374 CHAPTER 16: IP R OUTINGtarget protocol address, the receiving device places its MAC address inthe target hardware address field and sends the packet back to thesource hardware address.When the originating host or router receives this ARP reply, it places thenew MAC address in its ARP cache next to the corresponding IP address.See Figure 41.Figure 41 Example of ARP Cache Updated with ARP ReplyAfter the MAC address is known, the host or router can send the packetdirectly to the next hop.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 module resources. In thiscase, be sure to flush the ARP cache after your tests are complete.n You can keep ARP cache entries if you refresh the ARP cache;otherwise, the Multilayer Switching Module removes the entries afterthey reach their defined age time.ARP cache158.101.1.1158.101.2.1158.101.3.1IP address MAC address00308e3d00420080232b00ab0134650f3000
