Chapter 20 - Implementing OSPF

Question 1

What command(s) could you use to find OSPF neighbors off of an interface?

Answer 1

• show ip ospf neighbor (shows neighbors off all interfaces)
• show ip ospf neighbor <interface> (shows neighbors off a specific interface)</interface>

Question 2

What is the OSPF process-id?

Answer 2

• The OSPF process-id defines an OSPF process that will be used on the local router, it is the first part of the process of setting up OSPF on a router.
• This does not have to match between neighbors unlike the EIGRP AS-id.
• There can be multiple processes running on a single router.
• Can be any value between 1 and 65535.

Question 3

What is the OSPF network command used for?

Answer 3

• An alternate method of enable OSPF on interfaces to the ‘ip ospf’ method in interface configuration.
• Same as EIGRP network command
• The network command is used to indirectly identify which interface on the local router will be part of the relevant process-id and what area that interface will be part of.
• Matches the ip-address and wildcard-mask parameters to the IPs and subnets configured on interfaces. Whichever interfaces match the parameters will be part of the defined process and area.
• The area part of this command can be anything between 0 and 4294967295.

Question 4

What details are displayed by the ‘show ip ospf neighbor’ command?

Answer 4

• Interface - The local router’s interface that is connected to a particular neighbor.
• Address - The IP address of the interface of the particular neighbor at the other end of a link.
• State - Shows the role of the neighboring router in this neighbor relationship and whether the relationship is full.
• Neighbor ID - Shows the router-ID of this particular neighbor.

Question 5

When using the ‘show ip ospf neighbor’ command what are all the possible values of the State field?

Answer 5

• FULL/- - The neighbor has been fully negotiated. The ‘-‘ means this link does not use a DR/BDR.
• FULL/DR - The neighbor has been fully negotiated and the neighbor is the DR.
• FULL/BDR - The neighbor has been fully negotiated and the neighbor is the BDR.
• FULL/DROTHER - The neighbor has been fully negotiated and the neighbor is not the DR or BDR. This also implies that the local router is the DR or BDR as the first part of the state is FULL.
• 2WAY/DROTHER - The neighbor is in a 2-way state and the neighbor is neither the DR nor BDR (A DROTHER router). This also implies that the local router is not the DR or BDR (and is a DROTHER) since the first part of the state is not FULL.

Question 6

What information will the ‘show ip ospf database’ command show?

Answer 6

• A Router LSA (Type 1 Router LSA) for each router in the same area.
• A Network LSA (Type 2 Network LSA) from a DR for each router in that DRs network (per area)

Question 7

True or False. The ‘show ip protocols’ command will list the OSPF configuration on the local router per area.

Answer 7

True.

Question 8

What command can you use to show what interfaces OSPF is currently enabled on? What other information can you find using this command?

Answer 8

show ip ospf interface brief

This command also shows:
- The process ID that enabled this interface
- The area the interface is part of
- The IP of the interface
- The number of neighbors found via this interface

Question 9

What command can you use to configure a loopback interface?

Answer 9

interface loopback <interface></interface>

Question 10

What is a loopback interface?

Answer 10

• A loopback interface is a virtual interface that is always in an up/up state unless configured as shutdown.
• Very useful for OSPF RIDs unlike physical interfaces that can go down and will have a physical state of down if no signal is detected.

Question 11

What are the two methods of enabling OSPF on an interface?

Answer 11

• Indirectly using the network command (Older)
• Directly using the ip ospf command in interface configuration mode (Newer)

Question 12

Which OSPF show commands would list different information depending on whether it was configured directly or indirectly? Which would show the same?

Answer 12

Commands that differ depending on whether OSPF has been configured directly or indirectly:
- show ip protocols - Will add ‘Routing on interfaces configured explicitly’ if configured using the ‘ip ospf’ command.
- show ip ospf interface <interface> - will show 'Attached via Interface enable' next to processes configured directly. Will show 'Attached via Network statement' next to processes configured indirectly.</interface>

Commands that don’t differ depending on whether OSPF has been configured directly or indirectly:
- show ip ospf int brief
- show ip ospf database
- show ip ospf neighbor
- show ip route

Question 13

What is a Passive OSPF Interface?

Answer 13

An OSPF enabled interface that advertises the interface’s connected subnet to other OSPF routers in the area but does not send OSPF Hellos or process received OSPF Hellos. This can be configured using:

‘passive-interface <interface>' in router configuration mode</interface>

Question 14

What command could you use to determine if an OSPF enabled interface is passive?

Answer 14

• ‘show ip ospf int <interface>'</interface>
• ‘show ip protocols’

Question 15

What command could you use to cause a router to advertise a default route to neighbors?

Answer 15

‘default-information originate’ from router OSPF configuration mode

Question 16

What are the three ways of changing an OSPF interface cost to influence the route cost of a particular route?

Answer 16

• Directly on an interface using the ‘ip ospf cost’ command (1 to 65535)
• Indirectly by changing the ‘interface bandwidth’ setting for an interface, thereby changing the final calculation
• Indirectly by changing the ‘reference bandwidth’ setting for an interface, thereby changing the final calculation

Question 17

True or False. The value configured using the ‘bandwidth’ command has to match the speed of an interface.

Answer 17

False. The bandwidth command will not affect the actual speed of an interface, but some IOS features (such as OSPF) will use this metric to make decisions.

Question 18

What does the ‘auto-cost reference-bandwidth <value>' command do?</value>

Answer 18

• Sets the default reference bandwidth to this value in mbps.
• For example if this value is set to 10000 then 10gbps links have a cost of 1, 1gbps links have a cost of 10, 100mbps links have a cost of 100 etc.
• By default this is set to 100(mbps) so anything faster than this has a default cost of 1.
• It is best practice to increase this so that slow links don’t have the same cost as faster links.

Question 19

What command could be used increase the number of routes that OSPF will allow ECMP to route traffic to a particular subnet over?

Answer 19

‘maximum-paths <number>' in router OSPF configuration mode</number>

Question 20

What command could you use to find the cost of an interface?

Answer 20

‘show ip ospf int brief’

Question 21

What rules must OSPF routers follow in order to become neighbors?

Answer 21

• Must be in the same subnet
• Must be in the same area
• OSPF Process must not be shutdown (done from OSPF config mode)
• Router IDs must be unique
• Hello and Dead timers must match
• Authentication settings must match

On routers where the below don’t match, they can still become neighbors but OSPF won’t function properly and neighbors may not form full adjacencies
- Layer 3 MTU settings must match
- OSPF network type

Question 22

What command can you use to change the Hello timer of an OSPF router?

Answer 22

‘ip ospf hello-interval <value>' in interface config</value>

Question 23

What command can you use to change the Dead timer of an OSPF router?

Answer 23

‘ip ospf dead-interval <value>' in interface config</value>

Question 24

What commands would you use to setup an OSPF password?

Answer 24

• ‘ip ospf authentication-key <password>' in interface config to create a password</password>
• ‘ip ospf authentication’ in interface config to enable password security

Question 25

What is the OSPF password used for?

Answer 25

Only allows routers with a matching password to become neighbors

Question 26

True or False. When using RIPv2 and EIGRP, you don’t have to use a wild card mask when using the ‘network’ command.

Answer 26

True. For EIGRP if the mask is not entered it will use the classful mask of the address you entered. RIPv2 doesn’t even allow you to enter a mask.