ARP timeout resolution is implemented in minutes

Under some circumstances, you might want to tune the ARP timers on the router (for example, when using ARP as a keepalive mechanism to detect whether the host is up). Unfortunately, although you can set the per-interface arp timeout in seconds, the actual timer resolution is in minutes. For example, if you set the ARP timeout to 10 seconds, the router will age the ARP entries once per minute.Here's a sample debugging output produced on a router running IOS release 12.4(9)T with the arp timeout 10 interface configuration command. As you can see from the timestamps, the ARP entry is aged and refreshed exactly once per minute.

Jun 7 16:34:49: IP ARP: sent req src 192.168.0.1 0016.c7fe.f150, dst 192.168.0.2 000c.293a.b455 FastEthernet0/0.100
Jun 7 16:34:49: IP ARP: rcvd rep src 192.168.0.2 000c.293a.b455, dst 192.168.0.1 FastEthernet0/0.100
Jun 7 16:34:49: IP ARP: creating entry for IP address: 192.168.0.2, hw: 000c.293a.b455
Jun 7 16:35:49: IP ARP: sent req src 192.168.0.1 0016.c7fe.f150, dst 192.168.0.2 000c.293a.b455 FastEthernet0/0.100
Jun 7 16:35:49: IP ARP: rcvd rep src 192.168.0.2 000c.293a.b455, dst 192.168.0.1 FastEthernet0/0.100
Jun 7 16:35:49: IP ARP: creating entry for IP address: 192.168.0.2, hw: 000c.293a.b455
Jun 7 16:36:49: IP ARP: sent req src 192.168.0.1 0016.c7fe.f150, dst 192.168.0.2 000c.293a.b455 FastEthernet0/0.100
Jun 7 16:36:49: IP ARP: rcvd rep src 192.168.0.2 000c.293a.b455, dst 192.168.0.1 FastEthernet0/0.100
Jun 7 16:36:49: IP ARP: creating entry for IP address: 192.168.0.2, hw: 000c.293a.b455

3 comments:

  1. Hi,

    ARP retry interval and ARP retry attempt is there standard value that vendor follow or is it up to them to define any value.

    With Regards
    Wan T

    ReplyDelete
  2. I'm not aware of any standardized value.

    ReplyDelete

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.