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JNCIS-SP practice questions

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Question 1OSPF (multi-area, area types, metrics, convergence, authentication)

In Junos OS, what is the default OSPF hello interval on a broadcast multiaccess interface?

  1. 40 seconds
  2. 120 seconds
  3. 10 seconds Correct answer.
  4. 30 seconds

Explanation

Broadcast and point-to-point interfaces default to a 10-second hello timer, and the dead interval defaults to four times that value. Neighbors must agree on the timers or the adjacency will not form.

Question 2OSPF (multi-area, area types, metrics, convergence, authentication)

Which IPv4 multicast address do OSPF routers use when sending hello packets to all OSPF routers on a segment?

  1. 224.0.0.6
  2. 224.0.0.10
  3. 224.0.0.5 Correct answer.
  4. 224.0.0.9

Explanation

OSPF uses 224.0.0.5 (AllSPFRouters) for packets destined to every OSPF router on the link. The address 224.0.0.6 (AllDRouters) is used only by the DR and BDR for flooding updates.

Question 3OSPF (multi-area, area types, metrics, convergence, authentication)

A Junos router has two loopback interfaces, 10.1.1.1/32 and 10.2.2.2/32, and no explicit router-id is configured. Which address becomes the OSPF router ID?

  1. 10.2.2.2 Correct answer.
  2. the highest physical interface address
  3. 10.1.1.1
  4. 127.0.0.1

Explanation

Without an explicit router-id, OSPF selects the highest loopback address on the device. Physical interface addresses are considered only if no loopback address exists.

Question 4OSPF (multi-area, area types, metrics, convergence, authentication)

In a standard multi-area OSPF domain, which area must every other area connect to?

  1. area 10
  2. area 0 Correct answer.
  3. area 1
  4. the area with the highest number

Explanation

Area 0 is the backbone, and all other areas must attach to it either directly or through a virtual link. Inter-area traffic is exchanged through the backbone.

Question 5OSPF (multi-area, area types, metrics, convergence, authentication)

Which LSA type is kept out of a stub area?

  1. AS external LSAs Correct answer.
  2. router LSAs flooded by every router
  3. summary LSAs sent by the ABR
  4. network LSAs generated by the DR

Explanation

A stub area blocks Type 5 AS external LSAs, and the ABR injects a default route instead. Type 1, Type 2 and Type 3 LSAs are still flooded normally inside the area.

Question 6OSPF (multi-area, area types, metrics, convergence, authentication)

On a broadcast segment, which value is compared first during the designated router election?

  1. the highest loopback address
  2. interface priority Correct answer.
  3. router ID
  4. the configured interface cost

Explanation

The DR election first compares interface priorities, and the highest priority wins. Router ID is used only as a tiebreaker when priorities are equal, and a priority of zero makes the router ineligible.

Question 7OSPF (multi-area, area types, metrics, convergence, authentication)

Using the default OSPF reference bandwidth of 100 Mbps, what is the OSPF cost of a 10 Mbps interface?

  1. 10 Correct answer.
  2. 1
  3. 100
  4. 5

Explanation

The cost is the reference bandwidth divided by the interface bandwidth, so 100 Mbps divided by 10 Mbps gives a cost of 10. The result is rounded down, and costs below 1 are set to 1.

Question 8OSPF (multi-area, area types, metrics, convergence, authentication)

An administrator changes the hello interval on a broadcast OSPF interface to 5 seconds and leaves the dead interval at its default relationship to hello. What is the resulting dead interval?

  1. 20 seconds Correct answer.
  2. 10 seconds
  3. 15 seconds
  4. 5 seconds

Explanation

By default the dead interval is four times the hello interval, so a 5-second hello yields a 20-second dead interval. Both values must match on neighboring routers.

Question 9OSPF (multi-area, area types, metrics, convergence, authentication)

Which statement describes an OSPF not-so-stubby area?

  1. It allows Type 5 LSAs but blocks Type 7 LSAs.
  2. It blocks Type 5 LSAs and carries externals in Type 7 LSAs. Correct answer.
  3. It blocks all summary LSAs and uses a default route only.
  4. It blocks Type 3 LSAs and carries external routes in Type 5 LSAs.

Explanation

An NSSA does not accept Type 5 AS external LSAs, but it can originate external information as Type 7 LSAs inside the area. The NSSA ABR later translates those Type 7 LSAs into Type 5 LSAs for the rest of the domain.

