Interarea-prefix LSAs for ABRs (Type 3)-Advertises internal networks to routers in other areas
(interarea routes). Type 3 LSAs may represent a single network or a set of networks summarized
into one advertisement. Only ABRs generate summary LSAs. In OSPF for IPv6, addresses for
these LSAs are expressed as prefix, prefix length instead of address, mask. The default route is
expressed as a prefix with length 0.
Reference:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide_chapter0918
6a0080145c56.html


Q7
Drag the OSPF area type icons on the left, and drop them into the centre target area that
adjoins the corresponding description on the right.

Ans:


Explanation:
o Stub Areas: These areas do not accept routes belonging to external autonomous systems
(AS); however, these areas have inter-area and intra-area routes. In order to reach the
outside networks, the routers in the stub area use a default route which is injected into the
area by the Area Border Router (ABR).
o Normal Areas: These areas can either be standard areas or transit (backbone) areas.
Standard areas are defined as areas that can accept intra-area, inter-area and external
routes.
o backbone area is the central area to which all other areas in OSPF connect.
o Totally Stub Areas: These areas do not allow routes other than intra-area and the default
routes to be propagated within the area. The ABR injects a default route into the area and
all the routers belonging to this area use the default route to send any traffic outside the
area.
o NSSA: This type of area allows the flexibility of importing a few external routes into the
area while still trying to retain the stub characteristic. Assume that one of the routers in
the stub area is connected to an external AS running a different routing protocol, it now
becomes the ASBR, and hence the area can no more be called a stub area. However, if
the area is configured as a NSSA, then the ASBR generates a NSSA external link-state
advertisement (LSA) (Type-7) which can be flooded throughout the NSSA area. These
Type-7 LSAs are converted into Type-5 LSAs at the NSSA ABR and flooded throughout
the OSPF domain
Reference:
http://www.cisco.com/en/US/tech/ CK3 65/ CK4 80/technologies_tech_note09186a0080094a74.shtml


Q8
In a multi-area OSPF network inter-area summarization produces type's 3 & 4 Link State
Advertisements (LSA's). At which router are these LSA's produced at?A. At the ABR.B. At the ASBR.C. At the backbone DR.D. Each internal router calculates and sends network updates.
Ans: A

Explanation: Network summary link entries, LSA type 3 or LSA type 4 are originated by
ABRs. They describe the links between the ABR and the internal routers of a local area. These
entries are flooded throughout the backbone area to the other ABRs.
Note: Type-3 describes routes to networks within the local area and are sent to the backbone
area. Type-4 describes reachability to ASBRs. These link entries are not flooded through totally
stubby areas.
Incorrect Anss
A:
B: Autonomous system external link entries, LSA type 5, are originated by ASBRs. They
describe routes to destinations external to the autonomous system.
C: Backbone DRs does not generate these LSAs.
D: Internal routers produce router link entries, LSA type 1. They describe the states of the
router's link to the area.


Q9
What does Cisco recommend for those who set up networks with multiple OSPF areas?
(Select two)A. There should not be more than three areas per route.B. Area 0 must be larger than any subsequent OSPF area.C. A router cannot be a DR or BDR for more than one LAN.D. You should not run more than one instance of the OSPF process on an ABR.
Ans: C, D

Explanation:
C: A router cannot be a DR or BDR for more than one LAN.
D: Running multiple OSPF processes on the same router is not recommended because it creates
multiple database instances that add extra overhead.
Incorrect Anss
A: There is no such limit. The hierarchical structure could be larger.
B: There is no requirement that Area 0 must be the largest OSPF area.


Q0
When designing and implementing OSPF in multiple areas; what should be taken in
consideration? (Select two)A. Each area must connect to area 0.B. Each area must have a unique AS number.C. Remote areas must be configured as stub or NSSA areas.D. Traffic between two areas must travel across the backbone area.
Ans: A, D

Explanation:
A: In OSPF, all areas must be connected to a backbone area, area 0.
Incorrect Anss
A: For OSPF we have no idea about AS number
C: Remote areas are not required to be either sub or Not so stubby areas.


Q1
In OSPF, a Designated Router (DR) floods LSAs to all the routers on the segment that it
has neighbour relationships with, Over what kind of link does this happen and what kind
of LSAT type is sent?A. router link, type 1.B. network link, type 2.C. external link, type 5.D. network summary link, type 3.
Ans: B

Explanation: Network Link, LSA type 2 is generated by DRs in multiaccess networks. They
describe the set of routers attached to a particular network. Flooded within the area that contains
the network only.
Incorrect Anss
A: Router link, LSA type 1 is generated by each router for each area it belongs to. It describes
the states of the router's link to the area.
C: External link, LSA type 5 is originated by the ASBR. It describes routes to destinations
external to the autonomous system. It is flooded throughout an OSPF autonomous system
except for stub and totally stubby areas.
D: Network summary link entry, LSA type 3 is originated by ABRs. It describes the links
between the ABR and the internal routers of a local area. These entries are flooded
throughout the backbone area to the other ABRs.


Q2
In an OSPF network, neighbour relations allow the topology of the network to scale. What
are two reasons as to why this can help a network topology scale? (Select two)A. Neighbor adjacencies control distribution of routing protocol updates.B. Routing table information does not flood the network until holddown timers have
expired.C. The hello protocol is a more efficient means of sending routing updates than table
exchanges used in RIPv1.D. Topological database is maintained with incremental updates, with full exchange
occurring only every 30 minutes.
Ans: A, C

Explanation: When a network topology changes and becomes more sophisticated, less routers
are burdened with the responsibility of having to relearn the entire network. The hello protocol is
also efficient because rather then focusing on knowing what all the routers know, the routers
only focus on who is still around, therefore available for routing.


Q3
In an OSPF network; which IP address multicasts all OSPF Designated Routers and
Backup Designated Routers?A. 224.0.0.5B. 224.0.0.6C. 224.0.0.9D. 224.0.0.11
Ans: B

Explanation:
224.0.06 is the address of all OSPF DRs and BDRs.
Reference: Building Scalable Cisco Networks (Ciscopress) page 114


Q4
What kind of OSPF Link Sttate Advertisement (LSA) is found in a type 3 network
summary link?A. Summary of routes in the AS.B. Summary of link states in an OSPF area.C. Summary of IP subnets in an OSPF area.D. Summary of metric costs from ABR to ASBR.
Ans: C

Explanation:
Type 3 are INTER-AREA advertisements and they are not used within the local area.
1 These are the router links
advertisements. They describe the
collected states of the router's
interfaces. For more information,
consult Section 12.4.1.
2 These are the network links
advertisements. They describe the set
of routers attached to the network. For
more information, consult
Section 12.4.2.
3 or 4 These are the summary link
advertisements. They describe
inter-area routes, and enable the
condensation of routing information at
area borders. Originated by area border
routers, the Type 3 advertisements
describe routes to networks while the
Type 4 advertisements describe routes to
AS boundary routers.
5 These are the AS external link
advertisements. Originated by AS
boundary routers, they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS external link
advertisement.
Table 15: OSPF link state
advertisements.
LS Type Link State ID
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.


Q5
You are the administrator of a large OSPF network and are considering dividing it to
multiple areas. What are two advantages of configuring multiple OSPF areas? (Select two)A. It eliminates the need for a DR or BDR in the OSPF network.B. It eliminates security concerns by segregating portions of the network.C. Type 1 and 2 LSAs are confined to a single area, reducing routing overhead.D. Area members have smaller topological databases than if the network was one large area,
requiring less CPU to derive routes.
Ans: C, D

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