Question 1. 1. (TCO 4) OSPF multi-area design is based on _____. (Points : 2) hi

Question

Question 1.1. (TCO 4) OSPF multi-area design is based on _____. (Points : 2)        hierarchical network design
       flat network design
       designated routers
       the number of SPF calculations required Question 2.2. (TCO 4) In multi-area OSPF all areas must connect to the _____. (Points : 2)        MBone area
       backbone area
       headquarters area
       designated router Question 3.3. (TCO 4) OSPF messages are managed by _____. (Points : 2)        LSAs
       border states
       only OSPFv3
       keep alive packets Question 4.4. (TCO 4) Which statement describes a characteristic of convergence in an OSPF network? (Points : 2)        After it has converged, a router can communicate with any network within or outside the OSPF autonomous system.
       A router is unstable.
       The neighbor table is copied.
       OSPF never converges. Question 5.5. (TCO 4) On which type of OSPF routers can route summarization be configured? (Points : 2)        ABR
       PDQ
       ARSI
       Internal Question 6.6. (TCO 4) What LSA type is used by ABRs to advertise networks from other areas? (Points : 2)        1
       2
       3
       4 Question 7.7. (TCO 4) End users are typically not found in what area? (Points : 2)        End users are in all areas
       The regular area
       The backbone area
       The transitory area Question 8.8. (TCO 4) The propagation of OSPF message concerning routes to networks outside the OSPF AS is controlled by _____. (Points : 2)        LSA Type 3
       LSA Type 5
       LSA Type 4
       LSA Type 2
       LSA Type 1 Question 1.1. (TCO 4) OSPF multi-area design is based on _____. (Points : 2)        hierarchical network design
       flat network design
       designated routers
       the number of SPF calculations required

Explanation / Answer

Note: The explanations are from CISCO OSPF page, wiki pedia and from few websites which explained the concept. Explanation is given so that you can understand how answer is got.

Question 1.1. (TCO 4) OSPF multi-area design is based on _____
Ans:        hierarchical network design

An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification.


Question 2.2. (TCO 4) In multi-area OSPF all areas must connect to the _____
Ans: Backbone area

Each OSPF network that is divided into different areas must follow these rules:

    A backbone area—which combines a set of independent areas into a single domain—must exist.

    Each non-backbone area must be directly connected to the backbone area (though this connection might be a simple logical connection through a virtual link, ).

    The backbone area must not be partitioned—divided into smaller pieces—under any failure conditions, such as link or router down events.

Question 3.3. (TCO 4) OSPF messages are managed by
Ans: LSAs

Each router in an OSPF autonomous system that runs OSPF software that is responsible for various tasks, such as setting timers to control certain activities that must occur on a regular basis, and the maintenance of important data structures, such as the link-state database (LSDB). Most importantly, each OSPF router must both generate and respond to OSPF messages. It is this messaging system that allows important routing information to be shared within an AS or area, which makes it crucial to understanding how OSPF works

Question 4.4. (TCO 4) Which statement describes a characteristic of convergence in an OSPF network? (
Ans: After it has converged, a router can communicate with any network within or outside the OSPF autonomous system.

OSPF detects changes in the topology, such as link failures, and converges on a new loop-free routing structure within seconds. It computes the shortest path tree for each route using a method based on Dijkstra's algorithm, a shortest path first algorithm.

The OSPF routing policies for constructing a route table are governed by link cost factors (external metrics) asso.
Exchange link-state advertisements: After adjacencies are established, routers then exchange link-state advertisements (LSAs). LSAs contain the state and cost of each directly connected link. Routers flood their LSAs to adjacent neighbors. Adjacent neighbors receiving the LSA immediately flood the LSA to other directly connected neighbors, until all routers in the area have all LSAs.

Question 5.5. (TCO 4) On which type of OSPF routers can route summarization be configured?
Ans:    ABR
Summarizing is the consolidation of multiple routes into one single advertisement. This is normally done at the boundaries of Area Border Routers (ABRs).

Question 6.6. (TCO 4) What LSA type is used by ABRs to advertise networks from other areas?
ANs: Type 3


    Type 1 - Router LSA - the router announces its presence and lists the links to other routers or networks in the same area, together with the metrics to them. Type 1 LSAs are flooded across their own area only. The link-state ID of the type 1 LSA is the originating router ID.

    Type 2 - Network LSA - the designated router (DR) on a broadcast segment (e.g. Ethernet) lists which routers are joined together by the segment. Type 2 LSAs are flooded across their own area only. The link-state ID of the type 2 LSA is the IP interface address of the DR.

    Type 3 - Summary LSA - an Area Border Router (ABR) takes information it has learned on one of its attached areas and summarizes it before sending it out on other areas it is connected to. This summarization helps provide scalability by removing detailed topology information for other areas, because their routing information is summarized into just an address prefix and metric. The summarization process can also be configured to remove a lot of detailed address prefixes and replace them with a single summary prefix, helping scalability. The link-state ID is the destination network number for type 3 LSAs.

    Type 4 - ASBR-Summary LSA - this is needed because Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas because it may be using a different routing protocol. This is solved by an Area Border Router flooding the information for the router (i.e. the Autonomous System Boundary Router) where the type 5 originated. The link-state ID is the router ID of the described ASBR for type 4 LSAs.


Question 7.7. (TCO 4) End users are typically not found in what area?
Ans:      The transitory area
Generally, end users are not found within a transit area.


Question 8.8. (TCO 4) The propagation of OSPF message concerning routes to networks outside the OSPF AS is controlled by
Ans: type 5
External LSA (Type 5) – Generated by ASBRs in OSPF, and contain
routes to destination networks outside the local Autonomous System.
Type 5 LSAs can also take the form of a default route to all networks
outside the local AS. Type 5 LSAs are flooded to all areas in the OSPF
system.

Type 5 - External LSA - these LSAs contain information imported into OSPF from other routing processes. They are flooded to all areas unchanged (except stub and NSSA areas). For "External Metric Type 1" LSAs routing decisions are made using the Type 1 metric cost sent, as the total cost to get to the external destination and includes the cost to the ASBR; while for "External Type 2" LSAs the metric sent is the cost from the ASBR to the External destination network and must be added to the OSPF cost to the ASBR advertising the Type 5. The link-state ID of the type 5 LSA is the external network number.

Note: The explanations are from CISCO OSPF page, wiki pedia and from few websites which explained the concept

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