JNCIS-ENT IS IS Practice Test 4 — 15 Questions

Question 1

Which level is enabled by default in Junos when configuring IS-IS on an interface?

Accepted Answer

Level 1 and Level 2. By default, interfaces in Junos IS-IS are enabled for both Level 1 and Level 2.

Answer

None

Answer

Level 2 only

Answer

Level 1 only

Question 2

You are configuring multi-topology IS-IS. What does this allow?

Accepted Answer

Running separate topologies for different address families.. Multi-topology IS-IS allows for separate routing topologies for different protocol families (e.g., IPv4 and IPv6).

Answer

Load balancing over different physical paths.

Answer

Creating multiple areas within a single instance.

Answer

Increasing the maximum MTU.

Question 3

What is the function of the DIS in an IS-IS broadcast network?

Accepted Answer

To generate pseudonode LSPs.. The DIS (Designated Intermediate System) is responsible for generating pseudonode LSPs to represent the broadcast network.

Answer

To authenticate all neighbors.

Answer

To perform path calculation for all routers.

Answer

To act as the routing controller for the area.

Question 4

What happens if you have an area address mismatch in a Level 1 adjacency?

Accepted Answer

The adjacency will fail to form.. Level 1 adjacencies require the routers to share at least one area address.

Answer

The DIS will override the mismatch.

Answer

The adjacency will form but no routes will be exchanged.

Answer

The router will automatically adopt the neighbor's area address.

Question 5

What is the purpose of the 'lsp-generation-interval'?

Accepted Answer

To limit the frequency of LSP updates.. The 'lsp-generation-interval' controls how often a router can generate new LSPs (Link State PDUs) in IS-IS. It sets a minimum interval between consecutive LSP generations, thereby limiting the frequency of LSP updates and reducing flooding and SPF churn.

Answer

To set the hello interval.

Answer

To define the authentication key rotation.

Answer

To set the time for database aging.

Question 6

How does an IS-IS router identify the DIS in its hello packets?

Accepted Answer

By the router's System ID followed by a non-zero pseudonode ID.. The DIS is identified by the System ID and a pseudonode number (the 7th byte of the LSP ID).

Answer

By a special flag in the header.

Answer

By the area address.

Answer

By the MAC address only.

Question 7

What does the 'show isis statistics' command reveal?

Accepted Answer

The number of received and transmitted PDUs.. This command displays the number of PDUs sent and received by the IS-IS process.

Answer

The current SPF calculation time.

Answer

The database size in bytes.

Answer

The adjacency hold-time.

Question 8

When should you consider increasing the 'csnp-interval'?

Accepted Answer

To reduce control traffic overhead on slow links.. Increasing the interval is useful on slow or congested links to reduce the frequency of full database synchronization traffic.

Answer

To improve convergence time.

Answer

To increase the maximum LSP size.

Answer

To fix adjacency flapping.

Question 9

Which command displays the IS-IS interface parameters?

Accepted Answer

show isis interface. 'show isis interface' shows the configuration and state of IS-IS interfaces.

Answer

show isis configuration

Answer

show interfaces isis

Answer

show protocols isis interface

Question 10

Which of the following are valid IS-IS PDU types? (Choose THREE)

Accepted Answer

IIH PDU. IS-IS defines three core PDU categories, and options A, B, and C map directly onto them. A: IIH (Intermediate System-to-Intermediate System Hello) PDUs are the valid IS-IS hello packets used to discover and maintain adjacencies on broadcast and point-to-point links, with LAN and P2P variants. B: LSP (Link State PDU) is the valid IS-IS packet that carries link-state information and is flooded to build the link-state database, analogous to an OSPF LSA. C: CSNP (Complete Sequence Number PDU) is a valid IS-IS PDU used to describe the complete set of LSPs in a database, sent by the DIS on broadcast networks and used for database synchronization. Options D and E are not IS-IS PDU types: IS-IS hellos are not acknowledged with a separate 'Hello ACK PDU' (acknowledgment is implicit via receipt of a subsequent IIH), and there is no 'Routing Update PDU' in IS-IS—routing information is distributed through LSPs, CSNPs, and PSNPs.

