KB.15.15

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 HP routing switches, can be configured

to reply to ARP requests from one network on behalf of devices on another network. For more

information, see “About enabling proxy ARP” (page 119).

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.

Configuring IP parameters for routing switches 119