The Clear Arp Cisco command

The ‘clear arp’ command is a powerful tool used in Cisco networking to manage and troubleshoot issues related to Address Resolution Protocol (ARP). It allows network administrators to remove all the dynamic ARP entries from a device’s ARP cache, effectively resetting the ARP table. This can be useful in several scenarios to resolve ARP-related problems and maintain network stability.

Syntax:

clear arp [cache]

Parameters:

• cache: This optional keyword can be used to specify a specific ARP cache to be cleared. By default, the command clears the main ARP cache. If you want to clear a specific cache, such as a proxy ARP cache or a specific VLAN’s ARP cache, you can use the ‘cache’ keyword followed by the cache name or VLAN ID.

Function:

When the ‘clear arp’ command is executed, it removes all the dynamically learned ARP entries from the specified ARP cache. This means that all the IP-to-MAC address mappings that were automatically populated through ARP requests and responses are cleared. As a result, the device will have to relearn these mappings the next time it needs to communicate with the affected hosts.

Usage:

The ‘clear arp’ command is commonly used in the following situations:

• Troubleshooting ARP Issues: If you encounter ARP-related problems, such as connectivity issues or duplicate IP addresses, clearing the ARP cache can help resolve the problem. By removing the outdated or incorrect ARP entries, the device will be forced to repopulate the ARP cache with accurate information, potentially fixing the issue.

• Network Changes: When making network changes, such as changing IP addresses or subnet masks, clearing the ARP cache can ensure that the device quickly learns the new mappings and avoids potential conflicts or disruptions.

• Security Concerns: In some cases, clearing the ARP cache can help mitigate certain types of network attacks, such as ARP spoofing or ARP poisoning, by removing any malicious ARP entries that may have been introduced.

Supported Equipment:

The ‘clear arp’ command is supported on a wide range of Cisco devices, including routers, switches, and firewalls. The specific syntax and availability of the command may vary slightly depending on the device model and IOS version.

Additional Information:

• The ‘clear arp’ command does not affect static ARP entries, which are manually configured and stored in the device’s configuration. These entries will remain in the ARP cache until they are manually removed or the configuration is changed.

• Clearing the ARP cache can temporarily disrupt network connectivity for devices that are communicating with the affected hosts. However, the impact is usually minimal, and the ARP table will be repopulated quickly as the devices send and receive ARP requests and responses.

• The ‘clear arp’ command is a privileged EXEC command, meaning that only users with sufficient privileges can execute it. Typically, network administrators or authorized personnel will have the necessary permissions to use this command.

clear arp Usage Examples

Examples:

1. Clear ARP entries for all entries and all interfaces:

   clear arp *
   

2. Clear ARP entries for a specific interface:

   clear arp interface GigabitEthernet0/1
   

3. Clear ARP entries for a specific IP address:

   clear arp 10.1.1.1
   

4. Clear ARP entries for a range of IP addresses:

   clear arp 10.1.1.1 - 10.1.1.254
   

5. Clear ARP entries for a specific MAC address:

   clear arp 00:11:22:33:44:55
   

6. Clear ARP entries for a range of MAC addresses:

   clear arp 00:11:22:33:44:55 - 00:11:22:33:44:56
   

7. Clear ARP entries for a specific VLAN:

   clear arp vlan 10
   

8. Clear ARP entries for all VLANs:

   clear arp vlan *
   

9. Clear ARP entries for a specific subnet:

   clear arp subnet 10.1.1.0/24
   

10. Clear ARP entries for all subnets:

    clear arp subnet *