8 - Implementing OSPF IPV4

Question 1

What does the ‘ospf network ‘ do?

Answer 1

• finds the routers interfaces that match the network/wildcard mask
• enables OSPF on those interfaces
• DISCOVERS neighbors
• CREATES RELATIONSHIPS with neighbors
• ASSIGNS INTERFACES to AREAS

Question 2

What does wildcard 0.0.0.0 do?

Answer 2

all 4 octets must exactly match

Question 3

What does wildcard 255.255.255.255 do?

Answer 3

all addresses will match

Question 4

What are the 3 steps an OSPF router does to add routes to its routing table?

Answer 4

• create neighbor relationships
• build and flood LSAs so that each router in that area has identical LSDB
• computes its IP routes INDEPENDENTLY using SPF and adds them to routing table

Question 5

What show command verifies neighbors?

Answer 5

show ip ospf neighbors

Question 6

What show command verifies the LSDB?

Answer 6

show ip ospf database

Question 7

What show command verifies the routes?

Answer 7

show ip route

Question 8

What is the difference between a Local route and a Connected route?

Answer 8

Local is the local interface with /32 mask, Connected is the subnet the interface connects to.

Question 9

What are two ways to stop and start the OSPF process?

Answer 9

1. ) Reboot the router

2. ) Use the command ‘clear ip ospf process’

Question 10

What show command confirms a router’s OSPF ID?

Answer 10

show ip ospf

Question 11

If an interface is configured to be passive what happens?

Answer 11

• Quits sending OSPF Hellos on the interface.
• Ignores received Hellos on the interface.
• Does not form neighbor relationships over the interface.

Question 12

Does a subnet whose interface is passive still get advertised?

Answer 12

Yes

Question 13

What are two ways to configure interfaces as passive?

Answer 13

1) in router-config mode use ‘passive-interface fa0/1’ or 2) in router-config mode use ‘passive-interface default’ and then for each interface you don’t want to be passive use ‘no passive-interface fa0/0’

Question 14

What show command will reveal a passive-interface?

Answer 14

show ip ospf interface fa0/0

Question 15

What kind of routers have interfaces in more than one area?

Answer 15

ABRs, Area Boundary Routers

Question 16

What show commands reveal what area an interface is in?

Answer 16

• show ip ospf interfaces fa0/0

* show ip ospf interfaces brief

Question 17

Which show commands identifies a router as a DR or BDR

Answer 17

• show ip ospf interface brief
• show ip ospf interface
• show ip ospf neighbor

Question 18

How are the DR and BDR elected?

Answer 18

The highest Router ID wins and is DR, the second highest Router ID is BDR

Question 19

How do Intra-area Routes appear in the ‘show ip route’ cmd output?

Answer 19

They show up as an O route (OSPF) and WITHOUT the IA code

Question 20

What is the formula OSPF uses to calculate the cost of an interface?

Answer 20

Convert to kilobits/sec and use Reference Bandwidth / interface_bandwidth

Question 21

How does an engineer change the cost of a particular interface?

Answer 21

Method 1: go on the interface and configure the cost using ‘ip ospf cost ‘.
Method 2: Change the bandwidth of the interface using ‘bandwidth or
Method 3: Go into OSPF router-config and change the reference bandwidth using the command ‘auto-cost reference-bandwith 10000’ changes reference bandwidth to 10Gbps.

Question 22

What is the command to enable OSPF directly on the interface?

Answer 22

interface fa0/0

ip ospf 1 area 23

Question 23

If OSPF is enabled using a network command in router-config what does the output of the ‘show ip ospf interface fa0/0’ show?

Answer 23

Attached via Network Statement

Question 24

If OSPF is enabled using a ‘ip ospf 1 area 23 command in Interface config mode what does the output of the ‘show ip ospf interface fa0/0’ show?

Answer 24

Attached via Interface Enable

Question 25

What 4 items make up the OSPF Configuration Checklist

Answer 25

* router ospf * router-id * network 10.2.2.0 0.0.0.255 area 0 * passive-interface gig0/1

Question 26

What 3 'show' cmds verify 1) neighbor relationships 2) builds and floods LSAs 3) add routes

Answer 26

* show ip ospf neighbor * show ip ospf database * show ip route

Question 27

What are some default interface costs?

Answer 27

* 1.54 Mbps = 64 * 10 Mbps = 10 * 100 Mbps = 1

Question 28

What is the command to manually configure the interface cost?

Answer 28

Interface subcommand: ip ospf cost

Question 29

How do you change the Reference Bandwidth?

Answer 29

In router subcommand mode: auto-cost reference-bandwidth example: auto-cost reference-bandwidth 10000 will accomodate links up to 10Gbps.

Question 30

How does the reference-bandwidth formula work?

Answer 30

reference_bandwidth / actual bandwidth = cost