IS-IS Overview
IS-IS protocol is an interior gateway protocol (IGP) that uses link-state information to make routing decisions.
IS-IS is a link-state IGP that uses the shortest path first (SPF) algorithm to determine routes. IS-IS evaluates the topology changes and determines whether to perform a full SPF recalculation or a partial route calculation (PRC). This protocol originally was developed for routing International Organization for Standardization (ISO) Connectionless Network Protocol (CLNP) packets.
 | Note: Because IS-IS uses ISO addresses, the configuration of IP version 6 (IPv6) and IP version 4 (IPv4) implementations of IS-IS is identical. |
This section discusses the following topics:
IS-IS Terminology
An IS-IS network is a single autonomous system (AS), also called a routing domain, that consists of end systems and intermediate systems. End systems are network entities that send and receive packets. Intermediate systems send and receive packets and relay (forward) packets. (Intermediate system is the Open System Interconnection [OSI] term for a router.) ISO packets are called network protocol data units (PDUs).
In IS-IS, a single AS can be divided into smaller groups called areas. Routing between areas is organized hierarchically, allowing a domain to be administratively divided into smaller areas. This organization is accomplished by configuring Level 1 and Level 2 intermediate systems. Level 1 systems route within an area; when the destination is outside an area, they route toward a Level 2 system. Level 2 intermediate systems route between areas and toward other ASs.
ISO Network Addresses
IS-IS uses ISO network addresses. Each address identifies a point of connection to the network, such as a router interface, and is called a network service access point (NSAP).
IS-IS supports multiple NSAP addresses on the loopback (lo0) interface.
An end system can have multiple NSAP addresses, in which case the addresses differ only by the last byte (called the n-selector). Each NSAP represents a service that is available at that node. In addition to having multiple services, a single node can belong to multiple areas.
Each network entity also has a special network address called a network entity title (NET). Structurally, an NET is identical to an NSAP address but has an n-selector of 00. Most end systems and intermediate systems have one NET. Intermediate systems that participate in multiple areas can have multiple NETs.
The following ISO addresses illustrate the IS-IS address format:
The first portion of the address is the area number, which is a variable number from 1 through 13 bytes. The first byte of the area number (49) is the authority and format indicator (AFI). The next bytes are the assigned domain (area) identifier, which can be from 0 through 12 bytes. In the examples above, the area identifier is 0001.
The next six bytes form the system identifier. The system identifier can be any six bytes that are unique throughout the entire domain. The system identifier commonly is the media access control (MAC) address (as in the first example, 00a0.c96b.c490) or the IP address expressed in binary-coded decimal (BCD) (as in the second example, 2081.9716.9018, which corresponds to IP address 208.197.169.18). The last byte (00) is the n-selector.
| Note: The system identifier cannot be 0000.0000.0000. All 0s is an illegal setting and the adjacency is not formed with this setting. |
To provide help with IS-IS debugging, the Junos OS supports dynamic mapping of ISO system identifiers to the hostname. Each system can be configured with a hostname, which allows the system identifier-to-hostname mapping to be carried in a dynamic hostname type length value (TLV) in IS-IS link-state protocol data units (LSPs). This permits ISs in the routing domain to learn about the ISO system identifier of a particular IS.
IS-IS Packets
IS-IS uses the following protocol data units (PDUs) to exchange protocol information:
- IS-IS hello (IIH) PDUs—Broadcast to discover the identity of neighboring IS-IS systems and to determine whether the neighbors are Level 1 or Level 2 intermediate systems.
- Link-state PDUs —Contain information about the state of adjacencies to neighboring IS-IS systems. Link-state PDUs are flooded periodically throughout an area.
- Complete sequence number PDUs (CSNPs)—Contain a complete list of all link-state PDUs in the IS-IS database. CSNPs are sent periodically on all links, and the receiving systems use the information in the CSNP to update and synchronize their link-state PDU databases. The designated router multicasts CSNPs on broadcast links in place of sending explicit acknowledgments for each link-state PDU .
- Partial sequence number PDUs (PSNPs)—Multicast by a receiver when it detects that it is missing an link-state PDU; that is, when its link-state PDU database is out of date. The receiver sends a PSNP to the system that transmitted the CSNP, effectively requesting that the missing link-state PDU be transmitted. That router, in turn, forwards the missing link-state PDU to the requesting router.
Persistent Route Reachability
IPv4 and IPv6 route reachability information in IS-IS link-state PDUs is preserved when you commit a configuration. IP prefixes are preserved to their original packet fragment upon LSP regeneration.
