Learn Cisco CLI – Part 7 – Address Resolution Protocol

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Table of Contents
Learn Cisco CLI - Part 7

Address Resolution Protocol

These blog posts I created to help me as a reference tool, as well as a way of retaining the knowledge. If you find it useful that’s just a plus.

The Address Resolution Protocol (ARP) is used by network devices to resolve IP addresses to MAC addresses. This process is fundamental to data communication on local area networks (LANs), where devices communicate directly with each other using MAC addresses. Understanding ARP and how to manage it on Cisco devices is essential for network troubleshooting and management.

Configuration Commands

Clear ARP

Deletes all entries from the ARP table. This is useful when you want to reset the ARP table and force the device to relearn the MAC addresses of devices on the network.

				
					Syntax:
clear arp

Example:

				
					Router1>enable
Router1#clear arp

Interface

Changes from global configuration mode to interface configuration mode, allowing you to configure a specific interface.

				
					Syntax:
interface [type] [number]

Example:

				
					Router1>enable
Router1#configure terminal
Router1(config)#interface GigabitEthernet0/1 
Router1(config-if)#

IP Address

Assigns an IP address to an interface. This is necessary for the ARP process to occur, as ARP maps IP addresses to MAC addresses.

				
					Syntax:
ip address [ip-address] [subnet-mask]

Example:

				
					Router1>enable
Router1#configure terminal
Router1(config)#interface GigabitEthernet0/1 
Router1(config-if)#ip address 192.168.1.1 255.255.255.0

No Shutdown

Enables an interface that was previously shut down, making it active and allowing it to participate in ARP requests.

				
					Syntax:
no shutdown

Example:

				
					Router1>enable
Router1#configure terminal
Router1(config)#interface GigabitEthernet0/1 
Router1(config-if)#no shutdown

Verification and Troubleshooting

Show ARP

Displays the entries in the ARP table, showing the mappings between IP addresses and MAC addresses.

				
					Syntax:
show arp

Example:

				
					Router1>enable
Router1#show arp

Output Example:

				
					Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.1.2             5   0011.2233.4455  ARPA   GigabitEthernet0/0
Internet  192.168.1.3             3   0011.2233.4466  ARPA   GigabitEthernet0/0

Show IP Interface Brief

Displays a brief summary of the interface status and configuration, including IP address assignments necessary for ARP.

				
					Syntax:
show ip interface brief

Example:

				
					Router1>enable
Router1#show ip interface brief

Output Example:

				
					Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     192.168.1.1     YES manual up                    up

Ping

Sends an ICMP echo request to a specified IP address to test connectivity. A successful ping indicates that ARP has correctly resolved the IP address to a MAC address.

				
					Syntax:
ping [ip-address]

Example:

				
					Router1>enable
Router1#ping 192.168.1.1

Output Example:

				
					Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms

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