| 1: | What does the acronym NSAP represent and what is an NSAP used for? |
| A: | NSAP means network service access point. This is OSI terminology for a network layer address. NSAP addresses provide the foundation for routing datagrams in a CLNP network, and IS-IS functions are designed around NSAPs. Therefore, IP routers need to be configured with NSAPs when using IS-IS for IP routing. |
| 2: | What are the three major components of an NSAP? Describe the significance of each. |
| A: | The three components are area address (area ID), system ID (SysID), and the NSAP selector (NSEL). The area address identifies the native area of a network node and helps determine the type of adjacencies one node forms with another. The SysID is a unique identifier of a node within an IS-IS area. The NSEL determines the higher level user of the network service that packets must be delivered to for processing at a node. The value of the NSEL is 0x00 for the routing layer. |
| 3: | What is the maximum length of an NSAP and what is the minimum length that can be configured on a Cisco router? |
| A: | The maximum length of an NSAP is 160 bits (20 bytes). The minimum size that can be configured on a Cisco router is 8 bytes. The 8 bytes includes 1 byte of NSEL, 6 bytes of SysID, and 1 byte of area ID. However, it is recommended that the AFI stands apart from the actual area value in the area ID. Therefore, 9 bytes makes more sense where 2 bytes are allocated for the area, one of which is the AFI. |
| 4: | What's the AFI field in an NSAP, and what is its significance? |
| A: | AFI stands for address and format identifier. The AFI designates the top-level address domain to which the NSAP belongs and also defines the syntax (binary, decimal, or character) of the domain-specific part of the NSAP. |
| 5: | How many OSI top-layer address domains exist? List them. |
| A: | There are seven top-layer OSI addressing domains: -
X.121 ” International plan for public data networks -
ISO DCC ” Data country code -
F.69 ” Telex -
E.163 ” Public Switched Telephone network -
E.164 ” ISDN -
ISO 6523 ” International code designator ( ICD ) for organizations -
Local ” For local use only within a network domain |
| 6: | Associate the following addresses with one of these top-level address domains: -
39.0005.1100.2200.432A.26CD.00 -
47.0001.2211.3311.5566.ACD7.2351.00AC.210700 |
| A: | You can associate an NSAP address with one top-level domain from the value of its AFI, which is the leftmost byte in the address: -
AFI value of 39 is ISO DCC. -
AFI value of 47 is ISO 6523(ICD). |
| 7: | How many bytes of the NSAP are allocated to the SysID on a Cisco router? What is the value specified by ISO 10589? |
| A: | Cisco follows the convention specified by the US GOSIP version 2 standard, which requires 6 bytes for the system ID field. ISO 10589 specifies a range of 1 to 8 bytes. |
| 8: | IS-IS has two levels of routing, Level 1 and Level 2. Elaborate on the relevance of the major fields of the NSAP to these routing levels in the ISO CLNS environment. |
| A: | Level 1 routing is based on only the System ID field in the NSAP, whereas Level 2 routing uses only the Area field. On a Cisco router, the combined length of the System ID and N-Selector fields is always 7 bytes, so the area address can easily be discerned as the remainder of the NSAP after stripping the trailing 7 bytes. |
| 9: | List some of the requirements and caveats for defining the system ID on a device. |
| A: | The system ID of all nodes in the routing domain must have the same length. On Cisco routers, the system ID length must be 6 bytes. Each node in an area must have a unique system ID. |
| 10: | How many NSAPs can you have per router according to ISO 10589? What is the purpose of having more than one NSAP per router? |
| A: | According to ISO 10589, a router can have up to three NSAPs, all of which must use the same system ID and 0x00 for the N-selector but different area prefixes. Multiple NSAPs per router might be necessary for renumbering NSAPs in an area or domain, partitioning an area or merging different areas in a network domain. |
| 11: | What does SNPA stand for and what is its relevance in the IS-IS routing environment? |
| A: | SNPA stands for subnetwork point of attachment. It has no relevance to the Subnetwork Access Point ( SNAP ) field associated with the Ethernet 802.3 SNAP frame format. SNPA is the ISO name for a data-link address, such as a MAC or a Frame Relay DLCI address. Layer 3 routes point to an outgoing data-link interface that is also described by its address, the SNPA. |
| 12: | Identify the area address, SysID, and NSEL values in the following address: |
| A: | 47.005.8001.443E.AB11.BD48.0C1F.00 The NSAP address components are as follows: Area: 47.005.8001.443E System ID: AB11.BD48.0C1F N-selector: 00 |
