IS-IS

Intermediate System-to-Intermediate System (IS-IS) is a link state routing protocol that is part of the OSI family of protocols. Like OSPF, it uses Dijkstra’s SPF algorithm to choose routes. IS-IS is a classless interior gateway protocol that uses router resources efficiently and scales to large networks, such as large Internet service providers (ISP). The following table lists some IS-IS terms, acronyms, and their meanings.


IS-IS Overview
Integrated IS-IS can carry IP network information, but does not use IP as its transport protocol. It uses OSI protocols CLNS and CLNP to deliver its updates. IS-IS sends its messages in PDUs. There are four IS-IS PDU types: Hello, LSP, PSNP, and CSNP.

Types of IS-IS Routers
Figure 4-1 shows an IS-IS network divided into areas. The IS-IS backbone is not a specific area, as in OSPF, but an unbroken chain of routers doing Level 2 routing. R3, R6, and R4 are the backbone in the following figure.

Within an area, routers can be one of three types:

Level 1 (L1) router
R1, R2, and R5 in the figure. Routes to networks only within the local area (intra-area routing). Uses a default route to the nearest Level 2 router for traffic bound outside
the area. Keeps one LSDB for the local area. When routing, compares the area of the destination to its area. If they are the same, routes based on system ID. If not, sends traffic to Level 1-2 router.

Level 2 (L2) router
R6 in the figure. Routes to networks in other areas (interarea routing). The routing is based on area ID. Keeps one LSDB for routing to other areas.

Level 1-2 (L1-2) router
R3 and R4 in this figure. Acts as a gateway into and out of an area. Does Level 1 routing within the area and Level 2 routing between areas. Keeps two LSDB: one for the local area and one for interarea routing.


NSAP Address Structure
In the Cisco implementation of integrated IS-IS, NSAP addresses have three parts: the area ID, the system ID, and the NSEL. They are written in hexadecimal and have a maximum size of 20 bytes.

Area IDs vary from 1 to 13 bytes. Those that begin with 49 designate private area addressing.
The Cisco system ID must be exactly six bytes. MAC addresses or IP addresses padded with 0s are often used as system IDs.
The NSEL is exactly one byte in size. A router always has a NSEL of 00.

Adjacency Formation in IS-IS
IS-IS routers form adjacencies based on the level of IS routing they are doing and their area number. This is a CLNS adjacency and can be formed even if IP addresses don’t match.

Level 1 routers form adjacencies only with L1 and L1-2 devices in their own area. (In Figure 4-1, R1 becomes adjacent with R2 and R3.)

Level 2 routers form adjacencies only with Level 2-capable devices (either L2 or L1-2 routers). These can be in the local area or in other areas. (In Figure 4-1, R6 becomes adjacent with R3 and R4.)

Level 1-2 routers form Level 1 adjacencies with L1 routers in their own area, and Level 2 adjacencies with routers in other areas. (In Figure 4-1, R4 has a L1 adjacency with R5 and a L2 adjacency with R6.)