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Nokia NRS II Composite (IS-IS) study materials
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Nokia NRS II Composite (IS-IS) practice questions

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Question 1IS-IS fundamentals and protocol operation

In IS-IS, which Protocol Data Unit is used by a router to advertise its own link-state information within an area?

  1. Intermediate System to Intermediate System Hello PDU (IIH)
  2. Complete Sequence Number PDU (CSNP)
  3. Partial Sequence Number PDU (PSNP)
  4. Level 1 Link State PDU (LSP) Correct answer.

Explanation

Link State PDUs carry the actual link-state payload: neighbor adjacencies, IP prefixes and reachability information. A Level 1 LSP stays within the area, while a Level 2 LSP is flooded in the backbone level. Hellos, CSNPs and PSNPs are control PDUs used for adjacency formation and database synchronization, not for carrying topology content.

Question 2IS-IS fundamentals and protocol operation

Which IS-IS PDU type is responsible for establishing and maintaining adjacencies between neighboring routers?

  1. Hello PDU (IIH) Correct answer.
  2. Routing Information PDU
  3. Link State PDU
  4. Sequence Number PDU

Explanation

IS-IS uses Intermediate System to Intermediate System Hello PDUs to discover neighbors on a circuit and to maintain the adjacency through periodic transmission. Hellos carry parameters such as area addresses, holding time, and the level being negotiated. Without a valid IIH exchange, no adjacency forms and no LSPs are exchanged.

Question 3IS-IS fundamentals and protocol operation

On a broadcast multi-access IS-IS segment, which mechanism is used to reduce the number of adjacencies and LSP flooding?

  1. Configuration of point-to-point network type
  2. Election of a Backup Designated Router
  3. Election of a Designated Intermediate System (DIS) Correct answer.
  4. Use of a virtual link

Explanation

On broadcast media IS-IS elects a Designated Intermediate System that originates a pseudonode LSP representing the segment. All other routers form adjacencies with the DIS and use it to distribute flooding, reducing the full mesh of adjacencies. The DIS role, unlike OSPF's DR, is preemptive and there is no backup DIS.

Question 4IS-IS fundamentals and protocol operation

What is the default IS-IS router type behavior when a Cisco or Nokia router is configured with a single Level 1-2 instance and no level restriction?

  1. Level 2-3, forming Level 2 and Level 3 adjacencies
  2. Level 1 only
  3. Level 2 only
  4. Level 1-2, forming both Level 1 and Level 2 adjacencies Correct answer.

Explanation

By default an IS-IS process operates as a Level 1-2 router, maintaining a Level 1 database for its area and a Level 2 database for the backbone. It forms Level 1 adjacencies with same-area neighbors and Level 2 adjacencies with other Level 2-capable routers. Level restrictions must be explicitly configured to change this.

Question 5IS-IS fundamentals and protocol operation

Which address family does the IS-IS NET (Network Entity Title) belong to?

  1. IPv4
  2. OSI/CLNS Correct answer.
  3. IPv6
  4. MAC

Explanation

The NET is an OSI Network Service Access Point address, so it uses CLNS addressing rather than IP. It consists of an area address, a system ID, and an NSEL of 00. IS-IS runs directly over Layer 2 and uses these OSI addresses for its own operation.

Question 6IS-IS fundamentals and protocol operation

What is the purpose of the System ID portion of an IS-IS NET?

  1. It defines the DIS priority on a segment.
  2. It uniquely identifies the router within the IS-IS domain. Correct answer.
  3. It selects the NSEL for the transport protocol.
  4. It identifies the area to which the router belongs.

Explanation

The System ID is a fixed-length field (typically 6 bytes) that uniquely identifies the intermediate system within the routing domain. The area address identifies the area, and the NSEL (usually 00) identifies the service. Two routers in the same domain must not share the same System ID.

Question 7IS-IS fundamentals and protocol operation

Which IS-IS PDU type carries a list of all LSPs in the sender's database with their sequence numbers and remaining lifetime?

