Types of IS-IS Routers and Their Role in Area Hierarchical Design

According to RFC 1195, pure IP domains are systems that use only IP as their primary routing protocol. Although IP domains route only IP traffic, they can also support the processing and forwarding of OSI packets necessary for IS-IS functionality. A pure ISO domain can transport only ISO traffic, which includes the traffic necessary to operate IS-IS. A dual domain, on the other hand, routes both IP and OSI CLNP traffic at the same time.

Integrated IS-IS is similar to many of the other routing protocols discussed in this BCSI course. Integrated IS-IS supports VLSM. Both the subnet mask and prefix are exchanged in routing updates. You can also redistribute routes in and out of IS-IS. It is feasible to configure a dual domain so that some areas route only IP and others route only CLNP. In addition, you can design other areas to route both IP and CLNP. The main objective, of course, is to have reliable routing information in an area by making sure that the databases are the same on all Level 1 link-state routers in the area. Therefore, at the area level, all the area routers must be identically configured either for IP only, CLNP only, or both protocols. In other words, an intermediate system is not permitted to have some connections devoted to IP only, another set dedicated to CLNP, and still another group configured for both protocols. At the domain level, however, there are no constraints on integrating IP-only areas with other CLNP-only or IP-CLNP areas. All links within an area must be configured in the same fashion, but backbone connections can configure the attached routers in a different way. An IS-IS area can be described as a contiguous group of Level 1 routers.

OSPF Area Borders Versus IS-IS Area Borders

Realize that with OSPF, the individual interfaces actually belong to the area. With Integrated IS-IS, the entire router is within the area and belongs to the area, not just the individual interfaces. Both IS-IS and OSPF use a hierarchical structure to facilitate design and addressing, yet IS-IS is a more scalable solution. As you learned in the previous two chapters, OSPF has a central Area 0 backbone to which all the other areas are physically attached. The border between the areas actually exists within the Area Border Routers (ABRs) so that each connection is in a particular area. This inherently introduces limitations in the network design. Integrated IS-IS builds its hierarchy with Level 1 and Level 2 routers, where the borders are in the connections as opposed to within the area routers themselves . In other words, every IS-IS router belongs to exactly one Level 2 area. This is different from OSPF, and permits an easier and more flexible solution to network scalability. With IS-IS you can extend the backbone area by simply adding more Level 2 routers.

An IS-IS network domain is the counterpart to an autonomous system (AS). A domain can be carefully planned or arbitrarily constructed into smaller units called areas. By choosing a hierarchical routing design, you can achieve more efficient intradomain routing. All routers in an IS-IS domain must be linked to a single physical area, which is designated by an area ID number in the network service address point (NSAP) address. (The NSAP is the point at which the OSI network services are made available to the Transport layer.) Routers are in the same area if they have an identical area ID number. In reality, an IS-IS router can have multiple NSAPs in multi-homing scenarios where the router is connected to multiple areas. We look closely at the NSAP addressing scheme later in this chapter. Multihoming combines all the areas into a single physical area. This is addressed later in this chapter.

Within the domain, a dual hierarchy exists where Level 1 routers (Intermediate Systems) perform the intra-area routing and Level 2 routers exchange data between areas ( inter-area ) only. Routers that operate at both levels ( L1L2 ) perform routing services between the backbone and other areas, which include intra-area and inter-area routing.

An IS-IS L1L2 intermediate system is comparable to an area border router (ABR) in an OSPF autonomous system.

The Level 1 routers hold their own link-state database that stores the area topology and the exit points to other neighboring areas as well. In CLNP routing, for example, Level 1 routing engages in the collection of SysID (SysID is explained in the next section) and adjacency information from all routers and hosts in the local area. Level 2 routers have a different link-state topology database that contains the inter-area routing information. With CLNP Level 2 routing, routers exchange information such as area prefixes with their neighbors. Therefore, an L1L2 router will keep two separate databases so that they can perform L1 and L2 duties as if they were logically two separate routers, as shown in Figure 6.1.

Figure 6.1. A sample physical IS-IS configuration between two areas linked by L1L2 routers.

An IS-IS backbone is different from an OSPF backbone in the sense that it is a group of areas that are linked by a chain of contiguous L1L2 and L2 routers, as shown in Figure 6.2.

Figure 6.2. An IS-IS backbone linked by a chain of contiguous L1L2 and L2 routers.

IS-IS does not have the Area 0 backbone mechanism that is used by OSPF, but rather a dual-level hierarchy that propagates the Level 1 LSPs and the Level 2 LSPs separately. As mentioned in Table 6.2, the DIS sends the separate L1 and L2 LSPs for the network. In Figure 6.2, you can see that Area-51 has two Level 1 routers and three L1L2 routers that link the neighboring areas to form a "backbone path ." The middle router has to provide Level 2 routing functions even though it does not directly connect to another area. If this router did not provide Level 2 routing or if it were to fail, the backbone would be brought down. Area-52 is a Level 2 area only because it has no intra-area routing to Level 1 routers within its area. This figure also reinforces the concept that IS-IS borders are maintained on the connections between the Level 2 routers, whereas OSPF creates the border within the ABRs connecting the multiple areas.