JNCIS-ENT IS IS — 20 Questions

Question 1

An administrator is configuring a new point-to-point link between two Junos routers running IS-IS. By default, how does IS-IS handle adjacency establishment on a point-to-point link compared to a broadcast link?

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

Accepted Answer

It uses a 3-way handshake TLV in IIH packets and bypasses the DIS election process entirely.. On point-to-point links, IS-IS uses the 3-way handshake extension (RFC 5303) via IIH packets to verify bidirectional connectivity without holding a DIS election.

Answer

It relies strictly on CSNPs sent every 10 seconds to establish the adjacency.

Answer

It requires manual mapping of neighbor system IDs using static ARP bindings.

Answer

It initiates a DIS election using priority values identical to broadcast segments.

Question 2

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?

Accepted Answer

Configure 'set protocols isis interface priority ' where a higher value wins.. DIS priority in Junos IS-IS is configured under the interface hierarchy using 'set protocols isis interface priority '. Higher priority wins; default is 64, max is 127.

Answer

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

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

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

Question 3

During the examination of IS-IS packet captures on a multi-access LAN interface, an engineer notices a specific PDU type being transmitted periodically by the Designated Intermediate System (DIS) to maintain synchronized link-state databases. Which PDU type performs this function?

Accepted Answer

Complete Sequence Numbers PDU (CSNP). The Complete Sequence Numbers PDU (CSNP) is sent periodically by the DIS on a multi-access network to list all the link-state database entries present on the router, ensuring all routers maintain database synchronization.

Answer

Partial Sequence Numbers PDU (PSNP)

Answer

Intermediate System Hello (IIH)

Answer

Level 1 Link State PDU (LSP)

Question 4

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. To enable a Junos router interface to participate in both Level 1 and Level 2 IS-IS routing, you configure the level statement under the interface with 'level 1/2 wide-metrics-only' or specify the levels. By default, interfaces are configured for level 1 and level 2.

Answer

set border-router level 1-2

Answer

set area-type l1-l2

Answer

set routing-level both

Question 5

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 6

A network engineer wants to verify IS-IS link-state database entries, specifically checking for TLVs that carry extended IP reachability information with wide metrics. Which Junos operational command should be used?

Accepted Answer

show isis database detail. To view the IS-IS link-state database with detailed TLV and metric information on Junos devices, the command 'show isis database detail' is used.

Answer

show isis adjacency extensive

Answer

show ospf database

Answer

show route protocols isis

Question 7

Which TWO characteristics describe the behavior and properties of Level 1 and Level 2 routing in IS-IS? (Choose two)

Accepted Answer

Level 2 routers form the backbone and can route traffic between different IS-IS areas.. Level 1 areas are responsible for intra-area routing, similar to OSPF non-backbone areas. Level 2 routers form the backbone network and route between different areas.

Answer

Level 1 routers are strictly prohibited from connecting to Level 2 routers under any circumstances.

Answer

Level 1 routing requires the explicit configuration of a designated Area 0 backbone.

Answer

Level 2 routers do not support wide metrics and only operate with narrow metrics.

Question 8

During an audit of an IS-IS routing network, an engineer examines the Type-Length-Value (TLV) structures used in IS-IS PDUs. Which TLV is specifically responsible for carrying IPv4 internal and external reachability information when using wide metrics?

Accepted Answer

TLV 135. In IS-IS, TLV 135 (Extended IP Reachability) is used to carry IPv4 prefixes when wide metrics are enabled, replacing the older narrow metric TLV 128.

Answer

TLV 128

Answer

TLV 1

Answer

TLV 2

Question 9

Which command is used to verify the IS-IS adjacency state on a Junos device?

Accepted Answer

show isis adjacency. The 'show isis adjacency' command provides the current status of IS-IS neighbors.

Answer

show isis interface

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show isis database

Answer

show protocols isis neighbors

Question 10

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.. IS-IS DIS election is non-preemptive (unlike OSPF, which is also non-preemptive by default unless changed, but IS-IS never preempts even if a higher priority router joins later), supports backup DIS implicitly via priority, and uses higher priority and then higher MAC address as a tie-breaker.

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 11

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 12

An engineer is troubleshooting IS-IS adjacency formation failures on a Junos device. Which THREE parameters must match or be compatible between neighboring routers to successfully establish an IS-IS adjacency? (Choose three)

Accepted Answer

Link-level MTU size, as oversized IIH packets may be dropped if they exceed the interface MTU.. IS-IS adjacencies require matching area addresses (for Level 1), compatible link/network types, and correct authentication settings if configured. System IDs must be unique, and maximum transmission unit (MTU) mismatches can prevent IIH packet processing.

Answer

Identical system IDs configured on both neighboring nodes

Answer

Identical DIS priority values on multi-access networks

Question 13

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 'Initializing' state indicates that one side sees the other, but the other side does not see the first, often due to mismatched area addresses or authentication.

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 14

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 15

You have a Level 1/Level 2 router connecting two areas. How does it advertise itself to Level 1 routers?

Accepted Answer

It advertises a default route using the ATT bit.. A Level 1/2 router sets the ATT bit in its Level 1 LSPs, signaling to Level 1 routers that it is an exit point to other areas.

Answer

It requires a manual default-route configuration on all L1 routers.

Answer

It floods the entire Level 2 LSDB into the Level 1 area.

Answer

It sends a specific inter-area TLV to all neighbors.

Question 16

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. TLV 22 (Extended IS Reachability) is used to carry wide metrics in IS-IS.

Answer

TLV 135

Answer

TLV 2

Answer

TLV 128

Question 17

In a multi-access network, which PDU is responsible for the DIS election?

Accepted Answer

IIH PDU. The IIH (IS-IS Hello) PDU contains the priority field used for DIS election on broadcast segments.

Answer

CSNP

Answer

LSP

Answer

PSNP

Question 18

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 19

What is the effect of changing the reference bandwidth in IS-IS?

Accepted Answer

It has no effect on IS-IS.. IS-IS does not have a reference bandwidth concept like OSPF; metrics are configured manually or via interface cost.

Answer

It automatically recalculates all interface metrics.

Answer

It increases the hello interval.

Answer

It forces all links to use narrow metrics.

Question 20

When using authentication in IS-IS, where can you apply the password?

Accepted Answer

At both the interface and global levels.. IS-IS authentication can be applied to the interface level or the global level (area/domain).

Answer

Only on the loopback interface.

Answer

Only at the global level.

Answer

Only at the interface level.