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ICND2 – OSPF Questions 4

October 13th, 2017 in ICND2 200-105 Go to comments

Question 1

Explanation

OSPF is a link-state routing protocol so it converges more quickly than distance-vector protocol. OSPF uses cost to determine the best route. The popular formula to calculate OSPF cost is: cost = 108 / Bandwidth [ in kbps] (in fact the formal formula is: cost = reference bandwidth / configured bandwidth of interface in kbps. On Cisco routers, the reference bandwidth defaults to 100000 kbps)

Question 2

Explanation

From the output of the “show ip ospf database”:

show_ip_ospf_database.jpg

We can see OSPF Router ID will be used as source of Type 1 LSA (1.1.1.1 & 2.2.2.2). Also the router will chose the highest loopback interface as its OSPF router ID (if available).

Question 3

Explanation

Unlike OSPFv2 which form adjacencies using their IPv4 interface addresses, OSPFv3 use the link-local addresses (FE80::/10) to form adjacencies. Therefore it can support multiple IPv6 subnets on a single link as an interface can have multiple IPv6 addresses (and two nodes can communicate with each other even if they do not share a common IP subnet.) -> B is correct.

IPv6 uses the term “link” instead of “subnet” or “network” to define a medium used to communicate between nodes at the link layer -> D is correct.

OSPFv3 can support more than 1 instance over a common link. For example you can run instance 1 on an interface with the command:

Router(config-if)# ipv6 ospf 100 area 0 instance 1

-> C is not correct.

Question 4

Explanation

The “show ipv6 route” displays the current contents of the IPv6 routing table. This device is running OSPF so we can deduce it is running OSPFv3 (OSPF for IPv6). An example of the “show ip v6 route” is shown below:

show_ipv6_route.jpg

Question 5

Explanation

By default, OSPF uses a 10-second hello timer and 40-second hold (dead) timer on broadcast and point-to-point links, and a 30-second hello timer and 120-second hold timer for all other network types.

Question 6

Explanation

In order to become OSPF neighbor following values must be match on both routers:

+ Area ID
+ Authentication
+ Hello and Dead Intervals
+ Stub Flag
+ MTU Size

Therefore we need to adjust the MTU size on one of the router so that they are the same. Or we can tell OSPF to ignore the MTU size check with the command “ip ospf mtu-ignore”.

Question 7

Explanation

In the router ospf command, the ranges from 1 to 65535 so o is an invalid number -> B is correct but A is not correct.

Question 8

Explanation

There are 2 segments on the topology above which are separated by Corp-3 router. Each segment will have a DR so we have 2 DRs.

To select which router will become DR they will compare their router-IDs. The router with highest (best) router-ID will become DR. The router-ID is chosen in the order below:

+ The highest IP address assigned to a loopback (logical) interface.

+ If a loopback interface is not defined, the highest IP address of all active router’s physical interfaces will be chosen.

In this question, the IP addresses of loopback interfaces are not mentioned so we will consider IP addresses of all active router’s physical interfaces. Router Corp-4 (10.1.40.40) & Branch-2 (10.2.20.20) have highest “active” IP addresses so they will become DRs.

Question 9

Explanation

Router_E learns two subnets subnets 208.149.23.64 and 208.149.23.96 via Router_A through FastEthernet interface. The interface cost is calculated with the formula 108 / Bandwidth. For FastEthernet it is 108 / 100 Mbps = 108 / 100,000,000 = 1. Therefore the cost is 12 (learned from Router_A) + 1 = 13 for both subnets -> B is not correct.

The cost through T1 link is much higher than through T3 link (T1 cost = 108 / 1.544 Mbps = 64; T3 cost = 108 / 45 Mbps = 2) so surely OSPF will choose the path through T3 link -> Router_E will choose the path from Router_A through FastEthernet0/0, not Serial1/0 -> C & D are not correct.

In fact, we can quickly eliminate answers B, C and D because they contain at least one subnet learned from Serial1/0 -> they are surely incorrect.

Question 10

Explanation

The ‘network … area …’ command under OSPF process has the following meaning: It searches all the active interfaces, if the IP address of that interface belong to the ‘network …’ configured under OSPF process then the router will run OSPF on that interface. Therefore when we configure ‘network 0.0.0.0 255.255.255.255 area 0’ command, all interfaces are matched -> OSPF is enabled on all active interfaces on the router.

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