IS-IS Overview
On the Services Router, Intermediate System-to-Intermediate System (IS-IS) protocol is an interior gateway routing protocol (IGP) that uses link-state information for routing network traffic. IS-IS uses the shortest path first (SPF) algorithm to determine routes. Using SPF, IS-IS evaluates network topology changes and determines if a full or partial route calculation is required. The protocol was originally developed for routing International Organization for Standards (ISO) connectionless network protocol (CLNP) packets.
This overview contains the following topics:
ISO Network Addresses
IS-IS uses ISO network addresses. Each address identifies a point of connection to the network, such as a router interface, which is called a network service access point (NSAP). NSAP addresses are supported on the loopback (lo0) interface. (For information about interface names, see Network Interface Naming.)
An end system can have multiple NSAP addresses, which differ by the last byte called an n-selector. Each NSAP represents a service that is available at the node. In addition to multiple services, a single node can belong to multiple areas.
Each network entity also has a special address called a network entity title (NET) with an identical structure to an NSAP address but an n-selector of 00. Most end systems and intermediate systems have one NET address, while intermediate systems participating in more than one area can have more than one NET address.
The following ISO addresses are examples of the IS-IS address format:
49.0001.00a0.c96b.c490.00
49.0001.2081.9716.9018.00
The first part of the address is the area number, which is a variable number from 1 to 13 bytes. The first byte of the area number, 49, is the authority and format indicator (AFI). The next bytes are the assigned area identifier and can be from 0 to 12 bytes. In the examples, 0001 is the area identifier.
The next 6 bytes are the system identifier and can be any 6 bytes unique throughout the entire domain. The system identifier is commonly the media access control (MAC) address, as shown in the first example, 00a0.c96b.c490. Otherwise, the system identifier is the IP address expressed in binary-coded decimal (BCD) format, as shown in the second example, 2081.9716.9018, which corresponds to 208.197.169.18. The last byte, 00, is the n-selector.
 | Note: The system identifier cannot be configured as 0000.0000.0000. Using all zeros as an identifier is not supported and does not form an adjacency. |
System Identifier Mapping
To provide assistance with debugging IS-IS, the Services Router supports dynamic mapping of ISO system identifiers to the hostname. Each router can be configured with a hostname that allows the system identifier-to-hostname mapping to be sent in a dynamic hostname type length value (TLV) in the IS-IS link-state PDU (LSP). The mapping permits an intermediate system in the routing domain to learn the ISO system identifier of another intermediate system.