AIS AI2524 user manual Configure Tarp on ISO Clns, Pdu

Chapter 9: AI2524 OSI/CLNP Configuration Steps

Configure TARP on ISO CLNS

Some applications (typically used by telephone companies) running on Synchronous Optical Network (SONET) devices identify these de- vices by a target identifier (TID). Therefore, it is necessary for the router to cache TID-to-network address mappings. Because these ap plications usually run over OSI, the network addresses involved in the mapping are OSI NSAPs.

When a device must send a packet to another device it does not know about (that is, it does not have information about the NSAP address corresponding to the remote device's TID), the device needs a way to request this information directly from the device, or from an interme- diate device in the network. This functionality is provided by an ad- dress resolution protocol called Target Identifier Address Resolution Protocol (TARP).

Requests for information and associated responses are sent as TARP protocol data units (PDUs), which are sent as CLNP data packets. TARP PDUs are distinguished by a unique N-selector in the NSAP ad- dress. Here are the five types of TARP PDUs:

zType 1—Sent when a device has a TID for which it has no match- ing NSAP. Type 1 PDUs are sent to all Le vel1 (IS-IS and ES-IS) neighbors. If no response is received within the specified time lim- it, a Type 2 PDU is sent. To prevent packet looping, a loop detec- tion buffer is maintained on the router. A Type 1 PDU is sent when you use the tarp resolve command.

zType 2—Sent when a device has a TID for which it has no match- ing NSAP and no response was received from a Type 1 PDU. Type 2 PDUs are sent to all Le vel1 and Lev el2 neighbors. A time limit for Type 2 PDUs can also be specified. A Type 2 PDU is sent when you use the tarp resolve command and specify the op- tion 2.

z	Type 3—Sent as a response to a Type 1, Type 2, or Type 5	PDU.
	Type 3 PDUs are sent directly to the originator of the request.
z	Type 4—Sent as a notification when a change occurs loca	lly (fo
	example, a TID or NSAP change). A Type 4 PDU is sent when you
	use the tarp query command.
z	Type 5—Sent when a device needs a TID that correspo nds to a
	specific NSAP. Unlike Type 1 and Type 2 PDUs that are sent to

all Leve l1 and Level 2 neighbors, a Type 5 PDU is sent only to a particular router.

In addition to the type, TARP PDUs contain the sender's NSAP, th sender's TID, and the target's TID (if the PDU is a Type 1 orType 2).

August 1997	Page 9-59
2524UM