JNCIS-ENT IS IS — 20 Questions

Question 1

An engineer needs to influence the DIS election on a multi-access broadcast segment running IS-IS on Junos devices. Which configuration parameter and hierarchy should the engineer modify to guarantee a specific router wins the DIS election?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Configure 'set protocols isis interface priority ' where a higher value wins.. The DIS election in IS-IS is influenced by the interface priority. Configuring 'set protocols isis interface priority ' with a higher value increases the likelihood of winning the DIS election. The default priority is 64, and the highest priority wins; if tied, the highest MAC address wins.

Answer

Configure 'set protocols isis reference-bandwidth <value>' globally.

Answer

Configure 'set protocols isis lsp-interval <value>' to speed up PDU transmission.

Answer

Configure 'set protocols isis level <1|2> metric ' to lower the interface cost.

Question 2

An engineer needs to configure a Junos device to operate in an IS-IS routing domain where the router should connect both Level 1 and Level 2 areas, forming a boundary between them. Which configuration command under the [edit protocols isis interface] hierarchy correctly enables dual-level operations on the interface?

Accepted Answer

set level 1-2 wide-metrics-only. On Junos, dual-level IS-IS operation on an interface is enabled by setting the interface's level to '1-2' under [edit protocols isis interface]. The 'wide-metrics-only' keyword is a valid modifier that tells the interface to advertise only wide (TLV 22) metrics for both levels, which is the modern recommended style. Together, 'set level 1-2 wide-metrics-only' both enables L1/L2 adjacency formation on the interface and constrains metric advertisement to wide metrics.

Answer

set border-router level 1-2

Answer

set area-type l1-l2

Answer

set routing-level both

Question 3

A network administrator troubleshooting an IS-IS adjacency issue notices that two routers are failing to form an adjacency over a point-to-point link. One router has wide metrics enabled globally, while the other router has wide metrics disabled and only supports narrow metrics. Which parameter or feature must be matching or handled for IS-IS link-state PDUs to be successfully processed between these neighbors?

Accepted Answer

The metric-style configuration must be compatible (such as enabling wide metrics on both sides or using transition modes) to ensure LSAs containing TLVs 128/130 or 22/135 are understood.. IS-IS supports narrow metrics (default in older implementations, max weight 63 per link) and wide metrics (supporting up to 2^24-1). If metric styles do not match or if compatibility modes (such as traffic-engineering metrics or metric-style transition settings) are not configured, routers may experience issues exchanging reachability, although point-to-point adjacencies can technically form, routing tables will not converge correctly without metric style alignment or wide-metrics configuration.

Answer

The DIS priority must be manually set to identical non-zero values on a point-to-point link.

Answer

The area addresses configured on both routers must be completely distinct from one another.

Answer

The manual system ID must be identical across both routing nodes.

Question 4

Which THREE statements are true regarding the Designated Intermediate System (DIS) election and its operational differences compared to OSPF's Designated Router (DR) election? (Choose three)

Accepted Answer

The IS-IS DIS election is non-preemptive; if a router with a higher priority comes online later, it does not take over until the current DIS fails or is restarted.. Option A is correct because IS-IS DIS election is non-preemptive: once a DIS is elected, a newly arriving router with a higher priority will not displace it until the DIS fails or the interface/adjacency is reset, unlike OSPF DR election which can be influenced by priority and re-election events. Option B is correct because IS-IS breaks priority ties by comparing the highest interface MAC address on broadcast media (or the highest System ID on point-to-point/non-broadcast media), which is a deterministic tie-breaker. Option D is correct because IS-IS has no Backup DIS role analogous to OSPF's BDR; instead, other routers on the segment maintain awareness of priorities and can assume the DIS role if the current DIS fails. Option C is not correct because DIS election is per-level and does not require all routers on the segment to share identical area addresses regardless of Level 1 or Level 2 status. Option E is not correct because the DIS floods LSPs using multicast (and on point-to-point links uses specific mechanisms), not exclusively unicast to all neighbors.

Answer

IS-IS DIS election requires all routers on the segment to have identical area addresses regardless of whether they are Level 1 or Level 2.

Answer

The DIS is responsible for flooding individual LSPs using unicast packets exclusively to all neighbors.

Question 5

You have two routers connected via a point-to-point link. You need to ensure they form an IS-IS adjacency. Which configuration is required?

