55 questions · IS IS · All types, answers revealed
How do you configure an interface to be point-to-point when using IS-IS?
This is the correct Junos CLI path.
Why this answer
You use the 'set protocols isis interface <name> point-to-point' command to force point-to-point mode.
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?
Point-to-point links do not require a DIS. They use the 3-way handshake mechanism to ensure reliable adjacency formation.
Why this answer
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.
Which factors influence the DIS election on a broadcast link? (Choose TWO)
Secondary tie-breaker.
Why this answer
The DIS is determined by the configured priority and, as a tie-breaker, the SNPA (MAC) address.
What happens if you have an area address mismatch in a Level 1 adjacency?
Area address matching is a hard requirement for L1.
Why this answer
Level 1 adjacencies require the routers to share at least one area address.
What is the purpose of the 'lsp-generation-interval'?
It prevents excessive updates during flapping.
Why this answer
This command controls how frequently the router generates a new LSP following a topology change.
Which of the following are true regarding the IS-IS 'hello-interval'? (Choose TWO)
This defines the timer.
Why this answer
The hello-interval controls how often Hellos are sent and directly impacts how quickly an adjacency times out.
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?
TLV 135 carries extended IP reachability information, supporting wide metrics and sub-TLV attributes.
Why this answer
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.
Which TLV is used to carry IP prefixes in IS-IS?
TLV 128 (IP Reachability) carries prefix info.
Why this answer
TLV 128 and 135 are used for IP reachability information.
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?
CSNPs contain the complete list of all LSPs in the router's database and are sent periodically by the DIS on broadcast networks.
Why this answer
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.
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?
TLV 22 is the standard for extended (wide) metrics.
Why this answer
TLV 22 (Extended IS Reachability) is used to carry wide metrics in IS-IS.
Which of the following are true regarding IS-IS authentication? (Choose TWO)
This is the primary benefit.
Why this answer
Authentication can be applied to PDUs (Hello/LSP/SNP) and can be configured globally or per interface.
Which command is used to verify the IS-IS adjacency state on a Junos device?
This is the correct operational command.
Why this answer
The 'show isis adjacency' command provides the current status of IS-IS neighbors.
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?
Point-to-point links do not require a DIS election and form adjacencies simply by being enabled.
Why this answer
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.
You have a Level 1/Level 2 router connecting two areas. How does it advertise itself to Level 1 routers?
The ATT (Attached) bit is used to signal the presence of an inter-area path.
Why this answer
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.
Which of the following are valid IS-IS interface types? (Choose TWO)
Standard for serial/P2P media.
Why this answer
Junos supports both broadcast and point-to-point interface types for IS-IS.
You are configuring multi-topology IS-IS. What does this allow?
This is the core purpose of multi-topology.
Why this answer
Multi-topology IS-IS allows for separate routing topologies for different protocol families (e.g., IPv4 and IPv6).
How does a router determine its System ID?
The administrator defines this value.
Why this answer
The System ID is manually configured in the 'set protocols isis system-id' hierarchy.
What does the 'show isis statistics' command reveal?
It shows PDU counts by type.
Why this answer
This command displays the number of PDUs sent and received by the IS-IS process.
Which command allows you to view the IS-IS database summary?
This displays the contents of the database.
Why this answer
The 'show isis database' command shows the LSPs currently in the database.
What happens if you have an LSP with an expired lifetime?
Expired LSPs are flushed from the database.
Why this answer
When an LSP's remaining lifetime reaches zero, it is purged from the database after a short period.
Which actions occur during an IS-IS SPF calculation? (Choose THREE)
Core SPF step.
Why this answer
SPF involves building a tree from the LSDB, calculating the shortest path, and updating the RIB.
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)
DIS election is strictly non-preemptive to maintain network stability.
Why this answer
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.
What is the effect of changing the reference bandwidth in IS-IS?
IS-IS relies on explicit metric configuration, not reference bandwidth.
Why this answer
IS-IS does not have a reference bandwidth concept like OSPF; metrics are configured manually or via interface cost.
What are valid ways to configure IS-IS on a Junos interface? (Choose TWO)
Alternative valid path.
Why this answer
You can enable IS-IS under the 'protocols isis interface' hierarchy or by adding the interface to the hierarchy directly.
Why would you set an interface to 'passive' in IS-IS?
This is the primary purpose of passive interfaces.
Why this answer
Setting an interface to passive allows the router to advertise the network as reachable without attempting to form an adjacency on that interface.
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?
The DIS priority ranges from 0 to 127, with the default being 64. The router with the highest priority is elected DIS.
Why this answer
DIS priority in Junos IS-IS is configured under the interface hierarchy using 'set protocols isis interface <interface-name> priority <value>'. Higher priority wins; default is 64, max is 127.
You observe that an IS-IS adjacency is stuck in 'Initializing'. What is the most likely cause?
Area address mismatch prevents the Level 1 adjacency from fully establishing.
Why this answer
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.
What are the consequences of a mismatch in metric style (narrow vs wide) between IS-IS neighbors? (Choose THREE)
Networks are not properly advertised.
Why this answer
Metric mismatch prevents the exchange of reachability info, leads to partial SPF calculation failure, and results in broken reachability.
Which level is enabled by default in Junos when configuring IS-IS on an interface?
