How ARP Works
The Layer 3 switch encapsulates IP packets in Layer 2 packets regardless of whether the ultimate destination is locally attached or is multiple router hops away. Because the Layer 3 switch IP route table and IP forwarding cache contain IP address information but not MAC address information, the Layer 3 switch cannot forward IP packets based solely on the information in the route table or forwarding cache. The Layer 3 switch needs to know the MAC address that corresponds with the IP address of either the locally attached destination of the packet or the next-hop router that leads to the destination.
For example, to forward a packet with a destination that is multiple router hops away, the Layer 3 switch must send the packet to the next-hop router toward its destination, or to a default route or default network route if the IP route table does not contain a route to the packet destination. In each case, the Layer 3 switch must encapsulate the packet and address it to the MAC address of a locally attached device, the next-hop router toward the IP packet destination.
To obtain the MAC address required for forwarding a datagram, the Layer 3 switch first looks in the ARP cache (not the static ARP table) for an entry that lists the MAC address for the IP address. The ARP cache maps IP addresses to MAC addresses. The cache also lists the port attached to the device and, if the entry is dynamic, the age of the entry. A dynamic ARP entry enters the cache when the Layer 3 switch receives an ARP reply or receives an ARP request (which contains the sender IP address and MAC address). A static entry enters the ARP cache from the separate static ARP table when the interface for the entry comes up.
To ensure the accuracy of the ARP cache, each dynamic entry has its own age timer. The timer is reset to zero each time the Layer 3 switch receives an ARP reply or ARP request containing the IP address and MAC address of the entry. If a dynamic entry reaches its maximum allowable age, the entry times out and the software removes the entry from the table. Static entries do not age out and can be removed only by you.
If the ARP cache does not contain an entry for the destination IP address, the Layer 3 switch broadcasts an ARP request out all its IP interfaces. The ARP request contains the IP address of the destination. If the device with the IP address is directly attached to the Layer 3 switch, the device sends an ARP response containing its MAC address. The response is a unicast packet addressed directly to the Layer 3 switch. The Layer 3 switch places the information from the ARP response into the ARP cache.
ARP requests contain the IP address and MAC address of the sender, so all devices that receive the request learn the MAC address and IP address of the sender and can update their own ARP caches accordingly.
For a device to communicate with a second deive, knowing the IP address of that device is not sufficient; the MAC address is also required. ARP supplies the MAC address.