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| 7.3 |  | IS-IS Operation |
| 7.3.5 |  | IS-IS routing levels |

An IS-IS network is termed a domain, analogous to an autonomous system (AS) in TCP/IP. The following shows IS-IS within the domain as a two-level hierarchy:

- Level 1 (L1) ISs, closely equivalent to OSPF internal non-backbone routers, are responsible for routing to ESs inside an area. L1 ISs enable communication between ESs in an area.
- Level 2 (L2) ISs, closely equivalent to backbone routers in OSPF, route between areas only.
- Level 1 and Level 2 (L1L2) Intermediate ISs, closely equivalent to area border routers (ABRs) in OSPF, route between areas and the backbone. They participate in the L1 intra-area routing and the L2 inter-area routing.

L1 routers are also referred to as station routers because they enable stations to communicate with each other and the rest of the network. A contiguous group of L1 routers defines an area. The L1 routers maintain the L1 link-state PDU database (LSPD), which defines the picture of the area itself and the exit points to neighboring areas.

L2 routers are also referred to as area routers because they interconnect the L1 areas. L2 routers store a separate LSPD, which contains only the inter-area topology information.

L1L2 routers act as if they were two IS-IS routers. Physically, an L1L2 router connects to L1 routers inside its area and to L2 routers in the backbone. Notice that the boundary between areas in IS-IS exists on a link between routers and not on an ABR itself, as in OSPF. Logically, the L1L2 router acts, for the purposes of IS-IS routing, as if it were two logical routers. An L1L2 router operates an L1 routing process, with its own L1 topology table and adjacency table, to handle its association with other L1 routers and ESs. An L1L2 router also operates an L2 routing process, with a separate L2 topology table and a separate L2 adjacency table, to handle its association with its neighbor backbone routers.

L1L2 routers support an L1 function to communicate with the other L1 routers in their area and maintain the L1 LSP information in an L1 LSPD. They inform other L1 routers that they are an exit point for the area. They also support an L2 function to communicate with the rest of the backbone and maintain an L2 topology database separately from their L1 LSPD. IS-IS does not share the concept of an Area 0 with OSPF. An IS-IS domain appears as a set of distinct areas interconnected by a chain of L2 routers, weaving their way through and between the Level 1 areas.

An L1 area is a collection of L1 and L1L2 routers. The backbone area is a collection of L2 and L1L2 routers and has to be contiguous. IS-IS does not have a backbone area like OSPF Area 0. The IS-IS backbone is a contiguous collection of L2 capable routers, each of which can be in a different area.

> Note: By default, all Cisco routers behave as L1L2 routers. This default behavior can be overridden by using one or both of the

is-type, router global config mode, and isis circuit-type, interface config mode, commands.

Useful examples of the L1, L2, and L1L2 concepts are discussed in this section. Area 1 contains two routers. One router borders Area 2 and therefore is an L1L2 IS. The other router is L1 only.

Area 2 has many routers. Some routers are specified as L1 only and can route internally to that area only and to the exit points. L1L2 routers form a chain across the area linking to the neighbor areas. Even though the middle of these three L1L2 routers does not link directly to another area, it must support Level 2 routing so the backbone is contiguous. If that middle router fails then the other L1 only routers, though providing a physical path across the area, could not perform the L2 function. This will cause the backbone to fail.

Area 3 contains one router that borders Area 2 and Area 4, but has no intra-area neighbors, so this router is an L2 only router. In the event that another router is added to Area 3, the border router would revert to L1L2.

> Note: Previously, each Cisco router could participate in only one area. The router would perform L1, intra-area, routing locally. The router would perform L2, inter-area, routing to other areas in the network. This limitation meant that when the network was divided into a large number of L1 areas, a correspondingly large number of L1L2 routers were required to route between all areas. The need for redundancy increases the number of routers needed. With IOS release 12.0(5)T, support was added for configuration of multiple L1 areas within a single router. A single Cisco router can now participate in routing in up to 29 areas, as well as perform L2 routing in the backbone. Restated, expansion of an IS-IS network consisting of many small areas is simpler than before because multiple L1 areas can now be configured on the same Cisco router. This is possible without needing to add and configure physical units for each additional local area. This feature provides connectivity between L1 areas local to the router. Previously, L1 areas could only be connected using the L2 backbone. However, it should be emphasized that multiarea support per IS was introduced in later implementations of IS-IS to accommodate OSI telecommunications management networks needs. This functionality is generally not useful or recommended for IP network design.

| Interactive Media Activity Matching: IS-IS Level 1 Adjacencies Upon completion of this activity, the student will be able to identify the Level 1 adjacencies between routers in an IS-IS network.