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Chapter 4. Routing | 253
ProSafe® Gigabit L3 Managed Stackable Switches Software Administration Manual
hop router, if the destination is not on the same subnet. This is achieved by broadcasting an
ARP request packet, to which the intended recipient responds by unicasting an ARP reply
containing its MAC address. Once learned, the MAC address is used in the destination
address field of the layer 2 header prepended to the IP packet.
The ARP cache is a table maintained locally in each station on a network. ARP cache entries
are learned by examining the source information in the ARP packet payload fields,
regardless of whether it is an ARP request or response. Thus, when an ARP request is
broadcast to all stations on a LAN segment or virtual LAN (VLAN), every recipient has the
opportunity to store the sender’s IP and MAC address in their respective ARP cache. The
ARP response, being unicast, is normally seen only by the requestor, who stores the sender
information in its ARP cache. Newer information always replaces existing content in the ARP
cache.
The number of supported ARP entries is platform-dependent.
Devices can be moved in a network, which means the IP address that was at one time
associated with a certain MAC address is now found using a different MAC, or may have
disappeared from the network altogether (i.e., it has been reconfigured, disconnected, or
powered off). This leads to stale information in the ARP cache unless entries are updated in
reaction to new information seen on the network, periodically refreshed to determine if an
address still exists, or removed from the cache if the entry has not been identified as a
sender of an ARP packet during the course of an ageout interval, usually specified via
configuration.
From the ARP link, you can access the following pages:
• Basic on page 253
• Advanced on page 255
Basic
From the Basic link, you can access the following pages:
• ARP Cache on page 254
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