Question 10OSPF (multi-area, area types, metrics, convergence, authentication)

Two ABRs are joined by a virtual link across a transit area. What property must that transit area have?

  1. It must be the backbone area itself.
  2. It must be a normal area that reaches area 0. Correct answer.
  3. It must be a stub area.
  4. It must be an NSSA with a default route.

Explanation

A virtual link is carried through a regular transit area that has full LSA flooding and a path to the backbone. Stub, totally stubby and NSSA areas cannot serve as transit because they lack the required LSAs.

Question 11OSPF (multi-area, area types, metrics, convergence, authentication)

Which LSA types are blocked at the boundary of a totally stubby area?

  1. Types 1 and 2 only
  2. Types 3, 4 and 5 Correct answer.
  3. Types 4 and 7
  4. Type 5 only

Explanation

A totally stubby area blocks Type 3 summary LSAs, Type 4 ASBR summary LSAs and Type 5 external LSAs. The ABR replaces them with a single default route, while Type 1 and Type 2 LSAs continue to flood inside the area.

Question 12OSPF (multi-area, area types, metrics, convergence, authentication)

Which router on a broadcast segment originates the Type 2 network LSA?

  1. the ASBR
  2. the ABR
  3. every router on the segment
  4. the DR Correct answer.

Explanation

The designated router originates the Type 2 network LSA, which lists all routers that have formed a full adjacency on the segment. Every router still originates its own Type 1 router LSA.

Question 13OSPF (multi-area, area types, metrics, convergence, authentication)

Which OSPF authentication method transmits a keyed hash instead of a cleartext password in the hello packet?

  1. CHAP authentication
  2. simple password authentication
  3. MD5 authentication Correct answer.
  4. null authentication

Explanation

MD5 authentication places a keyed hash of the OSPF packet in the authentication field, so the key itself is never sent on the wire. Simple password authentication sends the password in cleartext and is therefore insecure.

Question 14OSPF (multi-area, area types, metrics, convergence, authentication)

How is the metric of a Type 2 external OSPF route calculated?

  1. external cost plus internal cost to the ASBR
  2. the highest internal cost along the path
  3. only the external metric Correct answer.
  4. the internal cost to the ASBR plus one

Explanation

A Type 2 external metric is used unchanged throughout the domain, so all routers regard the external cost the same regardless of their distance from the ASBR. Type 1 externals, by contrast, add the internal cost of reaching the ASBR.

Question 15OSPF (multi-area, area types, metrics, convergence, authentication)

What happens when an OSPF-enabled interface is configured as passive in Junos?

  1. Hellos stop but the subnet is advertised. Correct answer.
  2. It lowers the interface cost to zero for the subnet.
  3. It suppresses hellos but floods extra LSAs for the subnet.
  4. It removes the subnet from the routing domain entirely.

Explanation

A passive interface stops sending and processing hellos, so no adjacency can form over it. Its subnet is still placed into OSPF as a stub network and advertised in the router LSA.

Question 16OSPF (multi-area, area types, metrics, convergence, authentication)

Which activity defines the OSPF ExStart state?

  1. Hello packets are traded and neighbors are listed
  2. Link-state requests are sent to the neighbor
  3. Link-state databases are compared with DD packets
  4. Master/slave roles are negotiated Correct answer.

Explanation

In ExStart the two routers exchange empty database description packets to agree on a master and slave, using router ID as the deciding value. The actual database description exchange happens in the following Exchange state.

Question 17OSPF (multi-area, area types, metrics, convergence, authentication)

Which two Junos timers control how long OSPF waits before running SPF after a topology change?

  1. Retransmit and transit delay
  2. Hello and dead intervals
  3. LSA age and MaxAge
  4. SPF delay and SPF hold time Correct answer.

Explanation

The SPF delay sets the initial wait before the first SPF run, and the hold time caps the interval for subsequent runs during a flapping event. Together they throttle SPF without delaying neighbor failure detection.

Question 18OSPF (multi-area, area types, metrics, convergence, authentication)

An administrator wants OSPF to avoid one path without altering any interface bandwidth. Which action increases the OSPF cost of that interface in Junos?

  1. Raising the interface metric value Correct answer.
  2. Lowering the interface metric
  3. Increasing the reference bandwidth
  4. Adding a secondary address

Explanation

The OSPF metric is directly configurable on the interface, and a larger value makes the path less preferred. Changing the reference bandwidth would affect every interface on the router, not just one link.

