OSPF Questions 2

Question 1

Question 2

Explanation

To configure OSPFv2 under interface mode we must use the command “ip ospf {process-id} area {area-id}”. For example:
R1(config-if)#ip ospf 1 area 2

Question 3

Question 4

Question 5

Explanation

After the new election, R3 and R1 have highest priority (of 2) so they will be elected DR and BDR. R3 will be elected DR because its has higher router ID (of 3.3.3.3).

Question 6

Explanation

In this question, R1 & R2 will not establish OSPF neighbor relationship because the hello & dead intervals are not the same.
If we modify the hello & dead intervals to the same values then R1 & R2 will become OSPF neighbors. But they will establish OSPF adjacency too.

Answer B is not correct as the process ID is only locally significant.

Answer C is partially correct as we need to change both “hello and dead” interval, not “hello” interval only.

Answer A is a good answer because if we change both the priorities to 0 then they cannot elect DR/BDR so the adjacency cannot be complete.

Answer D is a good answer too because if we change one of the network type to “point to point” then OSPF neighbor relationship can still come up but adjacency cannot (no routes are getting exchanged).

But two good answers are only correct after fixing the hello & dead intervals to the same value (so that OSPF neighbor relationship can come up) so answer C should be the best choice here.

Note:

Neighbors
Routers that share a common segment become neighbors on that segment. Neighbors are elected via the Hello protocol. Hello packets are sent periodically out of each interface using IP multicast (Appendix B). Routers become neighbors as soon as they see themselves listed in the neighbor’s Hello packet.

Two routers will become neighbors if they agree on the following: Same Area ID, same authentication (if used), same hello & dead intervals, same subnet mask and same stub area flag.

Adjacencies
Adjacency is the next step after the neighboring process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process.

The adjacency building process takes effect after multiple stages have been fulfilled. Routers that become adjacent will have the exact link-state database. The states an interface passes through before becoming adjacent to another router are: Down -> Attempt (optional) -> Init -> 2-Way -> Exstart -> Exchange -> Loading -> Full.

At Full state the adjacency is complete.

Reference: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/7039-1.html#t22

-> Two OSPF neighbors do not exchange any routing information – the only packets they exchange is Hello packets.

Adjacencies on Point-to-Point Interfaces
OSPF will always form an adjacency with the neighbor on the other side of a point-to-point interface such as point-to-point serial lines. There is no concept of DR or BDR. The state of the serial interfaces is point to point.

Question 7

Refer to the exhibit. What is the next hop address for traffic that is destined to host 10.0.1.5?

A. Loopback 0
B. 10.0.1.4
C. 10.0.1.50
D. 10.0.1.3

Answer: C

Explanation

Traffic destined to host 10.0.1.5 will match the entry “O 10.0.1.5/32 …” so the next-hop address will be 10.0.1.50.

Question 8

Explanation

We don’t need to configure hello and dead timers for OSPF neighbor as OSPF can auto negotiate the network type (with the same hello and dead timers on both sides). We just have to make sure both interfaces are running OSPF and they are in the same area ID.