True Statements About IS-IS Link-State Database (LSDB)

ABE

IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol that is used in large enterprise networks and service provider networks. In IS-IS, the network topology is represented by a link-state database (LSDB), which is maintained by each router in the network.

Here are the explanations for the statements that are true regarding the IS-IS link-state database:

A. An L1 router has a single database reflecting the connectivity of its area. An L1 (Level 1) router is a router that is only aware of the topology of its own area. The IS-IS network is typically divided into multiple areas to reduce the size of the LSDB and to minimize the flooding of link-state updates. Each L1 router maintains a separate LSDB that reflects the connectivity of its area only. Therefore, statement A is true.

B. An L1 router often relies on a default route generated by attached routers to reach interarea destinations. An L1 router doesn't have a complete view of the network topology, including the routes outside its own area. Therefore, an L1 router often relies on a default route generated by attached routers to reach interarea destinations. This default route is advertised by L1/L2 (Level 1/Level 2) routers to provide a way for L1 routers to reach destinations outside their area. Hence, statement B is true.

C. The Level 2 database contains only backbone area routes unless route leaking is configured. The Level 2 database contains the complete view of the network topology, including all Level 1 areas and the interconnecting links. By default, Level 2 routers only store information about the backbone area. However, route leaking can be used to advertise routes from Level 1 areas to Level 2 routers, which can optimize the routing and reduce the number of hops to reach the destination. Therefore, statement C is true.

D. Route leaking can be used to optimize Level 2 to Level 1 routing. Route leaking is a mechanism that allows the exchange of routing information between Level 1 and Level 2 areas. By default, Level 1 routers do not have any information about the routes outside their area. However, if route leaking is configured, Level 1 routers can receive routes from Level 2 routers, and vice versa. This can optimize the routing by reducing the number of hops to reach the destination. Therefore, statement D is true.

E. An attached router has at least two LSDBs: one for each Level 1 area and a single LSDB for the Level 2 backbone. An attached router is a router that has interfaces in multiple areas. An attached router maintains a separate LSDB for each Level 1 area it is connected to, and a single LSDB for the Level 2 backbone. The LSDBs are populated with link-state information received from other routers in the same area or backbone. Hence, statement E is true.

In summary, the true statements regarding the IS-IS link-state database are A, B, and C.