  1. Partial Sequence Number PDU
  2. Complete Sequence Number PDU Correct answer.
  3. Level 2 LSP
  4. Level 1 LAN IIH

Explanation

A Complete Sequence Number PDU summarizes the entire link-state database of the sender, listing each LSP ID, sequence number, checksum, and remaining lifetime. Receivers compare this list against their own database to detect missing or outdated LSPs. PSNPs, by contrast, reference only specific LSPs.

Question 8IS-IS fundamentals and protocol operation

Which metric type does IS-IS use by default when no wide metrics are configured?

  1. Wide metric with a 32-bit field
  2. Narrow metric with a 6-bit field per link Correct answer.
  3. TE metric with a 32-bit field
  4. Wide metric with a 24-bit field

Explanation

The original IS-IS narrow metric encodes each link cost in 6 bits, limiting values to 0-63, and the total path metric in 10 bits. Wide metrics (24-bit per link) were introduced later and are required for TE and for IPv6. Without explicit wide-metric configuration, the narrow format applies.

Question 9IS-IS fundamentals and protocol operation

A router receives an LSP with a higher sequence number than the copy in its database. What action does it take?

  1. It replaces the stored LSP with the newer one and floods it to other adjacencies. Correct answer.
  2. It waits for the LSP's remaining lifetime to expire before updating.
  3. It requests a full CSNP from the sender before updating.
  4. It discards the newer LSP and keeps its existing copy.

Explanation

IS-IS uses sequence numbers to determine which version of an LSP is most recent. When a received LSP has a higher sequence number, the router installs it, runs SPF, and floods it onward except on the interface where it arrived. This flooding ensures all routers converge on the same link-state database.

Question 10IS-IS fundamentals and protocol operation

What is the function of the overload bit in an IS-IS LSP?

  1. It signals that the originating router's resources are exhausted so other routers avoid using it as a transit hop. Correct answer.
  2. It reduces the metric of all links to zero.
  3. It disables the DIS election on all broadcast interfaces.
  4. It forces the router to become a Level 1-only router.

Explanation

When the overload bit is set in a router's LSP, other routers continue to reach prefixes directly attached to that router but do not use it as an intermediate hop for transit traffic. This is commonly used during startup or maintenance to avoid black-holing traffic. The bit is carried in the LSP header.

Question 11IS-IS fundamentals and protocol operation

Which TLV in an IS-IS LSP is used to advertise IPv4 interface addresses and prefixes?

  1. TLV 132 (IP Interface Address)
  2. TLV 22 (Extended IS Reachability)
  3. TLV 135 (Extended IP Reachability)
  4. TLV 128 (IP Internal Reachability Information) Correct answer.

Explanation

TLV 128 carries internal IPv4 reachability information in the classic narrow-metric format, advertising prefixes internal to the area. TLV 130 carries external IPv4 reachability. TLV 22 advertises neighbor reachability, and TLV 132 advertises interface addresses rather than prefix reachability.

Question 12IS-IS fundamentals and protocol operation

Which of the following best describes how a Level 1-2 router handles traffic destined outside its area?

  1. It sets the attached bit and drops the traffic.
  2. It forwards the traffic using its Level 2 database toward the nearest Level 2 router. Correct answer.
  3. It forwards the traffic using its Level 1 database only.
  4. It converts the Level 2 LSP into a Level 1 LSP.

Explanation

A Level 1-2 router maintains both databases and acts as an inter-area gateway. For destinations outside the local area, it uses its Level 2 topology to forward toward the backbone. Level 1-only routers rely on the default route learned from the attached bit to reach such a gateway.

Question 13IS-IS fundamentals and protocol operation

What does the attached bit in a Level 1 LSP indicate?

  1. That the originating router is in the overload state
  2. That the originating router has no IPv4 addresses configured
  3. That the originating router is the DIS for its segment
  4. That the originating router has at least one Level 2 adjacency and can reach other areas Correct answer.

