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Wednesday, September 28, 2011

What are the differences between clear arp-cache and clear ip arp

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 . The MAC address was from the original unit, so we did a clear arp-cache and then a clear mac-address-table dynamic interface . 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. What are the differences 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?

++ 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.

Please click here for documentation on IP Addressing commands

++ 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.

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.

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