。

为什么在各自的ARP超时过期后，ARP表中仍然存在一些动态ARP条目？

在Cisco IOS®^软件中，ARP缓存超时默认设置为四小时（240分钟），但可以在接口配置模式下修改。

输入show interfaces命令以显示ARP缓存超时：

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

ARP条目实际存储在ARP缓存中，即使超时已过。在本示例中，IP地址10.2.2.2的动态ARP条目在ARP缓存中已存在253分钟：

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

额外时间是创建每个动态ARP条目时添加的抖动。随机抖动会添加到ARP缓存超时，以避免ARP条目同步过期，这可能会触发ARP风暴。抖动应是0秒到30分钟之间的随机数，最大抖动为30分钟。

此过程介绍如何确认抖动是随机的：

1. 输入show arp IP address detail 命令以检查ARP条目详细信息：
   
   

   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. 清除ARP条目，并再次捕获show arp IP address detail命令的输出：
   
   

   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.

   注意计时器已重置。
   
   
3. 重复步骤2，注意结果不同：
   
   

   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.

相关信息

• 《Cisco IOS IP编址服务命令参考》中的show arp命令
• 技术支持和文档 - Cisco Systems