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"clear arp" can not clear arp in router or switch

Posted on 2013-06-16
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Last Modified: 2013-06-18
Hi Expert
Here is a question. After I used command "clear arp", I found the output of show arp is still same and did not have any change in router or switch. It looks like the command only can clear Age time, and cannot clear mac address. How to clear mac address ? Anyone can give me some suggestion ? Thank you

R1#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.0.0.11               0   c218.26d8.0000  ARPA   FastEthernet1/1
Internet  10.0.0.3                0   c219.26d8.0000  ARPA   FastEthernet1/1
Internet  10.0.0.1                -   c214.2070.f101  ARPA   FastEthernet1/1
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Question by:EESky
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Expert Comment

by:Sandeep Gupta
ID: 39253181
clear ip arp

clear counters
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Author Comment

by:EESky
ID: 39254540
The both commands can not work for the situation
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by:MarcusSjogren
MarcusSjogren earned 500 total points
ID: 39254616
If "clear arp" and "clear ip arp" doesn't clear the output for "show arp" and "show ip arp" it means that your router is communicating with the IP addresses.

Could for instance be via Cisco CDP or similar which is running automatically in the background with a discovery function.

Marcus
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Expert Comment

by:MarcusSjogren
ID: 39254625
show arp will show you arp-table for all protocols including apple talk etc.
Show ip arp will only show IP to MAC bindings.
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Author Comment

by:EESky
ID: 39255245
Yes, the command "clear arp" does not has the function which delete the record of relation of ip address and mac address. it can only update the age time of it. but it looks like that the cdp also cannot do that
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MarcusSjogren earned 500 total points
ID: 39255327
Hi,

May I ask why this is such a big issue for you? Both CDP and ARP feature automatic discovery functions and the discovery function cannot be removed.
Hence, the ARP table will have entries.

Type "arp -a", "arp -d *" and "arp -a" on your Windows PC, you will see a similar behavior.

Can you specify what devices 10.0 0.1,10.0.0.3 and 10.0.0.11 is?

Marcus
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Expert Comment

by:Sandeep Gupta
ID: 39255756
try this as well

clear arp-cache

remember default arp timeout is 4hrs.
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Author Comment

by:EESky
ID: 39257328
To Marcus, You are right. Thank you. And i found that once i changed ip address or mac address, the neighbor of the device can change the its own arp table immediately. it looks like that arp protocol send update message to neighbor so fast and frequently.
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