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arp caches

Could someone please refer me to where I can read up on the arp caches?



Re: arp caches

The Internet RFC826 "An Ethernet Address Resolution Protocol -- or -- Converting Network Protocol Addresses to 48.bit Ethernet Address for Transmission on Ethernet Hardware" is an excellent starting point. It refers to ARP "tables" where we tend to call them "caches" these days. Toward the end of the document it makes mention of aging and timeouts of entries.

For Cisco-specific commands related to ARP, check out "Cisco IOS Software Releases 12.2T: IP Addressing Commands".

Really, any IOS version worth running covers this in its documentation. Commands of general interest include:

arp (and no arp)

arp timeout

clear arp-cache

clear arp interface

show arp

show ip arp

Key to remember on Cisco equipment (routers, multilayer switches, PIX Firewalls) is that an IP-address-to-MAC-address mapping will persist in the ARP table/cache for four (4) HOURS by default. Use the "arp timeout" command if you'd like that information to age out sooner.

To accelerate the timeout process on demand you can use the "no arp" command for a single entry; the "clear arp interface" command to target all entries on a particular interface; or "clear arp-cache" to empty the entire cache and start fresh. And use "show arp" or "show ip arp" to see what's in the cache.

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