The local address or MAC address is known as a data link address because it is contained in the data link layer (Layer 2) section of the packet header and is read by data link (Layer 2) devices. To communicate with a device on Ethernet, the software must learn the MAC address of the device. The process of learning the MAC address from an IP address is called address resolution . The process of learning the IP address from the MAC address is called reverse address resolution . The device can use these forms of address resolution: • Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses.
Taking an IP address as input, ARP learns the associated MAC address and then stores the IP address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. • Proxy ARP helps hosts with no routing tables learn the MAC addresses of hosts on other networks or subnets. If the device (router) receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to the host through other interfaces, it generates a proxy ARP packet giving its own local data link address.
The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host. The device also uses the Reverse Address Resolution Protocol (RARP), which functions the same as ARP does, except that the RARP packets request an IP address instead of a local MAC address. Using RARP requires a RARP server on the same network segment as the router interface. Use the ip rarp-server address interface configuration command to identify the server. Proxy ARP Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no routing information to communicate with hosts on other networks or subnets.
The host assumes that all hosts are on the same local Ethernet and that they can use ARP to learn their MAC addresses. If a device receives an ARP request for a host that is not on the same network as the sender, the device evaluates whether it has the best route to that host. If it does, it sends an ARP reply packet with its own Ethernet MAC address, and the host that sent the request sends the packet to the device, which forwards it to the intended host. Proxy ARP treats all networks as if they are local, and performs ARP requests for every IP address.
ICMP Router Discovery Protocol Router discovery allows the device to dynamically learn about routes to other networks using ICMP router discovery protocol (IRDP). IRDP allows hosts to locate routers. When operating as a client, the device generates router discovery packets. When operating as a host, the device receives router discovery packets. The device can also listen to Routing Information Protocol (RIP) routing updates and use this information to infer locations of routers. The device does not actually store the routing tables that are sent by routing devices; it merely keeps track of which systems are sending the data.
The advantage of using IRDP is that it allows each router to specify both a priority and the time after which a device is assumed to be down if no further packets are received. Each device that is discovered becomes a candidate for the default router, and a new highest-priority router is selected when a higher priority router is discovered, when the current default router is declared down, or when a TCP connection is about to time out because of excessive retransmissions. IRDP packets are not sent while enabling or disabling IP routing. When interface is shutting down, the last IRDP message does not have a lifetime; it is 0 for all routers.
Configuring IP Unicast Routing Proxy ARP Configuring IP Unicast Routing 7