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    CCIE R&S 350-001 Q&As – Implement IPv4 Open Shortest Path First (OSPF) (21-26)

    By admin | June 18, 2014

    Tagged with:

    Section 5 – Implement IPv4 Open Shortest Path First (OSPF)

    QUESTION 21
    Based on the network provided in the exhibit, in these two areas, all routers are performing OSPF on all interfaces. After examining the OSPF database on R4, do you know which type of LSA will contain 10.1.5.0/24, and which router will have originated it?

    clip_image001

    A.    10.1.5.0/24 will be in a summary (type 3) LSA originated by R3.
    B.    10.1.5.0/24 will be in a router (type 1) LSA generated by R3.
    C.    10.1.5.0/24 will be in a network (type 2) LSA originated by R3.
    D.    10.1.5.0/24 will not be in any LSA in the OSPF database at R4, because R4 and R3 are in different areas.
    Answer: A
    Explanation:
    R3 is ABR hence LSA 3
    Depending on the point where a network is sourced, there are various types of routes that could be present in a OSPF domain.
    When there are multiple routes to a particular network in a OSPF domain, the type of the route influences the route that is selected and installed by the router in the routing table.
    In OSPF, routes that are learned by a router from OSPF sources within the same area are known as inta-area routes.
    Routes that originate from an OSPF router in a different area are considered as inter-area routes.
    Certain networks could belong to a domain outside OSPF, which could then be redistributed into the OSPF by an Autonomous System Boundary Router (ASBR).
    Such routes are considered external routes.
    They can be further divided into external type-1 or external type-2 routes, depending on how they are advertised while being redistributing on the ASBR.
    The difference between these two types is the way in which the metric for the route is calculated.
    OSPF-running routers use these criteria to select the best route to be installed in the routing table:
    1. Intra-area routes.
    2. Inter-area routes.
    3. External Type-1 routes.
    4. External Type-2 routes.
    B) If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route.
    The route with the lowest value for cost is chosen as the best route.
    C) If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.

    QUESTION 22
    Refer to the exhibit. Which two of the following statements are true? (Choose two.)

    clip_image002
    A:The distance commands have no effect as the specified routers are not directly connected.
    B:If either router A or B loses connectivity to the stub network via Domain 1, it is prevented from using a path via Domain 2.
    C:These configurations prevent a routing loop between routers A & B in the event that router C malfunctions and stops communicating.
    D:The configurations ensure that intra-domain routes to network 10.100.254.0 are preferred over inter-domain routes, no matter the actual cost to the stub network.
    E:The addition of the command distance ospf external 200 to both OSPF processes on both routers A & B will prevent routing loops and allow routing to the stub network via Domain 2 if either one of the routers loses connectivity via Domain 1.
    F:Either router A or router B will actively redistribute routes between the domains, the other router will be in standby. Since the administrative distances are the same, the active and standby roles are determined by which router becomes active first.
    Answers:  B, C
    Explanation:
    Numerically, an administrative distance is an integer from 0 to 255. In general, the higher the value, the lower the trust rating. An administrative distance of 255 means that the routing information source cannot be trusted at all and should be ignored.
    When the optional access list number is used with this command, it is applied when a network is being inserted into the routing table. This behavior allows filtering of networks according to the IP address of the router supplying the routing information. This option could be used, as an example, to filter out possibly incorrect routing information from routers not under your administrative control.

    QUESTION 23
    Which three statements accurately describe a link-state routing protocol? (Choose three.)
    A:Each router sends routing information to all nodes in the flooding domain.
    B:Each router sends all or some portion of its routing table to neighboring routers.
    C:Each router individually builds a picture of the entire flooding domain.
    D:Each router has knowledge of all other routers in the flooding domain.
    E:Each router is only aware of neighboring routers.
    F:Each router installs routes directly from the routing updates into the routing table.
    Answers:  A, C, D
    Explanation:
    Link State Routing Protocols
    The information available to a distance vector router has been compared to the information available from a road sign. Link state routing protocols are like a road map. A link state router cannot be fooled as easily into making bad routing decisions, because it has a complete picture of the network. The reason is that unlike the routing-by-rumor approach of distance vector, link state routers have firsthand information from all their peer7 routers. Each router originates information about itself, its directly connected links, and the state of those links (hence the name). This information is passed around from router to router, each router making a copy of it, but never changing it. The ultimate objective is that every router has identical information about the internetwork, and each router will independently calculate its own best paths.
    Link state protocols, sometimes called shortest path first or distributed database protocols, are built around a well-known algorithm from graph theory, E. W. Dijkstra’a shortest path algorithm. Examples of link state routing protocols are:
    Open Shortest Path First (OSPF) for IP
    The ISO’s Intermediate System to Intermediate System (IS-IS) for CLNS and IP
    DEC’s DNA Phase V
    Novell’s NetWare Link Services Protocol (NLSP)
    Although link state protocols are rightly considered more complex than distance vector protocols, the basic functionality is not complex at all:
    Each router establishes a relationship—an adjacency—with each of its neighbors.
    Each router sends link state advertisements (LSAs), some Each router stores a copy of all the LSAs it has seen in a database. If all works well, the databases in all routers should be identical.
    The completed topological database, also called the link state database, describes a graph of the internetwork. Using the Dijkstra algorithm, each router calculates the shortest path to each network and enters this information into the route table.
    Neighbors
    Neighbor discovery is the first step in getting a link state environment up and running. In keeping with the friendly neighbor terminology, a Hello protocol is used for this step. The protocol will define a Hello packet format and a procedure for exchanging the packets and processing the information the packets contain.
    At a minimum, the Hello packet will contain a router ID and the
    instance, an IP address from one of the router’s interfaces. Other fields of the packet may carry a subnet mask, Hello intervals, a specified maximum period the router will wait to hear a Hello before declaring the neighbor "dead," a descriptor of the circuit type, and flags to help in bringing up adjacencies.
    When two routers have discovered each other as neighbors, they Beyond building adjacencies, Hello packets serve as keepalives to monitor the adjacency. If Hellos are not heard from an adjacent neighbor within a certain established time, the neighbor is consider_ed unreachable and the adjacency is broken. A typical interval for the exchange of hello packets is 10 seconds, and a typical dead period is four times that.
    Link State Flooding
    After the adjacencies are established, the routers may begin sending out LSAs. As the term flooding implies, the advertisements are sent to every neighbor. In turn, each received LSA is copied and forwarded to every neighbor except the one that sent the LSA. This process is the source of one of link state’s advantages over distance vector. LSAs are forwarded almost immediately, whereas distance vector must run its algorithm and update its route table before routing updates, even the triggered ones, can be forwarded. As a result, link state protocols converge much faster than distance vector protocols converge when the topology changes.
    The flooding process is the most complex piece of a link state protocol. There are several ways to make flooding more efficient and more reliable, such as using unicast and multicast addresses, checksums, and positive acknowledgments. These topics are examined in the protocol-specific chapters, but two procedures are vitally important to the flooding process: sequencing and aging.

