288 C HAPTER 11: INTERNET P ROTOCOL (IP)ImportantConsiderationsKeep the following things in mind about this protocol:n Enter a static ARP entry when the ARP resolution does not result in anARP entry in the cache. For example, some applications do notrespond to ARP requests and, consequently, specific networkoperations may time out for lack of address resolution.n Enter a static ARP entry in a test environment if your test analyzercannot respond to an ARP request.n Setting an ARP cache age time of zero (no aging) is useful in themiddle of lengthy tests so that ARP requests do not have to be issued.If you do set an ARP cache age time of zero, be aware that the ARPcache can quickly grow in size and consume system resources. In thiscase, be sure to flush the ARP cache after your tests are complete.ARP Proxy ARP proxy allows a host that has no routing ability to determine the MACaddress of a host on another network or subnet.When ARP proxy is enabled and a workstation sends an ARP request for aremote network, the system determines if it has the best route and thenanswers the ARP request by sending its own MAC address to theworkstation. The workstation then sends the frames for the remotedestination to the system, which uses its own routing table to reach thedestination on the other network.ImportantConsiderationsConsider the following issues with ARP proxy:n Do not use ARP proxy if you are using VLSMs because ARP proxyworks by seeing the entire network configuration as one network.n ARP proxy increases ARP traffic to handle the increased mapping of IPaddresses to MAC addresses.Example In the following example, Server A cannot use the router as a gateway toServer B (if ARP proxy is disabled) because Server A has its subnet maskset to broadcast (using ARP) its IP network address as 158.101.0.0, whilethe IP network address of the router is 158.101.1.0.
