OSPF practice questions with answers

Answer: B

On a broadcast segment only the DR and BDR form full adjacencies with every router. Two DROthers remain in 2-Way with each other, which is the normal and correct state. It is not a fault, so there is no mismatched MTU or timer to fix. A router stuck in Init has not yet seen its own ID in the other router’s Hello. Full is reached only with the DR and BDR.

Answer: D

On broadcast and point-to-point network types the defaults are a 10-second Hello and a 40-second dead interval, four times the Hello. 5 and 20 are not defaults. 30 and 120 are the defaults on non-broadcast network types. A dead interval of 30 seconds would not be four times the Hello.

Answer: A

OSPF runs directly over IP as protocol number 89, as defined in RFC 2328. It does not use TCP or UDP, so there is no port number to open in a filter. UDP port 89 and TCP port 89 do not exist for OSPF. TCP port 179 belongs to BGP.

Answer: C

Without a configured router-id, OSPF uses the highest IPv4 address on an up loopback interface. 1.1.1.1 is used even though it is numerically lower than the physical addresses. Physical interfaces are considered only when no loopback is up, which is why 192.168.1.1 is not chosen. 10.0.0.9 is neither. 0.0.0.0 is not a valid router ID.

Answer: C

With the default reference bandwidth of 100 Mbps, Fast Ethernet and anything faster all cost 1, so the paths look equal. Raising auto-cost reference-bandwidth, for example to 10000 on every router, gives Gigabit a cost of 10 and Fast Ethernet 100. Changing Hello timers does not alter cost. The DR priority affects only the election. Changing the router ID has no effect on path selection.

Answer: A

INIT means R1 has received a Hello from R2 but that Hello does not list R1’s router ID, so Hellos from R1 are not reaching R2. Something is dropping traffic in that direction, such as an ACL or a one-sided configuration problem. An MTU mismatch shows in ExStart or Exchange, after the Hello exchange works in both directions. A timer mismatch would make R1 discard R2’s Hellos, so R2 would not be listed. A duplicate router ID also shows up later, during the database exchange.

Answer: D

Neighbors that stall in ExStart or Exchange usually have different interface MTUs, because the database description packets sized for one MTU are dropped by the other side. Making the MTU equal, or using ip ospf mtu-ignore as a workaround, resolves it. Changing priority affects only the DR election. Changing the Hello interval would not help because Hellos are already being exchanged. Changing the area would stop the Hello exchange altogether.

Answer: B

The highest priority wins the election, and the router ID is used only to break a tie. R2 has priority 100, so it becomes the DR, and R3 with priority 50 becomes the BDR. R1 has the highest router ID, but that applies only when priorities are equal, and R1 is left as a DROther. The other options choose by router ID or reverse the DR and BDR.

Answer: A

The DR election is not preemptive. R4 joins an existing election result and takes the DROther role, regardless of its higher priority. R1 remains the DR and R2 remains the BDR until one of them fails or the OSPF process is cleared. R4 does not become DR immediately, and the BDR is not replaced either. Priority only matters when an election takes place.

Answer: C

With a 100 Mbps reference, Fast Ethernet costs 1, 10 Mbps Ethernet costs 10 and Gigabit costs 1, because the cost never falls below 1. The path totals 1 + 10 + 1 = 12. A total of 3 treats the 10 Mbps link as cost 1. A total of 10 counts only the slowest link. A total of 11 leaves out the cost of R3’s interface on the destination network.

Answer: B

A router discards Hellos whose Hello or dead interval differs from its own, so the neighbor is never created. The neighbor does not appear in the table at all, not even in INIT, which is a state that needs a valid Hello to have been received. ExStart and 2-Way both come after a successful Hello exchange. Full requires a complete database exchange.

Answer: C

A network statement matches interface addresses using a wildcard mask. 172.16.4.0 0.0.0.3 matches 172.16.4.0 to 172.16.4.3, which includes 172.16.4.1, and area 0 matches R2. Using area 1 would put the interface in a different area and prevent the adjacency. 172.16.4.4 0.0.0.3 matches 172.16.4.4 to 172.16.4.7, which does not include R1’s address. 172.16.4.2 0.0.0.0 matches only R2’s address.

Answer: D

The passive-interface command stops Hellos on an interface while the network it belongs to is still advertised through a matching network statement. Removing the network statement would stop the subnet from being advertised. Setting priority to 0 only keeps the router out of the DR election and still sends Hellos. Raising the dead interval does not stop Hellos.

Answer: A

Setting the network type to point-to-point with ip ospf network point-to-point removes the DR and BDR election, and the neighbors move directly to Full. Priority 0 on both routers would leave no router able to become DR, and it does not change the network type. A passive interface sends no Hellos, so no adjacency forms. Setting the Hello interval to 40 changes timers only and the election still happens.

Answer: B

Priority 0 makes a router ineligible to be the DR or BDR, so R1 stays a DROther even though it has the highest router ID. R2 and R3 tie on priority 1, so the highest router ID decides: R3 (2.2.2.2) becomes the DR and R2 (1.1.1.1) the BDR. R1 cannot be DR or BDR. R2 has the lowest router ID, so it does not win the tie.