Networks Horizon

Inverse ARP

Inverse Address Resolution Protocol (Inverse ARP or InARP) is used to obtain Network Layer addresses (for example, IP addresses) of other nodes from Data Link Layer (Layer 2) addresses. It is primarily used in Frame Relay (DLCI) and ATM networks. On by default on FR network but can be explictly disabled but ARP response cannot be disabled.

Inverse ARP is implemented as a protocol extension to ARP: it uses the same packet format as ARP, but different operation codes.  InARP does not work without LMIs because it uses LMI messages to determine which PVC to map (remember that LMI carries information for all configured PVCs). If the DLCI is down, the Cisco router still processes and maps an InARP, but does not use it until DLCI is reported as active.

So Inverse ARP is primarily used to get layer 3 address information of far end that is associated with the virtual circuit or DLCI in ATM and FR network respectively.

Some of the key points in Inverse ARP:

• Access Router (CE) should initiate the InARP protocol and format InARP requests for each active PVC for which InARP is active.
• The Access router uses Inverse ARP responses to populate an Layer 3 address-to-DLCI mapping table.
• InverseARP request contains source hardware and source IP addresses and the known target hardware address.
• Enable by default on cisco router and can be explictly disable to faciliate static mapping.
• The router uses the RARP responses to populate an address-to-DLCI mapping table.