Regarding ARP

[CSCO10758684]

Hai Rick,

I had gone through your interview ..it would be helpfull for us if you could share the same once again ...(How ARP works....)

Thanks in advance

Lijesh

[bvsnarayana03 · Accepted solution]

ARP is used to associate IP addresses with media or MAC addresses. Taking an IP address as input, ARP determines the associated mac address by broadcating the ip add. The device corresponding to the Ip add replies with its own mac addres. Once a media or MAC address is determined, the IP address/mac address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network.

A device in the IP can have both a local address (which uniquely identifies the device on its local segment or LAN) and a network address (which identifies the network to which the device belongs). The local address is more properly known as a data link address because it is contained in the data link layer (Layer 2 of the OSI model) part of the packet header and is read by data-link devices (bridges and all device interfaces, for example). Also refered as MAC addresses, because the Media Access Control (MAC) sublayer within the data link layer processes addresses for the layer.

To see arp table on a device use "sh arp"

command.

Refer this link for further reading:

http://www.juniper.net/techpubs/software/erx/erx50x/swconfig-routing-vol1/html/ip-config8.html

pls rate all helpful posts.

[CSCO10758684]

Hai Narayan,

Thanks for the inputs ....that was a valuable one ...

[Richard Burts]

Lijesh

In addition to the previous response it may be helpful to consider the nature of ARP as a local network function. The design and intent of ARP is that it is a local network function and therefore ARP requests should be for local resources. An ARP request is generated on a local subnet/VLAN and should not be forwarded off of the local subnet/VLAN. If an IOS device receives an ARP request with a source address that is not in the local subnet it will treat the request as an error and will drop the request with no response.

As a local network function the destination address (the address you are looking for to resolve the relationship between IP address and MAC address) should be local. But if an IOS devices receives an ARP request and the destination address is not local the IOS device does not necessarily treat it as an error (as it does for source address). IOS implements the function of proxy-arp which gives the IOS device the ability to respond to an ARP request for a remote destination. If proxy-arp is enabled and the IOS device receives an ARP request for a remote destination then the IOS device will respond to the ARP request and send its own MAC address in the response, if the IOS device has a route to the remote network in its routing table.

HTH

Rick

[insccisco]

great stuff rick