CCNA – OSPF Questions 2

Note: If you are not sure about OSPF, please read our OSPF Tutorial.

Question 1

Explanation

A is not correct because configure EIGRP on these routers (with a lower administrative distance) will force these routers to run EIGRP, not OSPF.

B is not correct because the backbone area of OSPF is always Area 0.

C and D are correct because these entries must match on neighboring routers:

– Hello and dead intervals
– Area ID (Area 0 in this case)
– Authentication password
– Stub area flag

Question 2

Explanation

The well-known formula to calculate OSPF cost is

Cost = 10⁸ / Bandwidth

so B is the correct answer.

Question 3

Explanation

D is correct because these entries must match on neighboring routers:

– Hello and dead intervals
– Area ID (Area 0 in this case)
– Authentication password
– Stub area flag

In this case Ethernet0 of R1 has Hello and Dead Intervals of 5 and 20 while R2 has Hello and Dead Intervals of 10 and 40 -> R1 and R2 cannot form OSPF neighbor relationship.

Question 4

Question 5

Question 6

Explanation

To form an adjacency (become neighbor), router A & B must have the same Hello interval, Dead interval and AREA number.

Question 7

Explanation

OSPF LSA Type 1 (or Router LSA) is generated by all routers in an area to describe their directly attached links. An example below shows this type of LSA:

As you can see, the LSA Type 1 uses the router ID to advertise itself (1.1.1.1 or 2.2.2.2).

The Router ID (RID) is an IP address used to identify the router and is chosen using the following sequence:
+ 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.
+ The router ID can be manually assigned

Question 8

Explanation

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

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 9

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.