Accepted Answer

Configure the interface as point-to-point and enable IS-IS on both routers.. On point-to-point links, IS-IS will form an adjacency regardless of the level match, provided the interface is enabled in the correct protocol hierarchy.

Answer

Configure a DIS priority on the interface.

Answer

Disable the wide-metrics setting on both ends.

Answer

Configure a unique System ID for both routers.

Question 6

You observe that an IS-IS adjacency is stuck in 'Initializing'. What is the most likely cause?

Accepted Answer

The interface level is set to Level 1, but the neighbor has no Level 1 area address match.. An IS-IS adjacency stuck in 'Initializing' means the local router has received a valid IIH from the neighbor but the neighbor has not yet acknowledged the local router's IIH (or the neighbor rejected it). The most common cause is a level mismatch: if the local interface is configured as Level 1 but the neighbor has no matching Level 1 area address, the neighbor will not form a Level 1 adjacency, leaving the local side in Initializing.

Answer

The system IDs are identical.

Answer

The interface is configured as point-to-point.

Answer

The MTU size is too small on one link.

Question 7

You are troubleshooting a DIS election issue on a broadcast segment. What is the tie-breaker if the DIS priorities are equal?

Accepted Answer

The highest MAC address.. If DIS priorities are equal, the router with the highest Subnetwork Point of Attachment (SNPA) address (usually the MAC address) wins.

Answer

The lowest System ID.

Answer

The lowest MAC address.

Answer

The highest System ID.

Question 8

You want to influence the path selection for IS-IS traffic. You decide to change the metric. Which TLV is used to carry wide metrics?

Accepted Answer

TLV 22. IS-IS wide metrics are carried in TLV 22 (Extended IS Reachability TLV), which supports 24-bit or 32-bit metric values, allowing for more granular and larger metrics than the narrow 6-bit metrics in TLV 2. TLV 22 is used when wide metrics are enabled, which is the default in modern Junos for IS-IS.

Answer

TLV 135

Answer

TLV 2

Answer

TLV 128

Question 9

What is the default IS-IS metric for an interface in Junos?

Accepted Answer

10. The default IS-IS metric in Junos is 10.

Answer

1

Answer

0

Answer

100

Question 10

You are observing that your IS-IS routers are not synchronizing their databases. You check the CSNP settings. What is the role of the CSNP on a broadcast segment?

Accepted Answer

To provide a summary of the entire database to neighbors.. CSNPs (Complete Sequence Number PDUs) are sent by the DIS to ensure all routers have the same Link State Database.

Answer

To acknowledge individual LSPs.

Answer

To initiate the DIS election.

Answer

To request specific missing LSPs.

Question 11

Which of the following is a valid IS-IS PDU type?

Accepted Answer

Link State PDU. IIH, LSP, CSNP, and PSNP are the four standard IS-IS PDU types.

Answer

Routing Update PDU

Answer

Database Description PDU

Answer

Hello PDU

Question 12

How does a router determine its System ID?

Accepted Answer

It is manually configured.. The System ID is manually configured in the 'set protocols isis system-id' hierarchy.

Answer

It is learned via ARP.

Answer

It is automatically derived from the MAC address.

Answer

It is assigned by the DIS.

Question 13

How do you configure an interface to be point-to-point when using IS-IS?

Accepted Answer

set protocols isis interface point-to-point. You use the 'set protocols isis interface point-to-point' command to force point-to-point mode.

Answer

set protocols isis interface-type point-to-point

Answer

set interfaces <name> unit 0 family iso point-to-point

Answer

set protocols isis link-type p2p

Question 14

Which command allows you to view the IS-IS database summary?

Accepted Answer

show isis database. The command 'show isis database' displays the IS-IS link-state database, including all LSPs (Link State PDUs) received from other routers, with details such as sequence number, checksum, holdtime, and TLVs. This allows administrators to verify topology information and troubleshoot adjacency or routing issues. It is the direct way to view the IS-IS database summary.

Answer

show isis statistics

Answer

show isis route

Answer

show isis summary

Question 15

You need to redistribute OSPF routes into IS-IS. Where do you configure this?

Accepted Answer

Under the 'protocols isis export' hierarchy.. Redistribution is configured under the 'policy-options' and applied under 'protocols isis export'.

Answer

Under the OSPF configuration only.

Answer

It is not possible to redistribute OSPF into IS-IS.

Answer

Under the interface level.

Question 16

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 17

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 18

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 19

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 20

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.