Question 19OSPF (multi-area, area types, metrics, convergence, authentication)

What does the area range command accomplish on an ABR?

  1. Summarizes external routes before redistribution
  2. Translates Type 7 LSAs into Type 5 LSAs
  3. Summarizes intra-area routes into a Type 3 LSA Correct answer.
  4. Filters Type 1 LSAs inside the area

Explanation

Area range aggregates a set of intra-area prefixes into one summary and advertises it into other areas as a Type 3 LSA. It reduces the size of the link-state database outside the area.

Question 20OSPF (multi-area, area types, metrics, convergence, authentication)

Which statement about DR election on an OSPF point-to-point interface is true?

  1. The neighbors elect a DR and BDR as usual.
  2. The router with the lowest router ID becomes DR.
  3. No DR or BDR is elected on the link. Correct answer.
  4. The DR is elected only after a full adjacency forms.

Explanation

Point-to-point and point-to-multipoint interfaces form adjacencies directly and skip the DR/BDR election entirely. The network type is advertised accordingly so no Type 2 LSA is produced.

Question 21OSPF (multi-area, area types, metrics, convergence, authentication)

Which type of area is acceptable as the transit area for a virtual link?

  1. a stub area
  2. an NSSA
  3. a standard area Correct answer.
  4. a totally stubby area

Explanation

The transit area must carry full Type 3, Type 4 and Type 5 LSAs so the virtual link endpoints can reach each other and the backbone. Stub, totally stubby and NSSA areas are filtered and therefore cannot be used.

Question 22OSPF (multi-area, area types, metrics, convergence, authentication)

Which router originates a Type 4 summary ASBR LSA, and what does it describe?

  1. The DR, describing the segment's ASBR
  2. The backbone router, describing area 0 networks
  3. An ABR, describing the path to a remote ASBR Correct answer.
  4. The ASBR itself, describing its external links

Explanation

An ABR generates a Type 4 LSA to tell routers in other areas where the ASBR is located. Routers that receive it can then resolve the next hop for Type 5 external routes originated by that ASBR.

Question 23OSPF (multi-area, area types, metrics, convergence, authentication)

Inside an NSSA with more than one ABR, which device translates Type 7 LSAs into Type 5 LSAs?

  1. the ASBR that originated the Type 7 LSA
  2. the internal router nearest the ASBR
  3. the NSSA ABR Correct answer.
  4. the area 0 designated router

Explanation

The NSSA ABR performs the Type 7 to Type 5 translation when it floods the external information into the backbone. When several ABRs exist, the one with the highest router ID is chosen as translator.

Question 24OSPF (multi-area, area types, metrics, convergence, authentication)

At what age value is an OSPF LSA considered to have reached MaxAge?

  1. 600 seconds
  2. 1200 seconds
  3. 3600 seconds Correct answer.
  4. 1800 seconds

Explanation

MaxAge is 3600 seconds, one hour after the LSA was originated. An LSA is normally refreshed at 1800 seconds, and if it is not refreshed it is flushed from the database at MaxAge.

Question 25OSPF (multi-area, area types, metrics, convergence, authentication)

Which feature lets a restarting OSPF router keep forwarding packets while it rebuilds its adjacencies and database?

  1. graceful restart Correct answer.
  2. a stub area
  3. LSA throttling
  4. BFD

Explanation

Graceful restart preserves the forwarding table and keeps announcing the router as a helper-assisted restart so neighbors continue to use the paths. BFD and LSA throttling affect detection and flooding speed but do not preserve forwarding.

Question 26OSPF (multi-area, area types, metrics, convergence, authentication)

A router inside area 1 originates a Type 1 router LSA. To which routers is that LSA flooded?

  1. To all routers within area 1 only Correct answer.
  2. Only to the area border routers that border area 1
  3. To all routers in every area of the OSPF domain
  4. Only to the routers on the originating router's directly attached segments

Explanation

Type 1 router LSAs have area flooding scope: they describe a router's links and are flooded to every router inside that single area. Area border routers translate the information into Type 3 LSAs when advertising it into other areas, so the Type 1 LSA itself never leaves its own area.

Question 27OSPF (multi-area, area types, metrics, convergence, authentication)

Which area is defined as the backbone area of an OSPF domain?

  1. Area 0.0.0.0 Correct answer.
  2. Area 1.1.1.1
  3. The area with the highest router ID in the domain
  4. Area 0.0.0.1

Explanation

The backbone is always area 0.0.0.0, and all other areas are expected to reach it either directly or through a virtual link. Inter-area prefix information is distributed between non-backbone areas by way of the backbone.

