arp table

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Nov 5th, 2007
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Hi all, can anyone tell me why devices have an arp table. i was told it was only layer 3 devices, but if i got onto my access point and do show ip arp, i see mac addresses mapped to ip addresses.

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bvsnarayana03 Mon, 11/05/2007 - 04:03
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ARP table contains mac-add to IP add binding. At layer 2,frames are processed based on mac-addres but at layer 3, Ip address is used. So, devices would often keep track of the other devices by maintaining an arp table. Even the desktops maintains arp table.

sumesh_un Mon, 11/05/2007 - 06:46
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ARP basically gives you mac address to ip address mappings.Say host A wants to communicate with host B.Host A knows the ip address of B which would be L3 destination address.But when it has to frame the packet at Layer2 it needs the Mac address of B.ARP helps A get this by sending an ARP Broadcast on the LAN which asks the host B to identify itself.host B replies with its mac address which A uses to frame the packet.Mac tables in devices (say in a switch) are a way to help hosts get mac addresses.

Mac addresses tables are present in L3,L2 devices as ip mac resolutions are needed for both packet swicthing and frame forwarding

carl_townshend Mon, 11/05/2007 - 08:40
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if the router does not know the mac address for the ip, can the router itself send an arp broadcast for the mac address ?

bvsnarayana03 Mon, 11/05/2007 - 10:39
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Refer this link for how router process ARP request.


Router doesnt need to knw mac-add of all hosts connected to its segment instead it maintains mac-add of its directly connectd neighbors only.


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40.2cbe4a27/2#selected_message

sumesh_un Mon, 11/05/2007 - 23:43
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router doesnt need to know the mac of every ip behind it.Every host has something called as a default gateway which is the router interface ip.For any frame destined towards another host the destination mac is always of this interface.

So the router doesnt need to know the destination hosts mac..it just needs to know the mac-address of the next hop router

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