Chapter 21 - OSPF Network Types and Neighbors

Question 1

What are the three different OSPF network types?

Answer 1

• Point to Point - Enabled on PPP and HDLC interfaces by default.
• Broadcast (DR and BDR selection) - Enabled by default on Ethernet and FDDI (Fibre Distributed Data Interfaces) interfaces.
• Non Broadcast - Enabled on Frame Relay and X.25 interfaces by default.

Question 2

True or False. All OSPF Network Types use a DR and a BDR.

Answer 2

False. Only broadcast and non broadcast uses a DR and BDR.

Question 3

What is the default OSPF network type used on an OSPF enabled Ethernet interface?

Answer 3

Broadcast.

Question 4

When using the ‘show ip ospf interface brief’ command, what does the ‘nbrs F/C’ collumn mean?

Answer 4

This collumn lists two numbers.
- The first number lists the total number of Fully Adjacent (neighbors in a Full state with the local router) neighbors discovered out of a specified interface.
- The second number lists the total number of neighbors discovered out of a specified interface. This will include DROthers.

Question 5

True or False. If the current DR fails, the BDR will be come the DR and a new BDR will be elected.

Answer 5

True.

Question 6

What are the two configurable settings that can influence OSPF DR/BDR election?

Answer 6

• OSPF Interface Priority - Must be a value between 0 and 255. The highest value wins in an election.
• OSPF Router ID - If the interface priority is tied then the highest Router ID wins in an election.

Question 7

When would you use a Point to Point network type over a Broadcast network type?

Answer 7

• If you have two routers connected over a single link

Question 8

True or False. Broadcast network type increases OSPF convergence time.

Answer 8

True as all routers need to communicate with the DR and BDR to react to any changes in topology.

Question 9

What will the router do if you shutdown a routing protocol process? (e.g. shutdown when in router ospf process config mode)

Answer 9

• Brings down all neighbor relationships and clears the OSPF neighbor table
• Clears the LSDB
• Clears the IP routing table of any OSPF learned routes
• Retains OSPF configuration
• Lists OSPF enabled interfaces in the ‘show ip ospf interface’ command but in a DOWN state

Question 10

True or False. Neighboring routers will share routes even if their neighboring interfaces have different MTU values.

Answer 10

False. However the neighbors may still be visible when using the ‘show ip ospf neighbor’ command.

Question 11

What happens if two neighboring routers use different OSPF network types?

Answer 11

• The two routers become fully adjacent neighbors (reach a FULL state)
• They exchange their LSDBs
• They do not add IP routes to their routing tables as the routes are not learned

Question 12

What is an OSPF Network Type?

Answer 12

• Refers to the type of connection between OSPF neighbors.

Question 13

If there is only a single router on a network segment, what will happen in relation to DRs and BDRs?

Answer 13

Only a DR will exist and that router will become the DR.

Question 14

What is the order of priority for DR selection?

Answer 14

1. BDR - BDR will always become DR first
2. Highest OSPF interface priority (default of 1 on all interfaces) - Used to select BDR
3. Highest router ID - Used to select BDR

Question 15

What command could you use to find the Router State (DR/BDR/DROther), Router ID, and Interface Priority of an interface?

Answer 15

show ip ospf int <interface></interface>

Question 16

True or False. If the Interface Priority of an interface is 0 it will always be DR.

Answer 16

False. It can never be DR or BDR.

Question 17

What does Preemptive mean in the context of DRs and BDRs?

Answer 17

• It means that if a new router becomes highest priority in the segment, it will automatically become DR
• If it were non-preemptive, it would not become DR unless the OSPF process is reset or the current DR goes down

Question 18

What is a DR?

Answer 18

• A DR (Designated Router) is a router on a Broadcast type OSPF network (such as an Ethernet LAN) that receives LSAs from all other OSPF neighbors on the LAN and ensures that all neighbors have a copy, as opposed to all neighbors flooding all LSAs to each other.

Question 19

What is a BDR?

Answer 19

• A BDR (Backup Designated Router) will become the DR if the current DR fails. They exchange LSAs with the DR and DROthers the same as DRs do.

Question 20

What multicast address is used specifically to send traffic to a DR/BDR?

Answer 20

224.0.0.6

Question 21

True or False. OSPF routers in the Point to Point network type still use 224.0.0.5 to send Hellos to dynamically discover neighbors.

Answer 21

True. However a DR and BDR are not elected.

Question 22

What command do you use to change the OSPF network type statically?

Answer 22

ip ospf network <type></type>

Question 23

What is the default Hello and Dead timer of the non-broadcast OSPF network type?

Answer 23

• Default Hello is 30 seconds
• Default Dead is 120 seconds