JNCIS-ENT IS IS Practice Test 2 — 15 Questions

Question 1

Which TLV is used to carry IP prefixes in IS-IS?

Accepted Answer

TLV 128. TLV 128 and 135 are used for IP reachability information.

Answer

TLV 10

Answer

TLV 22

Answer

TLV 1

Question 2

Which of the following are true regarding IS-IS Level 1/Level 2 routers? (Choose TWO)

Accepted Answer

They maintain both a Level 1 and a Level 2 database.. Options A and B are correct. A Level 1/Level 2 (L1/L2) IS-IS router runs both routing levels simultaneously, so it maintains a separate Level 1 link-state database (for intra-area routing) and a Level 2 database (for inter-area/backbone routing), making A true. Because it is attached to both a Level 1 area and the Level 2 backbone, it sets the ATT (Attached) bit in its Level 1 LSPs to tell Level 1-only routers that a path to other areas exists via this router, making B true. Option C is wrong because DIS election is per-segment and based on interface priority and highest SNPA/MAC address, not on a router being L1/L2. Option D is wrong because an L1/L2 router forms both Level 1 and Level 2 adjacencies. Option E is wrong because all IS-IS routers, including L1/L2 routers, run SPF (Dijkstra) calculations for each level's database.

Answer

They are always the DIS for the segment.

Answer

They only form Level 1 adjacencies.

Answer

They do not perform SPF calculations.

Question 3

What is the default behavior when an IS-IS router receives an LSP it already has?

Accepted Answer

It discards the LSP if the sequence number is lower.. If the sequence number is the same, the router discards it. If newer, it updates; if older, it sends a copy of the newer one back to the sender.

Answer

It always floods it to all neighbors.

Answer

It restarts the SPF calculation immediately.

Answer

It requests the LSP from the source again.

Question 4

Which factors influence the DIS election on a broadcast link? (Choose TWO)

Accepted Answer

SNPA (MAC) address. On a broadcast link, IS-IS elects the Designated Intermediate System (DIS) primarily by the highest interface priority (Option C), so the router with the numerically greatest priority value on that interface becomes DIS. When priorities tie, the election is broken by the highest SNPA (MAC) address (Option B), which is the subnetwork point of attachment address on that LAN, ensuring a deterministic winner. System ID (Option A) is not the tiebreaker used for DIS election on a broadcast link, and Area Address (Option D) is irrelevant to DIS selection since all routers on the link share the same area. LSP sequence number (Option E) is used for LSP freshness and flooding, not for choosing the DIS.

Answer

System ID

Answer

Area Address

Answer

LSP sequence number

Question 5

What are the consequences of a mismatch in metric style (narrow vs wide) between IS-IS neighbors? (Choose THREE)

Accepted Answer

Partial reachability issues.. A mismatch in IS-IS metric style (narrow vs. wide) between neighbors does not prevent the adjacency from forming, but it does prevent the routers from correctly interpreting each other's TLVs, so option D is correct: the adjacency may come up, yet the routes will not be accepted because the metric TLVs (narrow uses old-style TLVs 2/128/130, wide uses TLVs 22/135) are not mutually understood. Because the metric information is not properly exchanged, option A is correct: partial reachability issues occur, as some prefixes or paths become unreachable across the mismatched link. Option B is also correct: since the link-state database lacks consistent metric data, the SPF calculation cannot find all paths, resulting in incomplete or suboptimal routing. Option C is wrong because a metric-style mismatch is a configuration/compatibility issue, not a fatal error that crashes the routers. Option E is wrong because DIS election on a broadcast network is based on priority and MAC address, not on metric style, so it is unaffected by the mismatch.

Answer

The routers crash immediately.

Answer

The DIS election will fail.

Question 6

Which of the following are valid ways to influence path selection in IS-IS? (Choose THREE)

Accepted Answer

Modifying interface cost. Option A is correct because IS-IS uses interface cost (metric) as the primary tie-breaker in SPF path selection, so modifying the interface cost directly changes which path is preferred. Option B is correct because export policies can manipulate route attributes and metric values redistributed into IS-IS, thereby influencing path selection. Option D is correct because wide metrics allow larger metric values and finer granularity, which affects how paths are compared and selected. Option C is incorrect because the area address defines the router's area membership and Level 1/Level 2 boundaries, not path preference. Option E is incorrect because the System ID is a fixed identifier used for adjacency and DIS election, not a path-selection parameter.

