ARP cache and static ARP table

The ARP cache contains entries that map IP addresses to MAC addresses. A static ARP table contains entries that are user-configured.

The ARP cache entries are generally for devices that are directly attached to the Layer 3 device.

An exception is an ARP entry for an interface-based static IP route that goes to a destination that is one or more router hops away. For this type of entry, the MAC address is either the destination device MAC address or the MAC address of the router interface that answered an ARP request on behalf of the device, using proxy ARP.

ARP cache

The ARP cache can contain dynamic (learned) entries and static (user-configured) entries. The software places a dynamic entry in the ARP cache when the Layer 3 device learns a device MAC address from an ARP request or ARP reply from the device.

The software can learn an entry when the Layer 2 device or Layer 3 device receives an ARP request from another IP forwarding device or an ARP reply. Here is an example of a dynamic entry:

IP Address          MAC Address         Type        Age     Port        0000.00fc.ea21      Dynamic     0         6

Each entry contains the destination device IP address and MAC address.

Static ARP table

In addition to the ARP cache, Layer 3 devices have a static ARP table. You can add entries to the static ARP table regardless of whether or not the device the entry is for is connected to the Layer 3 device.

Layer 3 devices have a static ARP table. Layer 2 switches do not.

The software places an entry from the static ARP table into the ARP cache when the entry interface comes up.

Here is an example of a static ARP entry.

Index  IP Address     MAC Address       Port
1    0000.003b.d210    1/1/1

Each entry lists the information you specified when you created the entry.