Question 28OSPF (multi-area, area types, metrics, convergence, authentication)

Two Junos routers share an Ethernet segment and both run OSPF on that interface. Router A is configured for area 0.0.0.1 and Router B for area 0.0.0.2. What is the result?

  1. They do not become neighbors because the area IDs differ Correct answer.
  2. They remain in the 2-Way state permanently
  3. They form an adjacency only after the DR election completes
  4. They form a full adjacency but do not exchange any routes

Explanation

The area ID is carried in the hello packet, and all hello parameters except the router priority, dead interval and DR/BDR fields must match before an adjacency can form. A mismatch in area ID causes the hellos to be rejected, so the routers never leave the Down/Init condition.

Question 29OSPF (multi-area, area types, metrics, convergence, authentication)

Which router originates a Type 3 summary LSA, and what does it advertise?

  1. An ASBR, advertising routes redistributed into the domain
  2. An ABR, advertising prefixes learned in one area into another area Correct answer.
  3. Any router in the area, advertising its own directly connected links
  4. The DR on a broadcast segment, advertising the segment prefix

Explanation

An area border router takes the prefix information it learned from one area's Type 1 and Type 2 LSAs and re-advertises it into other areas as Type 3 summary LSAs. Type 1 and Type 2 LSAs are never flooded across an area boundary, so the ABR must translate them.

Question 30OSPF (multi-area, area types, metrics, convergence, authentication)

Which statement describes how a stub area handles external routing information?

  1. Type 3 LSAs are not flooded into it, and the ABR advertises a default route instead.
  2. Type 5 LSAs are flooded into it, but the ABR suppresses the default route.
  3. Type 5 LSAs are not flooded into it, and the ABR advertises a default route instead. Correct answer.
  4. External routes are carried in it as Type 7 LSAs and translated at the ABR.

Explanation

A stub area blocks Type 5 external LSAs, so routers inside it cannot see individual external prefixes. The ABR instead injects a default route into the stub area, which gives internal routers a path to any destination outside the area.

Question 31OSPF (multi-area, area types, metrics, convergence, authentication)

How is the OSPF cost of an intra-area route to a destination network computed?

  1. As the sum of the outbound interface costs toward the destination Correct answer.
  2. From the highest outbound interface cost on the path
  3. As the reference bandwidth divided by the destination subnet size
  4. As the lowest outbound interface cost on the path

Explanation

OSPF accumulates cost as a packet travels: each router adds the cost of the interface it uses to leave toward the destination. The total is therefore the sum of the outbound interface costs along the shortest path, and the least-cost path is installed in the routing table.

Question 32OSPF (multi-area, area types, metrics, convergence, authentication)

A Junos router redistributes a connected route into OSPF with no metric type configured. Which external metric type is used?

  1. Type 1
  2. Type 4
  3. Type 2 Correct answer.
  4. Type 3

Explanation

Junos defaults redistributed routes to external metric type 2, which means the advertised metric is the external cost only and does not grow as the route is passed deeper into the domain. Type 1 must be configured explicitly if internal costs are to be added.

Question 33OSPF (multi-area, area types, metrics, convergence, authentication)

Two Junos OSPF neighbors are configured with MD5 authentication. Which values must be identical on both routers for the adjacency to form?

  1. The router IDs of both routers and the key string
  2. Both the key ID and the key string Correct answer.
  3. Only the key string; the key ID is locally significant
  4. Only the key ID; the key string may differ on each router

Explanation

A Junos router selects the MD5 key by key ID, so both the key ID and the key string must match on the two neighbors, and authentication must be enabled on both interfaces. If either value differs, the hello packet fails verification and the adjacency never forms.

Question 34OSPF (multi-area, area types, metrics, convergence, authentication)

Which two pieces of information identify a virtual link when it is configured on a Junos ABR?

  1. The transit area ID and the remote router's interface IP address
  2. The backbone area ID and the remote ABR's router ID
  3. The local router ID and the remote router's interface IP address
  4. The transit area ID and the remote ABR's router ID Correct answer.

Explanation

A virtual link is defined by the non-backbone area it tunnels through and by the router ID of the ABR at the far end. No interface addresses are used, because the virtual link behaves as an unnumbered point-to-point link between the two routers.

Question 35OSPF (multi-area, area types, metrics, convergence, authentication)

On a broadcast segment, two routers are both neither the DR nor the BDR. Which neighbor state do they reach with each other?