Answer

Hello ACK PDU

Answer

Routing Update PDU

Question 11

Which of the following are true regarding IS-IS authentication? (Choose TWO)

Accepted Answer

It secures the PDUs exchanged between neighbors.. IS-IS authentication is used to secure the PDUs exchanged between neighbors, such as Hello PDUs, LSPs, and SNPs, which is why option C is correct. It can also be configured at different levels, including per interface, which makes option D correct. IS-IS authentication is not mandatory for all deployments, so option A is incorrect. It is not limited to the entire area, since it can be applied at the interface, area, or domain level, so option B is incorrect. It does not encrypt the entire link traffic; it only authenticates routing protocol PDUs, so option E is incorrect.

Answer

It is mandatory for all IS-IS deployments.

Answer

It can only be configured for the entire area.

Answer

It encrypts the entire link traffic.

Question 12

Which of the following are valid IS-IS interface types? (Choose TWO)

Accepted Answer

point-to-point. In IS-IS, the two valid interface (circuit) types are point-to-point and broadcast, so options C and D are correct. A point-to-point interface (C) is used on links such as serial or point-to-point Ethernet subinterfaces, where IS-IS forms an adjacency without a DIS election and uses the Point-to-Point Three-Way Handshake (RFC 5303). A broadcast interface (D) is used on multi-access media like Ethernet, where IS-IS elects a Designated Intermediate System (DIS) and uses Level 1/Level 2 LAN IIHs. The other options are not IS-IS interface types: mesh (A) and star (B) describe topology shapes, not IS-IS circuit types, and passive (E) is an interface state/configuration (passive-interface) that suppresses adjacency formation, not a valid IS-IS interface type.

Answer

mesh

Answer

star

Answer

passive

Question 13

Which actions occur during an IS-IS SPF calculation? (Choose THREE)

Accepted Answer

Calculating the shortest path to each destination.. Option B is correct because the core purpose of the IS-IS SPF (Dijkstra) computation is to determine the shortest path from the calculating router to every reachable destination in the topology. Option D is correct because SPF operates on the Link State Database, using it to build a Shortest Path Tree rooted at the local router before deriving routes. Option C is correct because once the SPT is built and best paths are selected, the results are installed into the IP routing table (RIB), making the routes usable for forwarding. Option A is not part of SPF: IIH (IS-IS Hello) packets are generated by the Hello protocol to form and maintain adjacencies, which happens independently of the SPF calculation. Option E is also not part of SPF: DIS election on broadcast networks is driven by Hello packet exchanges and priority values, not by the SPF algorithm.

Answer

Generating new IIH packets.

Answer

Re-electing the DIS.

Question 14

Which of the following are true about IS-IS Level-1 and Level-2 areas? (Choose TWO)

Accepted Answer

Level 2 is for connecting Level 1 areas.. Option A is correct because IS-IS Level-2 routing operates as the backbone that interconnects separate Level-1 areas, allowing prefixes from one area to be advertised to others via Level-2 (or Level-1/Level-2) routers. Option E is correct because Level-1 routing is confined within a single area and handles intra-area prefix reachability using Level-1 LSPs and the Level-1 link-state database. Option B is wrong because Level-1 and Level-2 are distinct routing levels with separate link-state databases and different flooding scopes. Option C is wrong because a pure Level-1 router only knows its own area's topology and must use an attached Level-1/Level-2 router (with the ATT bit set) to reach other areas. Option D is wrong because Level-2 is the inter-area backbone routing level, not a mechanism for local interface management.

Answer

Level 1 and Level 2 are the same thing.

Answer

Level 1 routers can communicate directly with routers in other areas.

Answer

Level 2 is for local interface management only.

Question 15

Which of the following are true regarding the IS-IS 'hello-interval'? (Choose TWO)

Accepted Answer

It determines how often Hello PDUs are sent.. Option B is correct because the IS-IS hello-interval directly controls the frequency with which a router transmits IS-IS Hello PDUs (IIHs) on an interface, which is its fundamental purpose. Option D is correct because the adjacency hold time is derived from the hello-interval multiplied by the hello-multiplier (by default, hold time = hello-interval × 3), so changing the hello-interval changes how long a neighbor waits before declaring the adjacency down. Option A is wrong because the SPF interval is governed by separate SPF timers (e.g., spf-interval), not the hello-interval. Option C is wrong because the default hello-interval varies by interface type and network type (for example, 10 seconds on broadcast and 3 seconds on point-to-point by default on many platforms), so it is not always 10 seconds. Option E is wrong because Hello PDUs and the hello-interval apply to both broadcast and point-to-point links, not only point-to-point.

Answer

It is used to calculate the SPF interval.

Answer

It is always 10 seconds.

Answer

It is only used on point-to-point links.