Inverse ARP (InARP), UNARP, and DirectedARPInverse ARP is a feature used in ATM and Frame Relay dynamic address mapping. It is used to request the next-hop protocol address given a data-link connection identifier (DLCI) or ATM virtual path identifier/virtual circuit identifier (VPI/VCI) pair. The result is an address-to-DLCI or -VPI/VCI mapping table. In addition, static mappings can be configured. Inverse ARP is a multiprotocol mechanism on Cisco routers and turned on per default for Frame Relay (all protocols). Go to Cisco.com for example configurations.Examples 7-8 and 7-9 show a brief hub-and-spoke Frame Relay configuration example to emphasize this point. Keep in mind that this setup requires an intermediate Frame Relay switch; hence the name "Frame Relay" or "cell relay" for ATM networks. This will not work in a crossover fashion. Every Layer 2 WAN protocol deals with ARP issues in its own way.Example 7-8. Cisco Frame Relay Point-to-Multipoint Example (Hub Configuration Fragment)

scar# show running-config

...

!

interface Serial1

no ip address

encapsulation frame-relay

!

interface Serial1.1 multipoint

ip address 10.1.1.3 255.255.255.0

frame-relay map ip 10.1.1.1 160 broadcast

!

...

Example 7-9. Cisco Frame Relay Point-to-Multipoint Example (Spoke Configuration Fragment)

simba# show running-config

...

!

interface Serial0

ip address 4.0.1.1 255.255.255.0

encapsulation frame-relay

frame-relay map ip 10.1.1.2 140 broadcast

!

...

Interesting future developments (quoted from the RFCs) include the following:UNARP (RFC 1868):The Address Resolution Protocol allows an IP node to determine the hardware (datalink) address of a neighboring node on a broadcast network. The protocol depends on timers to age away old ARP entries. This document specifies a trivial modification to the ARP mechanism, not the packet format, which allows a node to announce that it is leaving the network and that all other nodes should modify their ARP tables accordingly.DirectedARP (RFC 1433):Directed ARP is a dynamic address resolution procedure that enables hosts and routers to resolve advertised potential next-hop IP addresses on foreign IP networks to their associated link level addresses.