Explanation

A Level 1-2 router sets the attached bit in its Level 1 LSP to tell intra-area Level 1 routers that it provides connectivity to the backbone. Level 1-only routers use this to install a default route toward the nearest attached router. Multiple attached routers can exist, and the closest is chosen by metric.

Question 14IS-IS fundamentals and protocol operation

How does IS-IS elect the DIS on a broadcast segment when two routers advertise the same interface priority?

  1. The router with the highest MAC address wins.
  2. The router that has been up the longest wins.
  3. The router with the lowest System ID wins.
  4. The router with the highest System ID wins. Correct answer.

Explanation

DIS election first compares interface priority, and the highest priority wins. When priorities tie, the router with the highest System ID is selected. Unlike OSPF, the election is preemptive, so a newly arriving higher-priority router immediately takes over as DIS.

Question 15IS-IS fundamentals and protocol operation

What is the purpose of the pseudonode LSP generated by the DIS?

  1. It synchronizes the database between Level 1 and Level 2.
  2. It carries the DIS's own loopback prefix to the backbone.
  3. It represents the broadcast segment as a virtual node so the SPF tree models the multi-access topology correctly. Correct answer.
  4. It replaces the DIS's own LSP in the database.

Explanation

The DIS originates a pseudonode LSP whose System ID is the DIS System ID with a non-zero pseudonode ID. It represents the shared segment as a single virtual node, and all routers on the segment list the pseudonode as a neighbor. This lets SPF compute paths that correctly reflect the broadcast topology.

Question 16IS-IS fundamentals and protocol operation

Which condition must be satisfied before two IS-IS routers on the same broadcast segment form a Level 1 adjacency?

  1. Their Level 2 databases must already be synchronized.
  2. Their interface priorities must be equal.
  3. Their System IDs must be identical.
  4. Their area addresses must match and their level capabilities must be compatible. Correct answer.

Explanation

Level 1 adjacency requires matching area addresses because Level 1 is intra-area. The hello exchange also checks level compatibility, authentication, and MTU. System IDs must be unique, not identical, and Level 2 database state is irrelevant to forming a Level 1 adjacency.

Question 17IS-IS fundamentals and protocol operation

What happens when a router receives an LSP with the same sequence number but a different checksum than the copy in its database?

  1. It keeps its existing copy and ignores the received LSP. Correct answer.
  2. It resets the adjacency with the sender.
  3. It purges both copies from the database.
  4. It accepts the received LSP as newer.

Explanation

IS-IS considers an LSP with the same sequence number and checksum to be identical. If the checksum differs at the same sequence number, the received LSP is treated as a duplicate or corrupted and discarded. The router retains its existing entry and does not flood the received version.

Question 18IS-IS fundamentals and protocol operation

Which authentication mechanism in IS-IS provides per-PDU authentication using HMAC-MD5 and is carried in a dedicated TLV?

  1. Cleartext password in the IIH only
  2. Simple password in the LSP header
  3. MD5 authentication using the IP header option
  4. HMAC-MD5 authentication TLV Correct answer.

Explanation

IS-IS supports an authentication TLV that can carry cleartext, HMAC-MD5, or other digest types, and it can be applied to hellos, LSPs, CSNPs, and PSNPs. HMAC-MD5 provides stronger protection than cleartext and is carried in the TLV rather than in the fixed header. Authentication can be configured per level and per PDU type.

Question 19IS-IS fundamentals and protocol operation

Which IS-IS feature allows a router to advertise a prefix with a different metric than the interface metric, commonly used for route leaking?

