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I don't get why a machine on the network is asking the gateway for its MAC address when it already knows its MAC address:

ARP example

So here you can see the machine MAC ***80 (IP ..*.115) asks the gateway (Cisco_87), who has 10.1.10.1 ? In other words, where is the gateway? But it already knows who the gateway is, because it sent the ARP directly to it! I could understand this, if the query was a broadcast, ie, can ANYBODY tell me who the gateway is, but the packet was not broadcast, it was sent directly to the gateway (CISCO_87) and to nobody else, so obviously the machine already knows who the gateway is.

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How frequently does the ARP occur? –  generalnetworkerror Aug 14 at 9:16

2 Answers 2

up vote 9 down vote accepted

It's neighbor unreachability detection (NUD). The node makes sure that the gateway is still alive and reachable.

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Many devices will send these types of ARP requests after entries in the ARP table reach a "certain" age to refresh those entries.

The reason they do this is so they can maintain valid ARP entries without having to broadcast ARP for hosts once the entries age out. Directed ARP is much "friendlier" to a network than broadcast ARP.

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