  1. Exchange
  2. 2-Way Correct answer.
  3. Loading
  4. Full

Explanation

On a broadcast segment every router synchronizes its database with the DR and BDR, so non-DR routers stay in 2-Way with one another. This avoids building a full mesh of adjacencies on a shared segment while still guaranteeing that every router receives all LSAs.

Question 36OSPF (multi-area, area types, metrics, convergence, authentication)

On a broadcast network, to which address does a DROther router send its link-state update packets?

  1. 255.255.255.255
  2. 224.0.0.1
  3. 224.0.0.6 Correct answer.
  4. 224.0.0.5

Explanation

DROther routers send updates to the AllDRouters address 224.0.0.6, which only the DR and BDR listen to. The DR then re-floods the information to 224.0.0.5 so that every other router on the segment receives it.

Question 37OSPF (multi-area, area types, metrics, convergence, authentication)

Assuming the topology does not change, at what interval does an OSPF router refresh its own self-originated LSAs?

  1. Every 5 minutes
  2. Every 15 minutes
  3. Every 20 minutes
  4. Every 30 minutes Correct answer.

Explanation

Each LSA carries an age that a router increments, and the originating router re-floods a new instance with a higher sequence number when the age reaches the 1800-second refresh interval. If an LSA instead reaches MaxAge at 3600 seconds without being refreshed, it is flushed from the database.

Question 38OSPF (multi-area, area types, metrics, convergence, authentication)

An ABR advertises a Type 3 LSA for a subnet in area 1 into area 0. What does the metric in that LSA represent, and how is it used by a receiving area 0 router?

  1. The cost from the ASBR to the subnet, excluding all intra-area costs
  2. The bandwidth of the subnet's interface divided by the reference bandwidth
  3. The ABR's cost to reach the subnet; the receiver adds its own cost to reach the ABR Correct answer.
  4. The receiver's cost to reach the subnet; the ABR always sets the metric to zero

Explanation

The ABR places its own cost to the advertised prefix in the Type 3 LSA, because it is the one that knows the topology of area 1. Receiving routers then add the cost of their own shortest path to the ABR, so the total metric reflects both parts of the journey.

Question 39OSPF (multi-area, area types, metrics, convergence, authentication)

A router receives an updated Type 1 LSA in which only the stub network entries changed; no transit links were added or removed. What does the router do?

  1. Runs a partial SPF calculation that recalculates only the affected stub networks Correct answer.
  2. Runs a full SPF calculation and rebuilds the entire shortest-path tree
  3. Flushes its link-state database and re-forms all adjacencies in the area
  4. Waits until the LSA reaches MaxAge before recalculating any routes

Explanation

When the change is confined to stub links, the shape of the shortest-path tree is unaffected, so the router performs a partial SPF and recomputes only the prefixes attached to the affected nodes. A full SPF is reserved for changes to transit links or to the set of routers and networks in the area.

Question 40OSPF (multi-area, area types, metrics, convergence, authentication)

A Type 5 LSA for an external prefix carries a non-zero forwarding address. How do routers in the OSPF domain use that address?

  1. They send traffic to the ASBR, which then forwards it to that address
  2. They forward traffic directly to that address without transiting the ASBR Correct answer.
  3. They ignore it unless the same prefix also appears in a Type 7 LSA
  4. They use it as a next hop only when it matches the ASBR's router ID

Explanation

A non-zero forwarding address points to a router on the external network that is also reachable through the OSPF domain. Packets are sent straight to that address, which removes the extra hop through the ASBR and lets traffic take the optimal entrance to the external network.

Question 41IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

In an IS-IS network entity title (NET), which portion identifies the routing area to which the router belongs?

  1. The last six bytes of the NET, which follow the area address
  2. The NSEL byte appended to the end of the address
  3. The area address, which precedes the system ID Correct answer.
  4. The system ID, because it is unique across the whole domain

Explanation

A NET is composed of an area address, a six-byte system ID, and an NSEL that is always 00 for a router. Everything before the system ID forms the area address, and routers sharing that value belong to the same Level 1 area.

Question 42IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

When IS-IS is enabled on a Junos interface without an explicit level statement, which levels are activated on that interface?

  1. Neither level until a level is configured
  2. Level 2 only
  3. Level 1 only
  4. Level 1 and Level 2 Correct answer.

