Configuring Address Resolution Protocol

Information About Address Resolution Protocol

The Address Resolution Protocol (ARP), a network layer protocol is used by the Internet Protocol (IP) to enable communications on an internetwork. Layer 3 devices need ARP to map an IP network address to MAC hardware address so that the IP packets can be sent across networks.

ARP Operating Process

Before a device sends a datagram to another device, it looks in its ARP cache to see if there is a MAC address and corresponding IP address for the destination device. If there is no entry, the source device sends a broadcast message to every device on the network. Each device compares the IP address to its own. Only the device with the matching IP address replies (unicast message) to the sending device with a packet containing the MAC address for the device. The source device adds the destination device MAC address to its ARP table for future reference, creates a data-link header and trailer that encapsulates the packet, and proceeds to transfer the data.

ARP Table

Each entry in the ARP Table consists of an IP Address, the corresponding MAC Address and port number.

An ARP entry is classified as:

  • Static ARP entry:

    A Static ARP entry is manually configured and maintained. It cannot be aged or overwritten by Dynamic ARP entry. A Static ARP entry can be a long or a short entry.

    Long Static ARP entry comprises the VLAN and egress interface details along with the IP address and MAC address. Long Static ARP entries can be directly used for packet forwarding.


    Note

    When you manually configure a Long Static ARP entry, the IP address in the entry must be in the same network segment as the IP address of the VLAN interface on which the egress interface resides.

    A Short Static ARP entry comprises the IP Address and the MAC Address. Short Static ARP entry cannot be directly used for packet forwarding. A Short Static ARP request packet is sent by the host. If the source IP address and the source MAC address in the received response packet are the same as the configured IP address and MAC address, the ARP entry will be completed. Then it can be used for packet forwarding.

  • Dynamic ARP entry:

    A Dynamic ARP entry is automatically generated and maintained by the ARP protocol, through ARP packets. A dynamic ARP entry can be aged, updated by new ARP packets and can also be overwritten by a static ARP entry.

Configuring ARP Table

The following are the steps to configure an ARP table:

Procedure

  Command or Action Purpose
Step 1

configure terminal

Example:

Device#configure terminal

Enters the global configuration mode.
Step 2

arp ipaddressmac mac

Example:

Device(config)#arp 10.1.1.1 mac 00.02.9a.3b.94.d9

Configures a static ARP entry.
Step 3

arp ipaddress mac mac [ vid vid | port port]

Example:

Device(config)#arp 10.1.1.1 mac 00.02.9a.3b.94.d9 vid 4 port 56

Configures a dynamic ARP entry.
Step 4

arp aging-time aging-time

Example:

Device(config)#arp aging-time 50

Sets the length of time an ARP entry stays in the cache. The default time is 20 minutes.
Step 5

no arp { dynamic | static | all | ipaddress }

Example:

Device(config)#no arp dynamic 10.1.1.1

Deletes the specified ARP entries from the ARP table.
Step 6

show arp { dynamic| static | all}

Example:

Device(config)#show arp dynamic

(Optional) Displays the requested ARP entries.