  1. TLV 132 interface address
  2. Wide metric TLV 135 with an up/down bit Correct answer.
  3. Narrow metric TLV 128 with an external bit
  4. TLV 22 extended IS reachability

Explanation

Route leaking from Level 2 into Level 1 uses extended IP reachability TLVs (135 for IPv6, 135 or 128 variants for IPv4) with the up/down bit set to prevent the leaked prefix from being re-advertised back into Level 2. The prefix can be advertised with a specific metric. This enables optimal intra-area routing to destinations outside the area.

Question 20IS-IS fundamentals and protocol operation

Which of the following is a valid reason to enable wide metrics in an IS-IS network?

  1. To allow the router to operate as Level 1 only
  2. To support link costs greater than 63 and to advertise IPv6 prefixes Correct answer.
  3. To reduce the LSP flooding scope
  4. To disable the DIS election

Explanation

Narrow metrics limit link costs to 6 bits (0-63) and the total path to 10 bits, which is insufficient for large or TE-enabled networks. Wide metrics provide 24-bit link costs and are required for IPv6 reachability TLVs. Enabling wide metrics is common when TE or large cost values are needed.

Question 21IS-IS fundamentals and protocol operation

A Level 1-2 router has multiple Level 2 adjacencies. How does it choose the next hop for a prefix learned via Level 2?

  1. It uses the Level 1 default route.
  2. It selects the neighbor with the lowest interface priority.
  3. It selects the neighbor with the highest System ID.
  4. It runs SPF on the Level 2 database and selects the lowest-cost path. Correct answer.

Explanation

Level 2 routing uses the Level 2 link-state database and SPF to compute shortest paths to destination prefixes. Among equal-cost paths, ECMP may install multiple next hops. Level 1 default routes are irrelevant once the packet is being forwarded within Level 2.

Question 22IS-IS fundamentals and protocol operation

Which scenario causes a router to generate a PSNP rather than a CSNP?

  1. When it wants to summarize its entire database to a new neighbor
  2. When it needs to request a specific missing LSP or acknowledge a specific received LSP Correct answer.
  3. When it elects itself as DIS on a broadcast segment
  4. When it sets the overload bit in its own LSP

Explanation

PSNPs reference individual LSPs and are used to request a missing LSP or to acknowledge receipt of a specific LSP. CSNPs summarize the whole database and are typically sent by the DIS on broadcast segments or at adjacency bring-up on point-to-point links. This distinction keeps flooding efficient.

Question 23IS-IS fundamentals and protocol operation

Consider a Level 1-2 router that loses all its Level 2 adjacencies. What is the immediate effect on its Level 1 LSP?

  1. It stops flooding Level 1 LSPs entirely.
  2. It changes its System ID to reflect the loss.
  3. It sets the overload bit for all prefixes.
  4. It clears the attached bit, and Level 1 routers remove the default route toward it. Correct answer.

Explanation

The attached bit reflects whether the router has at least one Level 2 adjacency and can reach other areas. When the last Level 2 adjacency is lost, the router clears the attached bit and re-originates its Level 1 LSP. Level 1-only routers then remove the default route pointing to it, provided no other attached router exists.

Question 24IS-IS fundamentals and protocol operation

Which statement accurately describes the difference between IS-IS and OSPF regarding DIS behavior on broadcast segments?

  1. IS-IS has a non-preemptive DIS with a backup, while OSPF has a preemptive DR with no backup.
  2. Both protocols elect a preemptive DIS with a backup.
  3. IS-IS has a preemptive DIS with no backup, while OSPF has a non-preemptive DR with a BDR. Correct answer.
  4. Both protocols elect a non-preemptive DIS with no backup.

Explanation

IS-IS elects a DIS that can be preempted immediately by a newly arriving higher-priority router, and there is no backup DIS. OSPF elects a DR and BDR, and the election is non-preemptive unless the DR fails. These differences affect convergence behavior and design considerations on broadcast segments.

Question 25IS-IS fundamentals and protocol operation

A network designer wants to prevent a specific router from being used as a transit node during a maintenance window while still allowing its directly connected prefixes to be reachable. Which IS-IS mechanism best achieves this?

