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Understanding ARP cache

Posted on 2014-04-13
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Last Modified: 2014-04-17
Is my understanding correct?

ARP is a table of devices which are currently connected on the network.

So when a new network is setup the router or managed switch stores the ip addresses and MAC addresses in a table. The table is created when a single device sends out an ARP request (i.e. broadcast) to check if a device is on the network using a MAC address.

thats the first thing.

second thing is if we flush the ARP - does that mean we reset the ARP cache which would result in the devices losing connectivity temporarily?

please advise in both scenarios.
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Question by:Ikky786
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Dave Baldwin earned 250 total points
ID: 39998338
No, ARP is Address Resolution Protocol, a method to turn IP addresses basically into MAC addresses on the LAN.  When an address is converted, a copy of the info is stored in the ARP cache.  When you clear the cache, it just means that the process has to be repeated instead of the information coming from the cache.  ARP is strictly for LAN access.  

http://en.wikipedia.org/wiki/Address_Resolution_Protocol#Example
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by:OomWall
ID: 39998392
It might be easier if you let us know what you are trying to troubleshoot.
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Expert Comment

by:jmathon
ID: 39998418
I already did an arp cache flush on a switch without any connectivity loss.
Since the flush is done, you will have new arp requests
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by:skullnobrains
skullnobrains earned 250 total points
ID: 39998520
ARP is a table of devices which are currently connected on the network.
no arp is a protocol. hosts only maintain a cache containing the IPs and mac addresses of the devices they already communicated with

So when a new network is setup the router or managed switch stores the ip addresses and MAC addresses in a table. The table is created when a single device sends out an ARP request (i.e. broadcast) to check if a device is on the network using a MAC address.

not really
ARP requests are not sent to a MAC adress. they are intended to retrieve the mac address corresponding to an IP.

also note that although ARP works in shoutcast mode, using the term broadcast is confusing.

second thing is if we flush the ARP - does that mean we reset the ARP cache which would result in the devices losing connectivity temporarily?

no. the ARP protocol will be used for the next packet that is sent and the cache will be repopulated as needed. on a busy network, it will be back to the previous state before you have a chance to see it empty
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