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ARP entries are periodically refreshed if you use CEF switching

In a previous post I've been writing about the inability to clean the ARP cache due to cached CEF adjacencies. As it turns out, this behavior has another side effect: the router will automatically refresh all ARP entries (and CEF adjacencies) as they expire from the ARP cache. This might become a problem on high-end devices with a lot of directly connected hosts if you set the arp timeout to a low value.
Here is a sample debugging printout verifying this behavior:

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


  1. ARP entries are periodically refreshed even if CEF not running. At least with c2600-adventerprisek9-mz.123-26

  2. What happened in the morning???? Very curious to know the answer


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Ivan Pepelnjak, CCIE#1354 Emeritus, is an independent network architect. 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.