    QUESTION 24
    Which three of these statements correctly describe type 2 LSAs (network link advertisements)? (Choose three.)
    A:Network LSAs are generated by the DR.
    B:Network LSAs are flooded only in their originating areas.
    C:A network LSA lists all attached routers including the DR.
    D:Every router in an OSPF area generates type 2 LSAs, as well as type 1 LSAs.
    E:The link-state ID of the type 2 LSA is the loopback address of the DR.
    F:When a network LSA reaches an ABR it is converted to a type 5 LSA (AS external LSA) and then flooded to other areas.
    Answers:  A, B, C

    QUESTION 25
    Refer to the exhibit. Assume all link costs in this network are 10. R1 is redistributing 192.168.1.0/24 into area 1 with a next hop of 10.1.1.2, which is its local address on the link between R1 and R2. R2 and R3, Area Border Routers, are configured to advertise a summary, 10.1.0.0/16; R2 is advertising this summary with a cost of 20, while R3 is advertising this summary with a cost of 10. Will R4 choose R2 or R3 as its next hop towards 192.168.1.0/24?

    clip_image003
    A: R4 will choose the path through R2, because it will use the border router (type 4) LSA to calculate the shortest path to the next hop (10.1.1.2), which is through R2.
    B: R4 will use the path through R3 to reach 192.168.1.0/24, because the shortest path it has to reach the next hop (10.1.1.2) is through the summary route advertised by R3, with a cost of 10.
    C: R4 will use R3 to reach 192.168.1.0/24, because OSPF always chooses the closest path to the border with the area in which an external route is injected.
    D: R4 will not have a route to 192.168.1.0/24.
    Answers:  A
    Explanation:
    cost of path to 192.168.1.0 is less through R2 than through R3
    Depending on the point where a network is sourced, there are various types of routes that could be present in a OSPF domain.
    When there are multiple routes to a particular network in a OSPF domain, the type of the route influences the route that is selected and installed by the router in the routing table.
    In OSPF, routes that are learned by a router from OSPF sources within the same area are known as inta-area routes.
    Routes that originate from an OSPF router in a different area are considered as inter-area routes.
    Certain networks could belong to a domain outside OSPF, which could then be redistributed into the OSPF by an Autonomous System Boundary Router (ASBR).
    Such routes are considered external routes.
    They can be further divided into external type-1 or external type-2 routes, depending on how they are advertised while being redistributing on the ASBR.
    The difference between these two types is the way in which the metric for the route is calculated.
    OSPF-running routers use these criteria to select the best route to be installed in the routing table:
    1. Intra-area routes.
    2. Inter-area routes.
    3. External Type-1 routes.
    4. External Type-2 routes.
    B) If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route.
    The route with the lowest value for cost is chosen as the best route.
    C) If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.

    QUESTION 26
    Refer to the exhibit. For this network, if you examined the local OSPF database on R3, what would be contained in the router (type 1) LSA from R1?

    clip_image004
    A: R3 would not have a router (type 1) LSA from R1 in its local database.
    B: R1′s router (type 1) LSA would contain a link to R2 and a connection to 10.1.1.0/31.
    C: R1′s router (type 1) LSA would contain a link to R2, a connection to 10.1.1.0/31, and a link to 10.1.5.0/24.
    D: R1′s router (type 1) LSA would contain a link to R2, a connection to 10.1.1.0/31, and a link to 10.1.2.0/31.
    Answers:  A

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