Address Resolution Protocol (ARP) is a network layer member of the TCP/IP protocol suite. After an outbound interface is selected by IP routing process, a frame with an encapsulation appropriate for that interface must be formed. An important and necessary field of the frame is its destination MAC address. If the outbound interface is a LAN interface (such as Ethernet), ARP discovers a MAC address to be used for the outgoing frame through a broadcast-based request. The request includes the IP address for which the ARP process is attempting to find a MAC address. If no reply is received, frame delivery (and naturally the intended IP packet delivery) fails and an appropriate ICMP message is generated. If a reply is received with a MAC address, the MAC address will be used and also stored in the ARP table (cache) for a predetermined period of time. The purpose of the ARP cache is to prevent sending ARP requests repeatedly for the following frames that are part of the same conversation as the first (for which the original ARP request was performed). To display the Address Resolution Protocol (ARP) cache, use the show ip arp EXEC command:
show ip arp [ip-address] [hostname] [mac-address] [type number]
The following explains the syntax of the command:
• ip-address—(Optional) ARP entries matching this IP address are displayed.
• hostname—(Optional) ARP entries matching this hostname are displayed.
• mac-address—(Optional) ARP entries matching this 48-bit MAC address are displayed.
• type number—(Optional) ARP entries learned via this interface type and number are displayed.
Viewing the IP ARP table allows you to determine which device replied to the ARP request sent out for a particular IP address and whether that is a desirable behavior. Furthermore, you can check accuracy of any static ARP entries currently in the table. Remember that an interface that has IP proxy-ARP enabled on it responds to ARP requests that do not necessarily contain the replying interface's IP address. Hence, it is possible that an ARP table may contain multiple entries (with different IP addresses), all of which have the same MAC address in front of them.
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