NSAP AddresstoHost Name Mapping

NSAP Address “to “Host Name Mapping

As discussed in the preceding section, an NSAP can be long (up to 20 bytes, compared to 4 bytes in IP addresses). Also, the hexadecimal representation doesn't provide any operational convenience for router configuration, troubleshooting, or routine operations activities, such as inspection and maintenance of the IS-IS routing environment. Working with long NSAP addresses might be a daunting task, especially in high-pressure troubleshooting situations.

The SysID appears in several attributes, such as adjacency information, identifiers of link-state packets (LSPs) in the IS-IS database, and features in the output of many commands that are used by network operators to gather information about the network. Most people are better at working with symbolic names than with numeric representations. With the great success of the IP Domain Name System (DNS), it is obvious that a similar facility for translating between the hexadecimal representations of NSAP addresses and router host names is a necessary convenience for the network operations staff.

Cisco IOS Software provides two mechanisms for the router host name “to “NSAP address mapping. These mechanisms essentially provide a host name-to-SysID mapping. The first and older method, static host name mapping, uses static mapping statements in the router's configuration. The alternative method, dynamic host name mapping, was standardized only recently in the IETF. It employs a dynamic mechanism to achieve the same purpose. The following sections review these two methods .

Static Host Name Mapping

The IOS command clns host <hostname> <nsap> creates static host name “to “NSAP address (actually SysID) map tables on Cisco routers. This method has been available for a while and, despite its manual approach and usability challenges in large networks, it offers convenience to many network operations engineers who work with IS-IS.

Example 4-11 shows RTA and RTB configured with static CLNP host statements that enable the routers to resolve each other's NSAP (actually the SysID) to the corresponding name. This allows a router's name to be used in place of the SysID component in the Link-State Packet Identifier (LSP ID), providing tremendous convenience when troubleshooting or reviewing entries in the IS-IS Link-State database.

Example 4-11 Static Host Name “to “NSAP Address Mapping

 RTA Router isis Net 49.0001.1111.2222.3333.00 CLNS host RTA 49.0001.1111.2222.3333.00 CLNS host RTB 49.0001.4444.4444.6666.00 RTB Router isis Net 49.0001.4444.5555.6666.00 CLNS host RTA 49.0001.1111.2222.3333.00 CLNS host RTB 49.0001.4444.4444.6666.00 

Dynamic Host Name Mapping

The challenges involved in maintaining static CLNS host tables in large networks, and the obvious success and convenience of IP DNS, inspired the need for a dynamic approach for mapping and resolving host names to NSAP addresses. RFC 2763 specifies an enhancement to the IS-IS protocol that allows host name “to “NSAP address mapping information to be transported within the IS-IS protocol itself rather than an external DNS-like application. This automatic mechanism for gathering host name “to “NSAP address mapping information between IS-IS routers is significantly more convenient than the cumbersome manual approach that relies on static tables. RFC 2763 introduces an optional TLV (Type 137), which is carried in LSPs of participating routers, providing a simple and reliable mechanism for advertising host name information. Type, Length, and Value (TLV) fields are discussed in detail in Chapter 5.

TLV 137 is used for carrying a 7-bit ASCII representation of a router's host name, 1 byte per character. The length of this TLV ranges from 1 to 255 bytes. Routers running implementations of IS-IS that support optional TLV 137 read and install host names with corresponding SysID information in a host table that is used to resolve the SysID component of IDs of LSPs in the Link-State database. The dynamic host table also allows router host names to be used with the CLNS Traceroute and Ping troubleshooting tools.

Cisco routers running older releases of IOS, and other vendor routers that cannot interpret TLV 137, should just ignore it when encountered in an LSP. Obviously, for such routers, static host name mapping is the only alternative. Static entries must also be configured on all routers in the network for such routers that do not support TLV 137.