packet and address it to the MAC address of a locally attached device, the next-hop router toward the IP packet's
destination.
To obtain the MAC address required for forwarding a datagram, the routing switch does the following:
• First, the routing switch 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 routing switch receives an ARP reply or receives an ARP request (which contains the sender's
IP address and MAC address.) A static entry enters the ARP cache from the static ARP table (which is a
separate 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 routing 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 routing switch broadcasts an
ARP request out all of 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 routing switch, the device sends an ARP response
containing its MAC address. The response is a unicast packet addressed directly to the routing switch. The
routing 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.
NOTE:
The ARP request broadcast is a MAC broadcast, which means the broadcast goes only to
devices that are directly attached to the routing switch. A MAC broadcast is not routed to other
networks. However, some routers, including routing switches, can be configured to reply to ARP
requests from one network on behalf of devices on another network. For more information, see
NOTE:
If the routing switch receives an ARP request packet that it is unable to deliver to the final
destination because of the ARP time-out, and no ARP response is received (the routing switch
knows of no route to the destination address), the routing switch sends an ICMP Host Unreachable
message to the source.
About enabling proxy ARP
Proxy ARP allows a routing switch to answer ARP requests from devices on one network on behalf of devices in
another network. Since ARP requests are MAC-layer broadcasts, they reach only the devices that are directly
connected to the sender of the ARP request. Thus, ARP requests do not cross routers.
For example, if Proxy ARP is enabled on a routing switch connected to two subnets, 10.10.10.0/24 and
20.20.20.0/24, the routing switch can respond to an ARP request from 10.10.10.69 for the MAC address of the
device with IP address 20.20.20.69. In standard ARP, a request from a device in the 10.10.10.0/24 subnet cannot
reach a device in the 20.20.20.0 subnet if the subnets are on different network cables, and thus is not answered.
An ARP request from one subnet can reach another subnet when both subnets are on the same physical
segment (Ethernet cable), since MAC-layer broadcasts reach all the devices on the segment.
Proxy ARP and local proxy ARP behavior
When local proxy ARP is enabled, all valid ARP requests receive a response.
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