Chapter 9: AI2524 OSI/CLNP Configuration Steps
August 1997 Page 9-59
2524UM
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 map pin gs. 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 respo nses 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 Level1 (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 Level1 and Lev el2 neighbors. A time
limit for Type 2 PDUs can also be spe cified. A T ype 2 PDU is sent
when you use the tarp resolve command and specify the op-
tion 2.
zType 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.
zType 4—Sent as a notification when a change occurs locally (fo
example, a TID or NSAP change). A Type 4 PDU is sent when you
use the tarp query command.
zType 5—Sent when a device needs a TID that corresponds to a
specific NSAP. Unlike Type 1 and Type 2 PDUs that are sent to
all Leve l1 and Level 2 neighbors, a Typ e 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 or Type 2).