Answer

Changing the area address

Answer

Changing the System ID

Question 7

Which of the following scenarios would prevent an IS-IS adjacency from forming? (Choose TWO)

Accepted Answer

Mismatched authentication passwords. Option A is correct because IS-IS uses authentication TLVs (with MD5 or cleartext) in its Hello PDUs, and if the passwords or authentication types do not match between neighbors, the Hello packets are rejected and no adjacency can form. Option B is correct because Level 1 routers must share a common area address to form an L1 adjacency; if the area addresses are incompatible, the routers will not establish the L1 adjacency (though they may still form an L2 adjacency if configured as L1/L2). Option C is not correct because IS-IS does not require matching MTU to form an adjacency; MTU mismatches can cause problems with LSP flooding or adjacency establishment only in specific implementations, but it is not a standard adjacency-blocking condition like authentication or area mismatch. Option D is not correct because interface descriptions are purely local administrative labels and have no effect on IS-IS adjacency formation. Option E is not correct because hostnames are not used in IS-IS adjacency negotiation; the system ID (derived from the NET) matters, but the hostname itself is irrelevant to adjacency formation.

Answer

Different MTU settings

Answer

Mismatched interface description

Answer

Mismatched hostname

Question 8

Which of the following are true about pseudonode LSPs? (Choose THREE)

Accepted Answer

They represent the broadcast segment in the LSDB.. Option B is correct because a pseudonode LSP models the multi-access broadcast segment itself as a virtual node in the link-state database, so the segment appears in the LSDB topology. Option C is correct because on a broadcast segment the Designated Intermediate System (DIS) originates the pseudonode LSP on behalf of the segment. Option D is correct because representing the segment as a single pseudonode lets each router on the LAN form and track an adjacency only to the pseudonode rather than to every other router, reducing the number of adjacencies. Option A is wrong because point-to-point links do not use pseudonodes; they form direct adjacencies and use point-to-point LSPs. Option E is wrong because the pseudonode LSP is generated and flooded by the DIS, not by all routers on the segment.

Answer

They are used in point-to-point links.

Answer

They are flooded by all routers.

Question 9

What are valid ways to configure IS-IS on a Junos interface? (Choose TWO)

Accepted Answer

set interfaces ge-0/0/0.0 protocols isis. Option A is correct because Junos supports enabling IS-IS directly under the logical interface unit using the hierarchy set interfaces ge-0/0/0.0 protocols isis, which activates IS-IS on that interface. Option B is correct because the equivalent and more common configuration method is set protocols isis interface ge-0/0/0.0, which adds the logical unit to the IS-IS protocol configuration. Option C is wrong because IS-IS interface configuration does not live under routing-options; that hierarchy is used for routing policy, static routes, and other global routing settings. Option D is wrong because there is no isis enable statement under the interface hierarchy in Junos. Option E is wrong because the syntax set protocols isis enable ge-0/0/0.0 is not valid; the correct keyword is interface, not enable.

Answer

set routing-options isis interface ge-0/0/0.0

Answer

set interfaces ge-0/0/0.0 isis enable

Answer

set protocols isis enable ge-0/0/0.0

Question 10

Which of the following are true about CSNP and PSNP? (Choose THREE)

Accepted Answer

PSNPs are used to acknowledge LSPs.. Option A is correct because a Partial Sequence Number PDU (PSNP) serves as an acknowledgment for Link State PDUs (LSPs) received on a point-to-point link, confirming receipt and synchronizing the link-state database. Option C is correct because PSNPs are also used to request missing or newer LSPs, enabling partial updates of the link-state database rather than a full re-flooding. Option E is correct because on a broadcast network the Designated Intermediate System (DIS) originates Complete Sequence Number PDUs (CSNPs) periodically (default every 10 seconds) to describe its complete link-state database and keep all routers synchronized. Option B is wrong because CSNPs are used at both Level 1 and Level 2, not only by Level 1 routers. Option D is wrong because missing LSPs are requested via PSNPs (or CSNPs on broadcast links), not by CSNPs acting as a request mechanism.

Answer

CSNPs are used only by Level 1 routers.

Answer

CSNPs are used to request missing LSPs.