Explanation

Junos defaults an IS-IS interface to level 1-2, so the interface attempts to form both a Level 1 adjacency and a Level 2 adjacency. The active levels can be restricted later with level 1 disable or level 2 disable.

Question 43IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

How are IS-IS protocol data units carried across a link?

  1. Directly in data-link frames, with no IP header Correct answer.
  2. Inside UDP datagrams that use port 520
  3. Inside GRE tunnels established between neighbors
  4. Inside IP packets that use IP protocol number 89

Explanation

IS-IS is a CLNS routing protocol, so its PDUs are placed directly into link-layer frames such as 802.3 with LLC or PPP. This is why IS-IS can form adjacencies on interfaces that have no IP address assigned.

Question 44IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

What is the largest interface metric value that can be advertised in a wide-metric IS-IS reachability TLV?

  1. 16777215 Correct answer.
  2. 1023
  3. 63
  4. 255

Explanation

Wide metrics use a 24-bit field on Juniper devices, giving a maximum value of 2^24 minus 1, or 16777215. Narrow metrics are far more limited because they use a 6-bit interface metric field.

Question 45IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

Which statement describes a router configured as Level 1/2 in IS-IS?

  1. It forms only Level 2 adjacencies and never advertises a default route
  2. It runs Level 1 and Level 2 as two independent routing processes
  3. It keeps a Level 1 link in its own area and Level 2 links across areas Correct answer.
  4. It forms only Level 1 adjacencies but still forwards traffic between areas

Explanation

A Level 1/2 router maintains two link-state databases: a Level 1 database for its own area and a Level 2 database for the backbone. It connects the two by setting the attached bit in its Level 1 LSPs and by leaking selected prefixes between levels.

Question 46IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

Two directly connected IS-IS routers have been configured with different area addresses. Which adjacency can they form?

  1. A Level 1 adjacency, but only after route leaking is enabled
  2. A Level 1 adjacency, because the link is in the same subnet
  3. No adjacency at all, because the area addresses do not match
  4. A Level 2 adjacency, if both are enabled for Level 2 on the link Correct answer.

Explanation

Level 1 adjacencies require matching area addresses, but Level 2 adjacencies do not, because Level 2 is the inter-area backbone. Routers with mismatched area addresses therefore still become Level 2 neighbors when Level 2 is enabled on both sides.

Question 47IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

With default Junos settings on a point-to-point IS-IS interface, what are the hello interval and the hold time?

  1. 10 seconds hello, 10 seconds hold
  2. 30 seconds hello, 90 seconds hold
  3. 3 seconds hello, 9 seconds hold
  4. 10 seconds hello, 30 seconds hold Correct answer.

Explanation

Junos sends IS-IS hellos every 10 seconds by default and advertises a hold time of three times the hello interval, which is 30 seconds. A neighbor is declared down when no hello is received before the hold time expires.

Question 48IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

How does a Level 1-only router learn a path toward destinations in other areas?

  1. A static default route must be configured manually on every Level 1 router
  2. The DIS advertises the default route inside every CSNP that it generates
  3. The Level 2 router floods an explicit 0.0.0.0/0 prefix in its Level 1 LSP
  4. The attached bit set by the Level 1/2 router installs a default route Correct answer.

Explanation

A Level 1/2 router with at least one active Level 2 adjacency sets the attached bit in its Level 1 LSPs. Receiving Level 1 routers then install a default route pointing at that router, which is how intra-area routers reach the rest of the domain.

Question 49IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

On a broadcast IS-IS network, how is the Designated Intermediate System selected?

  1. Lowest interface priority first, then lowest router ID
  2. Whichever router sends the first hello on the segment
  3. Highest interface priority first, then highest source MAC address Correct answer.
  4. Highest system ID first, then highest interface priority

Explanation

The DIS is chosen by highest interface priority, with the highest SNPA (source MAC address) breaking ties. Election is preemptive, so a newly arrived router with better credentials can take over the role.

Question 50IS-IS (levels, addressing, metrics, multi-area behavior, convergence)

What is the purpose of the Complete Sequence Number PDUs that a DIS sends on a broadcast segment?

  1. To describe the DIS's entire link-state database to the segment Correct answer.
  2. To request one specific missing LSP from a neighbor
  3. To carry every IPv4 prefix that the DIS has redistributed into the area
  4. To acknowledge each LSP received on the segment

Explanation

The DIS periodically multicasts a CSNP that lists the LSP identifiers and sequence numbers of its whole database. Neighbors compare this list with their own databases and request anything they are missing with PSNPs.