How To Clear Arp Cache In Cisco Router

The Address Resolution Protocol (ARP) is a very important part of IP networking. ARP is used to connect OSI Network Later (Layer 3) to OSI Data-Link Layer(Layer 2).

To communicate with any device on the network, the device must have an Ethernet MAC address.

Table of contents

What is ARP Cache?
Reasons you should clear ARP cache from your Computer Device
1. How to Identify if there is ARP cache issue with your System?
How to clear ARP cache on Linux?
1. Clearing cache with arp command
2. Clearing cache with ip command (Fully flush arp cache)
How to clear ARP cache in Windows?
How to clear ARP Cache in macOS or OS X

What is ARP Cache?

ARP cache works as a library of dynamic ARP entries. ARP entries are created when IP addresses are resolved from the hostnames and then into MAC addresses. This process is responsible to allow the devices to communicate with the different devices using IP addresses.

Reasons you should clear ARP cache from your Computer Device

Clearing ARP cache is not a needed task, in most cases rebooting your router or switch is enough to fix the connection issues.

But, with the time, arp database becomes stale and the new entries might not always override the expired entries in the database. So it can affect the network performance and may throw the errors that can affect your system.

How to Identify if there is ARP cache issue with your System?

You can't ping certain IP addresses when you know those are functioning properly.
You can't load websites that are working well or working properly on different devices.

If you facing this type of signs then you should clear ARP cache right now!

Clearing ARP cache will force all the requests in your database to go through the whole ARP process again.

How to clear ARP cache on Linux?

To look for arp cache entries, we can use arp command utility in linux. Run the following command to list all available entries:

$ arp -n Address                  HWtype  HWaddress           Flags Mask            Iface 192.168.1.240            ether   92:83:c6:3c:2a:9f   C                     eth0 192.168.225.1            ether   a2:70:ce:72:65:13   C                     eth0 192.168.1.26             ether   42:58:3a:d4:31:95   C                     eth0

Observe that we have three arp cache entries available in the cache table. We will flush arp cache table further.

There is mainly two methods for clearing arp cache on Linux machine.

using arp command
using ip command

Both command utilities are available in most of Linux distributions like Kali linux, Ubuntu, Arch Linux, CentOS, Debian, Fedora, RHEL, etc.

Clearing cache with arp command

arp is a command-line utility available in Linux to manage the cache of the system. arp command will allow users to clear specific cache entries. We can select which cache entries should be flushed or cache cleared.

Just use -d option to flush the shown entries

$ arp -d 192.168.1.240

To confirm which enties are deleted, run the following command,

$ arp -n Address                  HWtype  HWaddress           Flags Mask            Iface 192.168.225.1            ether   a2:70:ce:72:65:13   C                     eth0 192.168.1.26             ether   42:58:3a:d4:31:95   C                     eth0

Observe that the cache entry for 192.168.1.240 is cleared.

which means the ARP entry will be refreshed whenever it is needed.

Issue: Sometimes the command arp -d <IP-address> is used to remove cache, it leaves the table with entries where HWaddress is marked as (incomplete) but the entry still remains there. So the question is how to completely remove the arp entry from the table OR make the table empty?
We can use ip command utility for this type of issue resolution. Read the next point on "Clearing cache with ip command (Fully flush arp cache)"

Clearing cache with ip command (Fully flush arp cache)

Clearing arp cache using ip command is the most preferable way in Linux, as it can clear out the full ARP cache in one go. ip command is built-in in newer Linux distributions.

use the following command to clear full arp cache using ip utility:

$ ip -s -s neigh flush all          192.168.1.26          dev          eth0          lladdr          42:58:3a:d4:31:95          ref 1 used 14/14/14 probes 4 REACHABLE          192.168.1.240          dev          eth0          lladdr          92:83:c6:3c:2a:9f          ref 1 used 18/18/18 probes 4 REACHABLE          192.168.225.1          dev          eth0          lladdr          a2:70:ce:72:65:13          used 246/246/215 probes 5 STALE  *** Round 1, deleting 3 entries *** *** Flush is complete after 1 round ***

The first -s will provide extra verbose output.

Second -s to select the neighbor table. The neighbor table with the ip command equals to both the ARP and NDISC cache.

The command will fully flush ARP cache from the system in one go.

To confirm it, just run the following command,

$ arp -n

How to clear ARP cache in Windows?

Clearing arp cache in Windows is a super easy task. We will use the command prompt to clear arp cache or flush arp cache from the windows system. This method can be applied to any Windows OS like Windows 7, Windows 8, Windows 10, etc.

Here the requirement is, you have to run the command prompt(cmd) as administrator. If you are on an administrator account then it's fine, you can run the cmd directly. But, if you are not logged in as a system administrator then you have to start cmd as administrator to allow the NetShell command to work.

To view windows arp cache, just run arp -a command in command prompt.

To flush arp cache, just run the following the netsh command,

          netsh interface IP delete arpcache

After clearing arp cache, confirm it by rerunning arp -a command.

Observe the message "No ARP Entries Found.", which means the windows arp cache is cleared or flushed.

We can also use arp -d <ip address> to clear arp cache for specific entry only in windows.

How to clear ARP Cache in macOS or OS X

To clear or flush ARP cache in the macOS, follow these steps:

Open your terminal app.
Insert this command to view the current arp cache available in the system.

$ sudo arp -a  ? (192.168.1.240) at 92:83:c6:3c:2a:9f on en0 ifscope [ethernet] ? (192.168.1.26) at 42:58:3a:d4:31:95 on en0 ifscope [ethernet] ? (192.168.225.1) at a2:70:ce:72:65:13 on en0 ifscope permanent [ethernet]

Now use the following command to clear or flush the whole arp cache available in the macOS system.

$ sudo arp -d -a  192.168.1.240 (192.168.1.240) deleted 192.168.1.26 (192.168.1.26) deleted 192.168.225.1 (192.168.225.1) deleted

That is it your ARP cache is cleared

Hope this article helps you…