IS-IS Overview

The Intermediate System-to-Intermediate System (IS-IS) protocol is a classless interior routing protocol developed by the International Organization for Standardization (ISO) as part of the development of the Open Systems Interconnection (OSI) protocol suite. Like OSPF routing, IS-IS uses hello packets that allow network convergence to occur quickly when network changes are detected. 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.

This topic contains the following sections:

IS-IS Areas

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 (routers) send, receive, and relay (forward) packets.

IS-IS does not force the network to use a hierarchical physical topology. Instead, a single AS can be divided into two types of areas: Level 1 areas and Level 2 areas. A Level 1 area is similar to an OSPF stub area, and a Level 2 area interconnects all Level 1 areas. The router and its interfaces reside within one area, and Level 2 routers share link-state information. No IS-IS area functions strictly as a backbone.

Level 1 routers share intra-area routing information, and Level 2 routers share interarea information about IP addresses available within each area. Uniquely, IS-IS routers can act as both Level 1 and Level 2 routers, sharing intra-area routes with other Level 1 routers and interarea routes with other Level 2 routers.

The propagation of link-state updates is determined by the level boundaries. All routers within a level maintain a complete link-state database of all other routers in the same level. Each router then uses the Dijkstra algorithm to determine the shortest path from the local router to other routers in the link-state database.

System Identifier Mapping

To provide assistance with debugging IS-IS, Junos OS 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.

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 network service access point (NSAP). NSAP addresses are supported on the loopback (lo0) interface.

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

NETs take several forms, depending on your network requirements. NET addresses are hexadecimal and range from 8 octets to 20 octets in length. Generally, the format consists of an authority and format Identifier (AFI), a domain ID, an area ID, a system identifier, and a selector. The simplest format omits the domain ID and is 10 octets long. For example, the NET address 49.0001.1921.6800.1001.00 consists of the following parts:

The system identifier must be unique within the network. For an IP-only network, we recommend using the IP address of an interface on the router. Configuring a loopback NET address with the IP address is helpful when troubleshooting is required on the network.

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.

IS-IS Path Selection

Level 1 routers store information about all the subnets within an area, and choose intranetwork paths over internetwork paths. Using the area ID portion of the NET address, Level 1 routers determine which neighboring routers are Level 1 routers within the same area.

If the destination address is not within the area, Level 1 routers forward the packet to the nearest router configured as both a Level 1 and Level 2 router within the area. The Level 1 and Level 2 router forwards the packet, using the Level 2 topology, to the proper area. The destination router, which is configured as a Level 1 and Level 2 router, then determines the best path through the destination area.

Protocol Data Units

IS-IS routers use protocol data units (PDUs) to exchange information. Each protocol data unit (PDU) shares a common header.

IS-IS Hello PDU

IS-IS hello PDUs establish adjacencies with other routers and have three different formats: one for point-to-point hello packets, one for Level 1 broadcast links, and one for Level 2 broadcast links. Level 1 routers must share the same area address to form an adjacency, while Level 2 routers do not have this limitation. The request for adjacency is encoded in the Circuit type field of the PDU.

Hello PDUs have a preset length assigned to them. The IS-IS router does not resize any PDU to match the maximum transmission unit (MTU) on a router interface. Each interface supports the maximum IS-IS PDU of 1492 bytes, and hello PDUs are padded to meet the maximum value. When the hello is sent to a neighboring router, the connecting interface supports the maximum PDU size.

Link-State PDU

A link-state PDU (LSP) contains information about each router in the network and the connected interfaces. Also included is metric and IS-IS neighbor information. Each LSP must be refreshed periodically on the network and is acknowledged by information within a sequence number packet.

On point-to-point links, each LSP is acknowledged by a partial sequence number PDU (PSNP), but on broadcast links, a complete sequence number PDU (CSNP) is sent out over the network. Any router that finds newer LSP information in the CSNP then purges the out-of-date entry and updates the link-state database.

LSPs support variable-length subnet mask addressing.

Complete Sequence Number PDU

The complete sequence number PDU (CSNP) lists all the link-state PDUs (LSPs) in the link-state database of the local router. Contained within the CSNP is an LSP identifier, a lifetime, a sequence number, and a checksum for each entry in the database. Periodically, a CSNP is sent on both broadcast and point-to-point links to maintain a correct database. Also, the advertisement of CSNPs occurs when an adjacency is formed with another router. Like IS-IS hello PDUs, CSNPs come in two types: Level 1 and Level 2.

When a device receives a CSNP, it checks the database entries against its own local link-state database. If it detects missing information, the device requests specific LSP details using a partial sequence number PDU (PSNP).

Partial Sequence Number PDU

A partial sequence number PDU (PSNP) is used by an IS-IS router to request LSP information from a neighboring router. A PSNP can also explicitly acknowledge the receipt of an LSP on a point-to-point link. On a broadcast link, a CSNP is used as implicit knowledge. Like hello PDUs and CSNPs, the PSNP also has two types: Level 1 and Level 2.

When a device compares a CSNP to its local database and determines that an LSP is missing, the router issues a PSNP for the missing LSP, which is returned in a link-state PDU from the router sending the CSNP. The received LSP is then stored in the local database, and an acknowledgement is sent back to the originating router.

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