Both levels are enabled by default.
Why this answer
By default, interfaces in Junos IS-IS are enabled for both Level 1 and Level 2.
What is the default behavior when an IS-IS router receives an LSP it already has?
The protocol ensures only the most recent information is kept.
Why this answer
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.
In a multi-access network, which PDU is responsible for the DIS election?
IIH PDUs carry the priority information for DIS election.
Why this answer
The IIH (IS-IS Hello) PDU contains the priority field used for DIS election on broadcast segments.
You are troubleshooting a DIS election issue on a broadcast segment. What is the tie-breaker if the DIS priorities are equal?
The highest SNPA (MAC address) is the secondary tie-breaker.
Why this answer
If DIS priorities are equal, the router with the highest Subnetwork Point of Attachment (SNPA) address (usually the MAC address) wins.
Which of the following are true regarding IS-IS Level 1/Level 2 routers? (Choose TWO)
Required for inter-area routing.
Why this answer
L1/L2 routers act as bridges between areas and must maintain both databases.
Which of the following scenarios would prevent an IS-IS adjacency from forming? (Choose TWO)
This causes authentication failure.
Why this answer
Adjacencies fail if authentication keys don't match or if the area address is incompatible (for Level 1).
Which TWO characteristics describe the behavior and properties of Level 1 and Level 2 routing in IS-IS? (Choose two)
Level 2 routing constitutes the backbone, connecting different areas together.
Why this answer
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.
Which of the following is a valid IS-IS PDU type?
LSP is a valid PDU type.
Why this answer
IIH, LSP, CSNP, and PSNP are the four standard IS-IS PDU types.
How does an IS-IS router identify the DIS in its hello packets?
The pseudonode ID makes the DIS unique for that segment.
Why this answer
The DIS is identified by the System ID and a pseudonode number (the 7th byte of the LSP ID).
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?
CSNPs summarize the database to maintain synchronization.
Why this answer
CSNPs (Complete Sequence Number PDUs) are sent by the DIS to ensure all routers have the same Link State Database.
Which of the following are valid ways to influence path selection in IS-IS? (Choose THREE)
Directly influences path metric.
Why this answer
You can influence paths by modifying the interface cost, using wide metrics, or using policies to adjust metrics during redistribution.
Which of the following are true about CSNP and PSNP? (Choose THREE)
Used for specific ACK.
Why this answer
CSNPs are sent periodically by the DIS to ensure database consistency; PSNPs are used to request/ack specific missing LSPs.
Which of the following is required for two routers to form a Level 2 adjacency?
Both routers must be configured to support Level 2.
Why this answer
For Level 2 adjacencies, the interfaces must be in the same level (or both be L1/L2) and share the same network segment.
When should you consider increasing the 'csnp-interval'?
It optimizes bandwidth usage.
Why this answer
Increasing the interval is useful on slow or congested links to reduce the frequency of full database synchronization traffic.
Which command displays the IS-IS interface parameters?
This command shows interface-specific IS-IS details.
Why this answer
'show isis interface' shows the configuration and state of IS-IS interfaces.
Which of the following are valid IS-IS PDU types? (Choose THREE)
Used for hello and adjacency.
Why this answer
The four PDU types are IIH, LSP, CSNP, and PSNP.
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?
The 'show isis database detail' command provides granular output regarding LSPs, including extended IP reachability TLVs and wide metric values.
Why this answer
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.
What is the default IS-IS metric for an interface in Junos?
10 is the default value.
Why this answer
The default IS-IS metric in Junos is 10.
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)
If the MTU is mismatched and padding causes IIH packets to exceed the smaller MTU, adjacency formation will fail.
Why this answer
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.
When using authentication in IS-IS, where can you apply the password?
Junos allows authentication at either level.
Why this answer
IS-IS authentication can be applied to the interface level or the global level (area/domain).
Which of the following are true about pseudonode LSPs? (Choose THREE)
This is their purpose.
Why this answer
Pseudonode LSPs are generated by the DIS, represent the broadcast segment, and effectively reduce the number of LSPs in the LSDB.
You need to redistribute OSPF routes into IS-IS. Where do you configure this?
This is where the export policy is applied.
Why this answer
Redistribution is configured under the 'policy-options' and applied under 'protocols isis export'.
What is the function of the DIS in an IS-IS broadcast network?
This reduces the number of LSPs needed in the database.
Why this answer
The DIS (Designated Intermediate System) is responsible for generating pseudonode LSPs to represent the broadcast network.
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?
The command 'set level 1-2 wide-metrics-only' correctly enables both Level 1 and Level 2 operations on the interface and enforces the use of wide metrics.
Why this answer
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.
What is the purpose of the 'lsp-lifetime' command?
This defines the expiration period.
Why this answer
The 'lsp-lifetime' command sets the duration after which an LSP is considered invalid if not refreshed.
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?
IS-IS uses different TLVs for narrow metrics (TLVs 128 and 130) versus wide metrics (TLVs 22 and 135). Mismatched metric styles prevent proper route calculation and database synchronization.
Why this answer
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.
Which of the following are true about IS-IS Level-1 and Level-2 areas? (Choose TWO)
Correct definition.
Why this answer
Level 1 areas are for intra-area routing, while Level 2 areas connect multiple Level 1 areas.
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