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XGS-4526/4528F/4728F User’s Guide
381
CHAPTER 43
ARP Learning
43.1 ARP Overview
Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol
address (IP address) to a physical machine address, also known as a Media Access
Control or MAC address, on the local area network.
An IP (version 4) address is 32 bits long. In an Ethernet LAN, MAC addresses are
48 bits long. The ARP Table maintains an association between each MAC address
and its corresponding IP address.
43.1.1 How ARP Works
When an incoming packet destined for a host device on a local area network
arrives at the Switch, the Switch looks in the ARP Table and if it finds the address,
it sends it to the device.
If no entry is found for the IP address, ARP broadcasts the request to all the
devices on the LAN. The Switch fills in its own MAC and IP address in the sender
address fields, and puts the known IP address of the target in the target IP
address field. In addition, the Switch puts all ones in the target MAC field
(FF.FF.FF.FF.FF.FF is the Ethernet broadcast address). The replying device (which is
either the IP address of the device being sought or the router that knows the way)
replaces the broadcast address with the target's MAC address, swaps the sender
and target pairs, and unicasts the answer directly back to the requesting machine.
ARP updates the ARP Table for future reference and then sends the packet to the
MAC address that replied.
43.1.2 ARP Learning Mode
The Switch supports three ARP learning modes: ARP-Reply, Gratuitous-ARP, and
ARP-Request.