David’s Cisco Networking Blog

Dec 15 2008   3:27AM GMT

What is ARP and how does it work in the Cisco IOS?

David Davis David Davis Profile: David Davis

The Address Resolution Protocol, or ARP, is a critical protocol in your network infrastructure. What is ARP? It’s an Internet protocol used to map an IP address to a MAC Address. Usually ARP just “works” and you don’t have to do anything about it. However, there are reasons that you may need to make ARP or configure ARP on your router.

Here is how you make a static ARP entry in the Cisco IOS:

In global configuration mode, use:

arp {ip-address | vrf vrf-name} hardware-address encap-type [interface-type]

Here is an example:

arp  10.28.6.14 0700.0800.1834 arpa 

To remove an ARP entry, use the no arp command.

To show arp entries, just type show arp and you will see results like this:

Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.4.2.4               10   0000.0c07.ac14  ARPA   FastEthernet0/0
Internet  10.3.3.3               10   0000.0c07.ac14  ARPA   FastEthernet0/0
Internet  10.3.2.7               10   0000.0c07.ac14  ARPA   FastEthernet0/0
Internet  10.4.2.1               10   0007.b400.1401  ARPA   FastEthernet0/0
Internet  10.3.3.7               10   0000.0c07.ac14  ARPA   FastEthernet0/0

As you can see, the IP addresses are mapped to the hardware/MAC addresses on Ethernet interfaces.

Just a tip, if your device’s MAC address isn’t showing up in the ARP table on a switch or router that it is communicating through, it will never be able to communicate. There must be some reason (like an interface down) that is preventing it from making that Layer 2 (Ethernet) to Layer 3 (IP) connection. You can also use the debug arp command to troubleshoot.

And, one final tip- on a Cisco IOS switch, you can use the show mac-address-table or show mac command to see MAC to IP address mappings.

For additional information on using ARP in the Cisco IOS, please see Cisco IOS IP  Addressing Service documentation.

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