Question 11

You have an IS-IS network with routers configured as Level 1, Level 2, and Level 1/2. A Level 1 router needs to reach a prefix in a different area. The network engineer has configured the Level 1/2 routers with the 'attached-bit' in their Level 1 LSPs. However, the Level 1 router is not using the default route toward the nearest Level 1/2 router. What is the most likely cause?

Accepted Answer

The Level 1/2 routers are not setting the attached-bit in their Level 1 LSPs.. The attached-bit is a flag in the IS-IS Level 1 LSP that indicates a Level 1/2 router can reach other areas. When set, Level 1 routers install a default route toward the nearest Level 1/2 router. If the attached-bit is not set, the default route is not installed, and the Level 1 router cannot reach inter-area prefixes. The scenario states the engineer configured the attached-bit, but if it is not actually set due to misconfiguration or suppression, the Level 1 router will not use the default route.

Answer

The Level 1 router has a more specific route to the destination prefix in its own Level 1 database.

Answer

The Level 1 router has a default route pointing to a different next-hop.

Answer

The Level 1 router is configured with 'ignore-attached-bit'.

Question 12

You are troubleshooting an IS-IS network where a router is not forming an adjacency with its neighbor. You have verified that the interfaces are up and IP addresses are correctly configured. Which two statements about IS-IS adjacency formation are true? (Choose two.)

Accepted Answer

The routers must have matching IS-IS levels configured on the interfaces.. For IS-IS adjacency formation, routers must agree on the level (Level 1, Level 2, or both) and, for Level 1, must be in the same area. The system ID length is fixed and not configurable, so it is not a factor. MTU must match but not necessarily be 1492. Hello intervals and multipliers should match for stability, but they are not absolute requirements for initial adjacency formation. The correct statements are that levels must match and area IDs must match for Level 1.

Answer

The routers must have the same system ID length configured.

Answer

The interface MTU must be at least 1492 bytes.

Answer

The routers must have the same hello interval and hello multiplier.

Question 13

You are configuring IS-IS on a Juniper router and need to change the metric for a specific interface to influence path selection. The interface is a Gigabit Ethernet interface named ge-0/0/0. Which configuration statement accomplishes this?

Accepted Answer

set protocols isis interface ge-0/0/0 level 1 metric 100. To set the IS-IS metric for an interface on a Juniper router, you use the 'set protocols isis interface level metric ' command. The level (1 or 2) must be specified before the metric value. This allows different metrics for Level 1 and Level 2. The correct command sets the Level 1 metric to 100 for ge-0/0/0, which will influence path selection for Level 1 routes.

Answer

set interfaces ge-0/0/0 isis metric 100

Answer

set protocols isis interface ge-0/0/0 metric 100 level 1

Answer

set protocols isis interface ge-0/0/0 metric 100

Question 14

You are deploying IS-IS in a large service provider network. You need to enable wide metrics to support traffic engineering with metrics greater than 63. Which command enables wide metrics on a Juniper router?

Accepted Answer

set protocols isis level 1 wide-metrics-only; set protocols isis level 2 wide-metrics-only. To enable wide metrics on a Juniper router for IS-IS, you must configure 'wide-metrics-only' under both Level 1 and Level 2. This command forces the router to use and advertise only wide metrics, which support values up to 2^24. Without this, the router uses narrow metrics limited to 63. The correct configuration enables wide metrics for both levels, ensuring compatibility and support for traffic engineering.

Answer

set protocols isis wide-metrics-only

Answer

set protocols isis level 1 wide-metrics-only

Answer

set protocols isis interface all level 1 metric 100

Question 15

You are troubleshooting an IS-IS adjacency issue on a Juniper EX Series switch. The interface is configured as point-to-point, but the adjacency remains in the 'Init' state. You verify that the interface is up and the IP addresses are correct. What is the most likely cause?

Accepted Answer

The authentication keys do not match.. In IS-IS, the 'Init' state indicates that a router has received hellos from a neighbor but the neighbor has not acknowledged its own hellos. This commonly occurs when authentication is enabled but the keys do not match. The router will drop hellos with incorrect authentication, so the neighbor never sees the router's hellos, preventing the adjacency from reaching 'Up'. Other issues like MTU or level mismatch would cause different symptoms, such as no adjacency or 'Down' state.

Answer

The interface MTU is too low.

Answer

The IS-IS hello interval is mismatched.

Answer

The router is configured as Level 1 only, but the neighbor is Level 2 only.