  1. Set the overload bit in the router's LSP. Correct answer.
  2. Change the router to Level 1 only.
  3. Increase all interface metrics to 63.
  4. Disable wide metrics on the router.

Explanation

The overload bit tells other routers to avoid the node for transit while still allowing traffic to prefixes directly attached to it. This is the standard mechanism for graceful maintenance and startup. Changing levels or metrics would not achieve the same controlled effect.

Question 26IS-IS fundamentals and protocol operation

In an IS-IS network, which protocol data units are used to establish and maintain adjacencies between routers?

  1. Complete sequence number PDUs
  2. Link state PDUs
  3. Hello PDUs Correct answer.
  4. Partial sequence number PDUs

Explanation

Hello PDUs are the only IS-IS PDUs whose job is adjacency formation and keepalive on a link. LSPs, CSNPs, and PSNPs all distribute or synchronize database content once adjacencies already exist.

Question 27IS-IS fundamentals and protocol operation

Which statement describes the default behavior of a Cisco IOS XR or Nokia SR OS router running IS-IS at Level 1 only when it needs to reach a destination in another area?

  1. It sends the packet toward the closest Level 1-2 router in its own area, which then forwards it into Level 2 Correct answer.
  2. It promotes itself to Level 2 for that flow and computes a path across the backbone
  3. It drops the packet because Level 1 routers never install a route toward other areas
  4. It forwards the packet to its nearest Level 2-capable neighbor using a default route learned from the attached bit

Explanation

A Level 1 router only knows its own area's topology; when it sees the attached bit set in a Level 1 LSP it installs a default route pointing at the nearest Level 1-2 router. That L1-2 router then forwards the traffic into the Level 2 backbone toward the destination area.

Question 28IS-IS fundamentals and protocol operation

A router running IS-IS receives an LSP whose remaining lifetime field is zero. What does the router do with this LSP?

  1. It holds the LSP in a hold-down queue for later re-advertisement
  2. It floods the LSP normally and resets the lifetime to the maximum value
  3. It purges the LSP from its database and floods the purge throughout the area Correct answer.
  4. It ignores the LSP because zero lifetime is an invalid value

Explanation

An LSP with zero remaining lifetime is a purge: the originator or any receiver removes it from the database and floods it so every router in the area also removes it. The sequence number is retained to prevent stale copies from being reinstalled.

Question 29IS-IS fundamentals and protocol operation

Two IS-IS routers are connected over a point-to-point link and both are configured for Level 1 and Level 2. Which adjacency types form?

  1. Separate Level 1 and Level 2 adjacencies over the same link Correct answer.
  2. A single Level 2 adjacency only
  3. An adjacency that alternates between Level 1 and Level 2 based on traffic
  4. A single Level 1 adjacency only

Explanation

On a point-to-point link IS-IS runs independent Hello exchanges for each configured level, so a Level 1 and a Level 2 adjacency coexist over the same physical circuit. Each level maintains its own database and SPF independently.

Question 30IS-IS fundamentals and protocol operation

A network operator enables wide metrics on all routers in an IS-IS domain. Which change in behavior should be expected for path selection compared with narrow metrics?

  1. Link metrics are advertised with 24-bit values, so links above 63 can be distinguished and TE-style asymmetric costs can be expressed Correct answer.
  2. Link metrics remain 6-bit but are aggregated across the area to give a wider view of path cost
  3. Link metrics are advertised only in Level 2 LSPs and ignored at Level 1
  4. Link metrics are replaced by hop count for SPF computation to avoid overflow

Explanation

Narrow IS-IS metrics are 6-bit (0-63), which caps per-link cost and prevents expressing high-bandwidth or asymmetric costs. Wide metrics use a 24-bit field in a dedicated TLV, so operators can assign granular costs and TE-style asymmetric metrics that SPF will honor.

Question 31IS-IS addressing, NSAP/NET structure and area design

In an NSAP address used by IS-IS, which component identifies the area to which the router belongs?

