4A0-112 Nokia IS-IS Routing Protocol Questions and Answers

In a dual-stack IPv4/IPv6 IS-IS environment, the SPF (Shortest Path First) algorithm is only run once per router for each protocol (IPv4 or IPv6). The topology is shared, but the SPF calculations for IPv4 and IPv6 are separate. This means that the SPF algorithm will run twice (once for IPv4 and once for IPv6) for each L1/L2 router, not four times.

In both broadcast and point-to-point interfaces, IS-IS routers identify neighbors using system IDs, not interface MAC addresses. The system ID is a unique identifier assigned to each router, and it is used to identify neighbors in both types of interfaces.

When a link-state update reaches its maximum age, it is removed from the router’s link-state database, not flooded to its neighbors. This prevents outdated information from continuing to affect the routing decisions. The router does not flood the aged update; instead, it will typically request a new link-state advertisement from the original source router if the information is still needed.

When a router receives a link-state advertisement (LSA), it does not update the age field before forwarding it. The age field in an LSA is typically updated by the originating router or during the process of forwarding the LSA within the network. Routers do not modify the age field upon receiving and forwarding an LSA.

IS-IS routers must have matching authentication settings to establish an adjacency. If one router is configured with authentication and the other is not, the routers will not be able to establish an adjacency because their authentication parameters do not match.

In the configuration on router R1, the static route is defined with the next-hop IP address of 139.120.121.1 .

However, the next-hop IP address 139.120.121.1 does not belong to the same subnet as the directly connected interface on R1, which is 172.31.1.1/30 . For the static route to work properly, the next-hop IP address must be reachable via a directly connected interface, meaning it must be within the same subnet.

Therefore, this mismatch in subnet adjacency is likely causing the failure to reach the destination (139.120.121.2).

Router R1 is configured as L1/L2, meaning it is part of both Level 1 and Level 2 IS-IS routing areas. This means R1 will generate two types of LSPs:

Level 1 LSP (for the local area 49.0001) to advertise its local topology to other Level 1 routers.

Level 2 LSP (for the backbone area 49.0002) to advertise the global network topology to Level 2 routers.

R1 also has two broadcast interfaces, and as the Designated Intermediate System (DIS) on these interfaces, it will generate an LSP for each interface (one per broadcast link).

This results in three total LSPs:

A Level 1 LSP for the local area (49.0001).

A Level 2 LSP for the backbone area (49.0002).

An LSP for each of the two broadcast interfaces, which may include interface-related topology information.

From the output, we can see that the usage column indicates whether a router is operating as an L1 or L2 router:

Routers R1 and R2 have a usage of L2 , meaning they are L2 routers.

Routers R3 and R4 have a usage of L1 , meaning they are L1 routers.

The L1/L2 designations refer to whether the routers participate in both Level 1 and Level 2 of IS-IS:

L1 routers only communicate within their own area.

L2 routers communicate between areas.

In this case, R1 and R2 are L2 routers, and R3 and R4 are L1 routers.