This chapter describes how to use the Address Resolution Protocol (ARP) and the Inverse Address Resolution Protocol (Inverse ARP) on your router. It includes the following sections:
The ARP Protocol is a low-level protocol that dynamically maps network layer addresses to physical medium access control (MAC) addresses. Given only the network layer address of the destination system, ARP locates the MAC address of the destination host within the same network segment.
For example, a router receives an IP packet destined for a host connected to one of its LANs. The packet contains only a 32-bit IP destination address. To construct the data link layer header, a router acquires the physical MAC address of the destination host. Then, the router maps that address to the 32-bit IP address. This function is called address resolution. Figure 47 illustrates how ARP works.
Figure 47. ARP Address Resolution Broadcast
When a router translates a network layer address to a physical address, the router accesses the ARP (translation) cache. The ARP cache contains the physical MAC address that corresponds to that network layer address. If the address is missing, the router broadcasts an ARP request to all hosts on the attached network segment to locate the correct physical MAC address. The node with the correct physical MAC address responds to the router. The router then sends the packet to the node and enters the physical MAC address into the translation cache for future use.
Inverse ARP, described in RFC 1293/2390, was created for Frame Relay networks. This protocol defines a method for routers on a Frame Relay network to learn the protocol addresses of other routers in a way that very efficiently reduces traffic by eliminating the need to use broadcast ARP packets for address resolution. Inverse ARP discovers a protocol address by sending Inverse ARP request packets to the hardware address (for Frame Relay circuits the circuit identifier is the Frame Relay equivalent of a hardware address), as soon as the circuit becomes active. The remote router responds with its protocol address and the resulting mapping is stored in the ARP cache.
The protocol address-to-hardware address entries learned by Inverse ARP do not time out when the ARP refresh timer expires. The mappings do not age at all except when the Frame Relay circuit goes down. This means that the router does not need to transmit any ARP broadcasts to update the ARP cache. However, the router permits updates to an entry when the other (remote) router changes its protocol address.
Support for both ARP and Inverse ARP greatly enhances the router's interoperability with other vendors' routers over Frame Relay for dynamic mapping of protocol and hardware addresses. If other Frame Relay-attached routers support Inverse ARP, then the mappings are dynamically learned as described above. If the attached routers do not support Inverse ARP but support "traditional" ARP on Frame Relay, then the mappings still could be learned dynamically using ARP exchanges (see Figure 47).
If needed, you can manually configure the protocol addresses of other routers using the Frame Relay configuration command add protocol-address. For additional information, see the chapter Configuring and Monitoring Frame Relay Interfaces in Access Integration Services Software User's Guide.