  1. The entire 20-byte NSAP, which must be unique per area
  2. The NSEL byte, which is always set to 0x00 for routers
  3. The System ID portion of the NSAP
  4. The Area Address, formed from the IDP plus the high-order DSP bytes Correct answer.

Explanation

The Area Address is the variable-length leading part of the NSAP consisting of the Initial Domain Part and the high-order bytes of the Domain Specific Part, and all routers sharing that prefix are in the same IS-IS area. The System ID identifies the individual node within the area, and the NSEL identifies the service (0x00 for the routing process).

Question 32IS-IS addressing, NSAP/NET structure and area design

What is the maximum length of an NSAP address as defined by ISO/IEC 8348?

  1. 8 bytes
  2. 32 bytes
  3. 16 bytes
  4. 20 bytes Correct answer.

Explanation

An NSAP may be up to 20 bytes (160 bits) long, of which the last byte is the NSEL. IS-IS implementations typically support both the full 20-byte form and shorter addresses as long as the System ID is 6 bytes.

Question 33IS-IS addressing, NSAP/NET structure and area design

A router's NET is configured as 49.0001.1921.6800.1001.00. What is the System ID in this NET?

  1. 00
  2. 49.0001.1921.6800.1001
  3. 49.0001
  4. 1921.6800.1001 Correct answer.

Explanation

In a NET, the System ID is the 6-byte field that follows the Area Address and precedes the NSEL. Here the Area Address is 49.0001 and the trailing 00 is the NSEL, leaving 1921.6800.1001 as the System ID.

Question 34IS-IS addressing, NSAP/NET structure and area design

Which statement correctly describes the relationship between a NET and an NSAP?

  1. A NET is the IP address of the loopback interface mapped into the NSAP space
  2. A NET is an NSAP whose NSEL is 0x00, used to identify a router's IS-IS process Correct answer.
  3. A NET is a 20-byte address while an NSAP is always 8 bytes
  4. A NET is an NSAP whose System ID is set to all zeros

Explanation

A Network Entity Title is simply an NSAP with the NSEL field set to 0x00, which is the value reserved for the network entity itself. It therefore carries the Area Address and System ID used by IS-IS while denoting the routing process rather than an upper-layer service.

Question 35IS-IS addressing, NSAP/NET structure and area design

Which NET is syntactically valid for an IS-IS router?

  1. 49.0001.1921.6800.1001.00.01
  2. 49.0001.192168001001.00
  3. 49.0001.1921.6800.1001.00 Correct answer.
  4. 49.0001.1921.6800.1001

Explanation

A valid NET has an Area Address, a 6-byte System ID, and a 1-byte NSEL of 00, with each byte written as two hexadecimal digits. Option 0 satisfies all of these: 49.0001 is the area, 1921.6800.1001 is the 6-byte System ID, and 00 is the NSEL.

Question 36IS-IS addressing, NSAP/NET structure and area design

Why must every router in the same IS-IS area share an identical Area Address?

  1. Because the Area Address determines the Level 2 metric type used in the area
  2. Because the Area Address is used as the LSP identifier for the area
  3. Because the Area Address is hashed into the DIS election priority
  4. Because area membership is derived from the Area Address, and mismatched prefixes place routers in different areas Correct answer.

Explanation

IS-IS classifies a router into an area based on the Area Address portion of its NET, so routers with different area prefixes form separate areas even if they share a physical link. Matching Area Addresses is what allows them to form the same Level 1 topology.

Question 37IS-IS addressing, NSAP/NET structure and area design

A router is configured with three NETs that share the same System ID but have different Area Addresses. What is the effect on IS-IS?

  1. The router cannot run IS-IS until the extra NETs are removed
  2. The router forms three separate Level 1 areas and elects three DISs
  3. The router is placed in the area of the first NET and the other NETs are rejected
  4. The router belongs to multiple areas, but only the first configured Area Address is used in Level 1 LSPs Correct answer.

