Cisco IOS Cookbook, 2nd Edition

Managing the Router’s ARP Cache

Problem

You want to adjust the ARP table timeout value.

Solution

To modify the ARP timeout value, use the arp timeout configuration command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Ethernet0
Router1(config-if)#arp timeout 600
Router1(config-if)#end 
Router1#

Discussion

Every LAN device has an Address Resolution Protocol (ARP) cache. This is a table that the device uses to map Layer 2 MAC addresses to Layer 3 IP addresses. Without this mapping, the device could build its IP packets, but couldn’t build the Layer 2 frames to carry these packets.

Devices discover the information in the ARP cache dynamically. If a device needs to send a packet to an IP destination, and it doesn’t have a corresponding MAC address, it sends out a broadcast ARP request packet. This packet reaches every device on the LAN segment, and the one that “owns” the IP address in question sends back an ARP response packet to complete the process.

Many LAN devices also automatically send a gratuitous ARP packet when they first connect to the network. A gratuitous ARP is a broadcast packet that is effectively an unsolicited ARP response. Every device on the LAN segment will receive this packet so it can update its ARP cache in case there is ever a need to talk to this new device.

The ARP request and response process obviously takes time to complete, introducing a delay in packet processing. Furthermore, because the ARP ...