Chapter 2 - Single-Area OSPFv2 Configuration

Question 1

what command is used to enable OSPFv2 in global config mode

Answer 1

router ospf -process-id-

Question 2

two situations that use router ID on OSPF-enabled router

Answer 2

• participate in OSPF database synchronization during Exchange State (higher router ID sends their DBD packets first)
• participate in election of DR (highest router ID is elected DR, second highest is BDR)

Question 3

all OSPF packets include the router ID of the…

Answer 3

originating router

Question 4

In preferential order of how router ID is derived:

Answer 4

1. Router ID configured using router-id *recommended method
2. Highest IPv4 address of any configured loopback interface
3. Highest active IPv4 address of any physical interface

Question 5

command to check Router ID

Answer 5

show ip protocols | include Router ID

Question 6

True or False:

OSPF does not need to be enabled on an interface for that interface to be chosen as the router ID

Answer 6

True

Question 7

command to config router id

Answer 7

In global config:
#router ospf 10
#router-id 1.1.1.1

Question 8

when can router be changed/modified?

Answer 8

router is reloaded or OSPF process is reset

Question 9

command to reset neighbor adjacencies to modify router ID

Answer 9

clear ip ospf process

Question 10

what command is used to enable OSPF on an interface on a point-to-point OSPF network

Answer 10

network -network-address wildcard-mask- area -area-id

Question 11

two ways to identify interfaces that will participate in OSPFv2 routing process

Answer 11

• by Wildcard Mask - any active interface configured with an IPv4 add belonging to that network will participate in OSPFv2 routing process
• by specifying the exact interface IPv4 add using 0.0.0.0 wildcard mask -> network 10.1.1.5 0.0.0.0 area 0 tells router to enable interface associated with that network add for routing process

Question 12

command to configure OSPF directly on interface

Answer 12

#interface g0/0
#ip ospf -process-id- area -area-id-

Question 13

How does unneeded OSPF messages sent out on all OSPF-enabled interfaces affect the network?

Answer 13

inefficient use of bandwidth
inefficient use of resources
increased security risk

Question 14

what command is used to prevent unwanted OSPF messages from transmitting but still allow that router interface to be advertised to other routers?

Answer 14

#router ospf 10
#passive-interface loopback 0

Question 15

command to see what interfaces are elected DR and BDR

Answer 15

show ip ospf interface

Question 16

what interface configuration command disables DR/BDR election?

Answer 16

#interface g0/0
#ip ospf network point-to-point

Question 17

what IP does DROTHERs use to send OSPF packets to the DR and BDR

Answer 17

224.0.0.6

and only DR and BDR listen on this address

Question 18

what info does show ip ospf interface -interface- display

Answer 18

shows router state (DR, BDR, DROTHER) - which is DR and BDR, their Router ID and IP add, how many adjacencies and who they are

Question 19

command to verify OSPFv2 adjacencies

Answer 19

show ip ospf neighbor

Question 20

how is DR elected?

Answer 20

Router with highest interface priority is elected DR.
0-255 value can be set
0 value interface can’t be elected DR
If all interface priorities are equal, router with highest Router ID is elected DR

Question 21

Router remains DR until one of these events occurs

Answer 21

• DR fails
• OSPF process on DR fails or is stopped
• the multiaccess interface on the DR fails or is shutdown

Question 22

what command is used to set interface priority?

Answer 22

ip ospf priority -value-

value btwn 0-255

Question 23

what command is used in privilege mode to see DR and BDR election process

Answer 23

debug ip ospf adj

Question 24

what gives indication of the overhead that is required to send packets across a certain interface?

Answer 24

a metric

Question 25

formula to calculate OSPF cost

Answer 25

cost = reference bandwidth / interface bandwidth

**default reference bandwidth is always 10^8 (100,000,000) bps

Question 26

command to change reference bandwidth on router configuration

how to verify OSPFv2 cost assigned to a router interface

Answer 26

auto-cost reference-bandwidth -Mbps-

show ip ospf interface -interface-

Question 27

how often are Hello Packets transmitted to multiaccess address 224.0.0.5?

Answer 27

every 10 seconds

Question 28

what is the period called that the router waits to receive a Hello Packet before declaring the nieghbor down?

time for multiaccess and non-broadcast multiaccess networks

Answer 28

Dead Interval

40 seconds for multiaccess (x4 of Hello Interval)

30 secs for Hello Packet and 120 secs for Dead Interval on non-broadcast multiaccess networks

Question 29

command to check Hello and Dead intervals

Answer 29

sh ip ospf interface

Question 30

command to see Dead Time countdown

Answer 30

show ip ospf neighbor

Question 31

a router connected to the internet is sometimes called:

Answer 31

edge router or gateway router

Question 32

name given to the router located btwn the OSPF routing domain and a non-OSPF network

Answer 32

Autonomous System Boundary Router (ASBR)

Question 33

what does O*E2 indicate on a route table

Answer 33

the route was learned through OSPFv2
* - means good candidate for default route
E2 - identifies this as an external route

Question 34

in order to propagate a default route, the edge router must be configured with the following:

Answer 34

• a default static route using ip route 0.0.0.0 0.0.0.0 either with [next-hop-address] but if going out to the internet use [exit-intf]
• default-information originate router config command; which tells router to be the source of the default route info and propagate the default static route in OSPF updates

Question 35

Some reasons routers may not form OSPF adjacency:

Answer 35

• subnet masks don’t match
• OSPF Hello or Dead Timers don’t match
• OSPF Network Types don’t match
• missing or incorrect OSPF network command