When an interface requests a mapping for an address not in the cache, the SNPA of any known Intermediate System is returned. If an Intermediate System is not known, then the
all end systems multicast address is returned. It is assumed that the intended recipient of the NPDU will immediately transmit a “hello” PDU back to the originator of the NPDU.
If an NPDU is forwarded by the End System, a redirect PDU will not be generated. However, redirect PDUs received will be processed. This processing consists of adding an entry in the routing table. If the redirect is towards an Intermediate System, then an entry is made in the routing table as well. The entry in the routing table will mark the NSAP address contained in the redirect PDU as the gateway for the destination system (if an NET is supplied), or will create a route with the NSAP address as the destination and the SNPA address (embodied as a link-level struct sockaddr) as the gateway.
If the System is configured as an End System, it will report all the NSAPs that have been configured using the
ifconfig(8) command, and no others. It is possible to have more than one NSAP assigned to a given interface, and it is also possible to have the same NSAP assigned to multiple interfaces. However, any NSAP containing an NSEL that is consistent with the
nsellength option (default one) of any interface will be accepted as an NSAP for this System.