Difference between clear arp-cache and clear ip arp

[onslaught99]

Hello,

We had an instance where a piece of equipment failed that was connected to our 6509 switch. When we replaced the unit with same make and model, the device could not ping outside the VLAN. We performed a sh arp and then a sh mac-address-table | i <mac>. The MAC address was from the original unit, so we did a clear arp-cache and then a clear mac-address-table dynamic interface <int #>. The switch still showed the original mac address and not the new one. We have a Catalyst 3750 hanging off of the 6509, the 3750 can't ping it as well until we do a clear arp-cache. Once we do that, the 3750 can ping the spare device but not the 6509. After troubleshooting some more, we did a clear ip arp on the 6509 and then we were able to ping the new device. But when you do a show ip arp and a show arp, it looks like the same table. So our question is, what is the difference between the clear arp and clear ip arp? And would adjusting the arp timeout work in this case or should we just perform a clear ip arp whenever a device goes down when we replace it?

Thanks

[Somasundaram Jayaraman]

Hi,

++ show arp will show you your Ethernet/ARPA/MAC  addresses which are mapped to IP addresses for the hosts

which have  previously ARPed your router.

++ They basically display the same info. The show arp will show you not only your IP-MAC, but other L3-MAC addresses, e.g. Appletalk.

++ While show ip arp , will by definition show you only your IP-MAC bindings.

http://www.cisco.com/en/US/docs/ios/12_0/np1/command/reference/1ripadr.html#wp1020730

++ Clear arp & clear ip arp removes the output of show arp & show ip arp respectively

clear ip arp

++ By default the arp timeout is 4 hours and mac address table timeout is 300 seconds.

++ when you issued clear arp-cache/clear arp/clear ip arp, the relevant arp table output should clear.

Hope this answers your questions.

Cheers

Somu

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[Peter Paluch]

Somu,

In addition to clearing the ARP cache, the clear arp-cache causes the particular device to send a gratuitous ARP reply regarding its own address - usable if for whatever reason, the MAC address of the device changes. The clear ip arp does not send gratuitous ARPs.

Best regards,

Peter

[onslaught99]

Thanks for the reply to both of you!!!

So why did the clear ip arp command work and not the clear arp-cache? Wouldn't the clear arp work as well? To make things more confusing, we did a clear mac-add-table dynamic interface and performed a show mac-add-table | i . When we did that, we saw that new MAC address but when we did a show arp | i , we still saw the old MAC. Is clear arp supposed to do the same thing as clear ip arp? Why did clear arp not do anything while clear ip arp solved our issue right away? Would adjusting our ARP timeout help remedy this in case the unit has to be swapped when there is no network personnel available (i.e. weekends, off hours, ect.)?

[seandilda]

In my testing, both 'clear ip arp' and 'clear arp-cache' send an ARP request to the MAC address that last held the IP to see if it still has it.  The difference I noticed is:

clear ip arp - clears the entry and sends a single request out to see if it should immediately restore the entry

clear arp-cache -  sends multiple arp requests out over a minute or so, and doesn't clear the local cache until they've all gone unanswered.

My guess is that clear arp-cache did work for you, but you checked the table too quickly.   I thought it was only working intermittently until I realized there was a bit of a delay.