At the most basic, ARP information helps identify which devices are physically connected to the router and their associated IP addresses. The provided ARP information can also help network administrators manage network troubleshooting and optimization. Incorrect or out-of-date entries in the ARP table can cause network issues. While the ARP cache cannot be manipulated directly, the ARP table information can give you an idea of where to find a specific networking challenge.
Aside from the Reachable state (which indicates the interface is confirmed and passing traffic normally), ARP resolution issues can be caused by the following:
Adverse states - The state column displays one of the following color-coded ARP entry states:
Stale - An ARP table entry is marked as Stale when a device on the network has not recently communicated with the device associated with that entry, and the entry's reachable time (or aging timer) has expired. The reachable time is a set period of time (30 seconds) during which the device considers the entry to be up-to-date and reliable.
Delay - A Delay status is applied when an ARP table entry has been listed for a period of time without being used, and was assigned a Stale state previously. When a packet is sent to a Stale interface, it is moved to the Delay status. The ARP table then waits for a period of time to see whether a responding TCP packet comes from the Stale interface. If ARP does see a response, the state is then moved to Reachable. If there is no detectable response from the interface, ARP will send out a new request to confirm the MAC address, and if there is still no response the device will be assigned a Failed state.
Failed - An entry shows as Failed when ARP has unsuccessfully tried to find the MAC address for a specific IP address. A number of requests have been sent out but no reply has been received. See Common Reasons for Failed ARP Entry State below.
Duplicate IP addresses - The table makes it easy to find duplicate IP addresses.
Erroneous static ARP entries - See Configuring Static IP Routes for more information about the topic.
Router configuration errors - Unknown or unidentified entries can help direct network error investigations.
Common Reasons for Failed ARP Entry State
Common reasons for a Failed state include the following:
The device is offline: The most obvious reason. If a device is powered off or disconnected from the network, it cannot respond to ARP requests.
Network issues: There might be a problem with the physical connection (a bad cable, a down port) or a broadcast storm that is preventing the ARP request or the response from getting through.
Incorrect subnet mask: If a device is configured with the wrong subnet mask, it might attempt to contact a destination wrongly assumed to be on the local network. ARP requests sent for that device fail because the device is actually on a different network and needs to be reached through a router.
Outdated entries: A Stale or Delay entry can eventually transition to a Failed state if the device that it was trying to reach never responds to the re-verification attempts. Typically, the reachable timer is 30 seconds, with three retries attempted before an entry is marked as Failed.