Dell PowerConnect 6248 User's Guide - Page 500

ARP The PowerConnect 6200 Series uses the ARP protocol to associate a layer 2 MAC address with a layer 3 IPv4 address. Additionally, the administrator can statically add entries into the ARP table. ARP is a necessary part of the internet protocol (IP) and is used to translate an IP address to a media (MAC) address, defined by a local area network (LAN) such as Ethernet. A station needing to send an IP packet must learn the MAC address of the IP destination, or of the next hop router, if the destination is not on the same subnet. This is achieved by broadcasting an ARP request packet, to which the intended recipient responds by unicasting an ARP reply containing its MAC address. Once learned, the MAC address is used in the destination address field of the layer 2 header prepended to the IP packet. The ARP cache is a table maintained locally in each station on a network. There are no specific requirements for the construction or maintenance of this cache, but at a minimum it needs to contain the information learned from processing ARP protocol packets, which for Ethernet are denoted by an 0x0806 EtherType field. ARP cache entries are learned by examining the source information in the ARP packet payload fields, regardless of whether it is an ARP request or response. Thus, when an ARP request is broadcast to all stations on a LAN segment or virtual LAN (VLAN), every recipient has the opportunity to store the sender's IP and MAC address in their respective ARP cache. The ARP response, being unicast, is normally seen only by the requestor, who stores the sender information in its ARP cache. Newer information always replaces existing content in the ARP cache. The ARP cache can have between 256 and 896 entries When multiple network interfaces are supported by a device, as is typical of a router, either a single ARP cache is used for all interfaces, or a separate cache is maintained per interface. While the latter approach is useful when network addressing is not unique per interface, this is not the case for Ethernet MAC address assignment so a single ARP cache is employed. Devices can be moved in a network, which means the IP address that was at one time associated with a certain MAC address is now found using a different MAC, or may have disappeared from the network altogether (i.e., it has been reconfigured, disconnected, or powered off). This leads to stale information in the ARP cache unless entries are updated in reaction to new information seen on the network, periodically refreshed to determine if an address still exists, or removed from the cache if the entry has not been identified as a sender of an ARP packet during the course of an ageout interval, usually specified through configuration. The ARP menu page contains links to web pages that configure and display ARP detail. To display this page, click Routing →ARP in the tree view. Following are the web pages accessible from this menu page: • ARP Create • ARP Table Configuration ARP Create Use the ARP Create page to add an entry to the Address Resolution Protocol table. To display the page, click Routing →ARP →ARP Create in the tree view. 500 Configuring Routing