Explanation

IS-IS allows a router to have multiple Area Addresses, which makes it a member of several areas, but the Level 1 LSP carries only the Area Addresses of the area the router is advertising into. In practice, only the first configured Area Address is used for Level 1 LSPs, while all Area Addresses can appear in Level 2 LSPs.

Question 38IS-IS addressing, NSAP/NET structure and area design

What is the purpose of the 6-byte System ID within a NET?

  1. It specifies the NSAP selector for the routing process
  2. It identifies the area in which the router resides
  3. It encodes the Level 2 backbone topology
  4. It uniquely identifies the IS within its area Correct answer.

Explanation

The System ID is a fixed 6-byte field that uniquely identifies each Intermediate System within an area, and it is the basis for LSP identifiers and pseudonode IDs. It must be unique among routers in the same area.

Question 39IS-IS addressing, NSAP/NET structure and area design

Which Area Address format is a private address that is recommended when connecting to the public OSI domain is not required?

  1. An address beginning with 49, the local format Correct answer.
  2. An address beginning with 39, the ISO DCC format
  3. An address beginning with 00, the reserved format
  4. An address beginning with 47, the ICD format

Explanation

Area Addresses beginning with the AFI value 49 are locally administered and are not registered with any authority, which makes them the usual choice for private IS-IS deployments. Formats such as 39 and 47 are globally administered and require proper registration.

Question 40IS-IS addressing, NSAP/NET structure and area design

A network designer wants to merge two IS-IS areas into one without readdressing every router. Which approach is valid?

  1. Change every System ID to the same value so the routers share an identity
  2. Assign a common Area Address to all routers and keep the existing System IDs unique Correct answer.
  3. Set the NSEL of every router to 0x01 to force area merging
  4. Remove the Level 2 backbone so the areas automatically combine

Explanation

Area membership is determined solely by the Area Address, so giving all routers the same Area Address places them in one area. The System IDs must remain unique within that merged area to avoid LSP identifier conflicts.

Question 41IS-IS addressing, NSAP/NET structure and area design

Which NSEL value is used in a NET for the IS-IS routing process?

  1. 0xFE
  2. 0x01
  3. 0xFF
  4. 0x00 Correct answer.

Explanation

The NSEL of 0x00 denotes the network entity itself, so a NET is an NSAP with an NSEL of 00. Other NSEL values would refer to different upper-layer services rather than the routing process.

Question 42IS-IS addressing, NSAP/NET structure and area design

An engineer configures a NET of 49.0001.1921.6800.1001.01. What is the consequence?

  1. The router runs IS-IS normally because the NSEL is ignored
  2. The router enters Level 2-only mode automatically
  3. The router rejects the NET because the NSEL must be 00 Correct answer.
  4. The router uses 01 as the pseudonode identifier

Explanation

A NET must have an NSEL of 00 to identify the network entity; any other value makes the address an NSAP for a different service and is not a valid NET. Implementations therefore reject such a configuration.

Question 43IS-IS addressing, NSAP/NET structure and area design

Two routers on the same broadcast link have Area Addresses 49.0001 and 49.0002. What happens at Level 1?

  1. They form a Level 1 adjacency and exchange Level 1 LSPs with each other
  2. They form a Level 1 adjacency only after the DIS is elected
  3. They do not form a Level 1 adjacency because they belong to different areas Correct answer.
  4. They form a Level 1 adjacency because the System IDs differ

Explanation

Level 1 adjacencies require the routers to share an Area Address, so differing area prefixes prevent a Level 1 adjacency from forming. The routers can still form a Level 2 adjacency because Level 2 is area-independent.

Question 44IS-IS addressing, NSAP/NET structure and area design

Which statement about the Area Address length in a NET is correct?

