642-891 Composite Exam Practice Exam

Demo Question 1.

Answer: A

Explanation: NETs and NSAPs are ISO addresses. The differences between the NET and NSAP addresses are subtle. The NET address is the address of the host, where the value in the NSEL field is set to 0x00. Therefore, there is no upper-layer protocol identified within the host. With no application identified within the end host, the packet can be routed to the destination, but it cannot be handed off to a process after it has been delivered. However, routers do not have upper-layer protocols to identify because they are transitory ISs. Therefore, the NSAP of the router is referred to as a NET because the NSEL field is set to 00. The NSAP is the full ISO address. It not only defines the area and destination host within the area, but also specifies where to send the incoming packet after it has reached the host. The NSEL field at the end of the ISO address specifies the upper-layer protocol and is similar to the Protocol field of the IP header. Network Entity Title (NET) In IS-IS, this is the ISO address of the system, but not to the process destination within the system. The NET describes both the area and system ID of a system in the IS-IS network but excludes the NSEL, which is set to 0x00. If the NSEL identifies the process within the system, the ISO address is called the "NSAP address." Network Service Access point (NSAP) In IS-IS, this describes a service at the network layer to which the packet is to be directed. The NSAP is the NET address with the NSEL field set to a positive value, a value other than 0x00. Network entity title (NET) The NET describes both the area and system ID of a system in the IS-IS network but excludes the NSEL, which defines the NSAP address of the system. Network service access point (NSAP) Describes a service at the network layer to which the packet is to be directed. The NSAP is the NET address with the SEL field set to a value other than 0x00.