  1. It is variable in length, from 1 to 13 bytes Correct answer.
  2. It must always be exactly 3 bytes
  3. It must be exactly 6 bytes to match the System ID
  4. It is fixed at 8 bytes for all AFI values

Explanation

The Area Address is variable in length, with the IDP and high-order DSP bytes forming a prefix that can range from 1 to 13 bytes depending on the AFI and format. The System ID is the fixed 6-byte field that follows it.

Question 45IS-IS addressing, NSAP/NET structure and area design

A router has a NET with Area Address 49.0001 and is connected to a Level 2 backbone. Which statement about its Level 2 LSP is correct?

  1. It advertises all Area Addresses configured on the router in the Area Addresses TLV Correct answer.
  2. It omits the Area Addresses TLV because Level 2 does not use areas
  3. It advertises the System ID in place of the Area Address
  4. It advertises only the Area Address 49.0001 in the Area Addresses TLV

Explanation

Level 2 LSPs carry the Area Addresses TLV listing all Area Addresses configured on the router, which allows the backbone to know which areas are reachable through that node. Level 1 LSPs, by contrast, carry only the Area Addresses of the area being advertised.

Question 46IS-IS addressing, NSAP/NET structure and area design

Which of the following is a valid reason to assign multiple Area Addresses to a single router?

  1. To enable Level 1-only operation on a backbone link
  2. To reduce the size of the Level 1 database
  3. To allow the router to be a member of more than one area during a migration Correct answer.
  4. To increase the maximum LSP lifetime

Explanation

Multiple Area Addresses let a router participate in more than one area, which is useful during area renumbering or migration so that the router remains reachable from both old and new areas. It has no effect on timer values or database size.

Question 47IS-IS addressing, NSAP/NET structure and area design

What is the correct interpretation of the NET 39.0f01.0002.0000.0000.0001.00?

  1. Area Address 39.0f01.0002, System ID 0000.0000.0001, NSEL 00
  2. Area Address 39, System ID 0f01.0002.0000, NSEL 0000.0001.00
  3. Area Address 39.0f01, System ID 0002.0000.0000, NSEL 00 Correct answer.
  4. Area Address 39.0f01.0002.0000, System ID 0000.0001, NSEL 00

Explanation

The last byte is the NSEL, the preceding 6 bytes are the System ID, and everything before that is the Area Address. Here the trailing 00 is the NSEL, 0002.0000.0000 is the 6-byte System ID, and 39.0f01 is the Area Address.

Question 48IS-IS addressing, NSAP/NET structure and area design

Which AFI value in an NSAP indicates an address administered by an International Code Designator authority?

  1. 47 Correct answer.
  2. 00
  3. 39
  4. 49

Explanation

The AFI value 47 identifies the International Code Designator format, which is allocated by an ICD registration authority. AFI 39 is the ISO DCC format and 49 is the local format.

Question 49IS-IS addressing, NSAP/NET structure and area design

A router in area 49.0001 needs to reach a prefix in area 49.0002. What is the role of the Level 2 backbone in this path?

  1. It is not needed because all routers share the same System ID space
  2. It performs NAT between the two area addresses
  3. It provides the path between areas because Level 1 routers only know their own area Correct answer.
  4. It carries Level 1 LSPs between the areas so prefixes propagate directly

Explanation

Level 1 routers only maintain a Level 1 database for their own area and use a default route toward the nearest Level 1/2 router. Inter-area reachability is provided by Level 2 routers, which advertise area prefixes across the backbone.

Question 50IS-IS addressing, NSAP/NET structure and area design

Which statement about the System ID in a NET is correct when a router has multiple NETs?

  1. The System ID may be omitted if an Area Address is present
  2. All NETs on the same router must use the same System ID Correct answer.
  3. The System ID must match the router's IP loopback address
  4. Each NET must use a different System ID to avoid conflicts

Explanation

When multiple NETs are configured on one router, they must all share the same System ID so that the router has a single identity in the topology. Only the Area Addresses may differ between the NETs.