OSPF

Question 1

What kind of routing protocol is OSFP?

Distance Vector
Link State

Answer 1

​Link State

Question 2

Does OSPF only know about it’s neighbor’s routes or have a picture of the entire network topology

Answer 2

OSPF is a Link State routing protocol so it knows about the while network topology.

A Distance Vector protocol like RIP or EIGRP only learns about routes from its directly connected neighbors.

Question 3

What kind of packets are used in OSPF to exchange routing infrormation?

Answer 3

LSA - Link State Advertisements are used to pass routing updates between routers.

Question 4

List steps taken in OSPF route learning.

Answer 4

Question 5

List OSPF Packet Types

Answer 5

1. Hello Packet - Router sends and listens for these on enabled interfaces
2. DBD - DataBase Descriptor - Info about the networks a router knows about
3. LSR - Link State Request - asking for more information about a link
4. LSA - Link State Advertisement - A routing update
5. LSU - Link State Update - contains a list of LSA’s that should be updated, typicially flooded out
6. LSAck - Recieving routers acknowledge LSAs

Question 6

What is the number called that is listed when enabling OSPF on a router?

i.e. R1(config)# router ospf 100

Answer 6

the Process ID

• This is a locally, not globaly significant number.
• This does not have to be the same on other neighbor routers.
• Usually only one Process ID is used on a router no matter how many interfaces it has.

Question 7

What is the number denote that is entered after the network address and wildcard mask in OSPF?

i.e:

R1(config)# router ospf 1
R1(config)# network 192.168.0.0 0.0.255.255 area 0
R1(config)# network 10.0.0.0 0.255.255.255 area 0

Answer 7

The area number will be the same on all routers that are sharing routing info in their databases with each other.

Question 8

In OSPF, if you enter the following commands and hit enter, what will happen reguarding the wildcard mask?

R1 (config) # router ospf 100
R1(config-router) network 10.0.0.0

Answer 8

You’ll get an % Incomplete command error because OSFP does not default to classful network/wildcard masks.

Question 9

In OSPF when entering the command:
R1(config-router) network 192.168.0.0 0.0.255.255 area 0

Will the network 192.168.0.0/16 be advertised?

Answer 9

Probably No.
Only if an interface on that router is configured on that exact network / mask combination. If it only has interfaces configured with 192.168.10.0/24 and 192.168.20.0/24, those spific networks will be advertised, not the entire 192.168.0.0/16 network.

Question 10

Command to show OSPF running config

Answer 10

R1# show run | section ospf

Question 11

command to show detailed information about OSPF:

Answer 11

R1# show ip protocols

Question 12

Command to show interfaces setup for OSPF?

Answer 12

R1# show ip ospf interface brief

Question 13

command to show nearby OSPF devices:

Answer 13

R1# show ip ospf neighbor

Question 14

Command to view OSPF database

Answer 14

R1# show ip ospf database

Question 15

How is the default Router ID chosen in OSPF?

Answer 15

Highest loopback interface IP.

If no loopback interfaces, then highest interface IP address.

Can also manually be set.

Question 16

Command to mamually config Router ID

Answer 16

R1(config) router ospf 100
R1(config-router)# router-id 2.2.2.2

(Doesnt have to be an IP address on that router but must be in the form of an IPv4 address)

Question 17

Commands to configure OSPF passive interface

Answer 17

Question 18

Commands to default to all passive OSPF interfaces and the allow some interfaces to not be passive:

Answer 18

Question 19

Command to configure OSPF default route:

Answer 19

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1
R1(config)# router ospf 100
R1(config-router)# default-information originate

Question 20

What does O*E2 indicate in the show ip routes table?

Answer 20

The route was a static route on a router that was injected (redistributed) into OSPF.

Question 21

What does O IA indicate in the show ip routes table?

Answer 21

The route was learned from an ABR - Area Boundry Router and is an ‘Inter Area’ Route that’s located in another OSPF area.

Question 22

In OSPF, does manually setting the router ID imediatly take effect?

Answer 22

No. OSPF must be manually restarted for the new update to take effect.

Question 23

How is the OSPF metric calculated?

Answer 23

Based on path ‘cost’.
Cost is derieved from the interface bandwidth.

Question 24

How is the ‘Cost’ in OSPF referenced and be modified?

Answer 24

Based on 100 / interface bandwidth.

If this is less than 1, the protocol rounds up to 1

should be adjusted by:

R1(config)# router ospf 1
R1(config-router)# auto-cost reference-bandwidth 100000 (in mbit)

Question 25

Command to adjust the reference bandwidth in OSPF:

Answer 25

R1(config)# router ospf 1 R1(config-router)# **auto-cost reference-bandwidth 100000** (in mbit)

Question 26

How can the OSPF metric be manipulated?

Answer 26

Manually changing the cost of the link. | (can also change the bandwidth setting but that is a bad idea and not recomended)

Question 27

Command to view OSPF Cost setting of a router's interfaces:

Answer 27

R1# show ip ospf interface brief

Question 28

What must be the same between two neighbor routers in order to from an adjacency in OSPF

Answer 28

The area number

Question 29

Types of OSPF routers

Answer 29

* Backbone routers i.e. Area 0 * ABRs - Area Boundry Routers - part of multipe areas (normally 2) * Normal Area Router - All in the same area * ASBR - Autonomous System Boundry Router - connects to outer routing protocols or static routes

Question 30

In OSPF, where is route summerazation done?

Answer 30

on the ABRs - Area Border Routers

Question 31

In OSPF, will routes be automaticly or manually summarised?

Answer 31

Manually.

Question 32

How many routes are shared if maunal summarization is not configured on ABRs - Area Border Routers that at the edge of differnet areas.

Answer 32

All routes will be shared between both areas if manual summarazation is not configured.

Question 33

What are DR and BDR routers in OSPF?

Answer 33

Designated Routers Backup Designated Router 3+ routers can be DROTHER i.e. DR Other DR and BDR relationships arn't estlibilished for serial links. A DR and BRD will be estlabished for each multiaccess network segment. i.e. broadcast domain or non-braodcast multidevice link

Question 34

How are DR and BRD elections decided in OSPF?

Answer 34

Highest roter priority wins and becomes DR (designated router). Next highest becomes backup DR. In case of tie, highest router ID is elected DR and then BDR. Priority 0 will force the router to be not be a DR or BDR. Not tested: If routers come online at different times, the first router online will be the DR, and then the next will be BDR, then others will be DRother.

Question 35

What are the 7 OSPF relationship states?

Answer 35

0. **Down -** no recent information received 1. **Init** – the router received a Hello message from the other OSFP router 2. **2-way** – the neighbor has received the Hello message and replied with a Hello message of his own. A neighborship has been estliblished — election for DR/BDR may now occur — 3. **Exstart** – beginning of the LSDB exchange between both routers. Routers are starting to exchange link state information. 4. **Exchange** – DBD (Database Descriptor) packets are exchanged. DBDs contain LSAs headers. Routers will use this information to see what LSAs need to be exchanged. 5. **Loading** – one neighbor sends LSRs (Link State Requests) for every network it doesn’t know about. The other neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process 6. **Full** – both routers have the synchronized database and are fully adjacent with each other.

Question 36

In EIGRPv6, where should the network command be entered to enable each patrisapating interface/port?

Answer 36

The interface section in EIGRPv6

Question 37

In EIGRPv4, where should the network command be entered to enable each patrisapating interface/port?

Answer 37

The router EIGRP section in EIGRPv4 unlike EIGRPv6 which requires the network info to be entered into the interface config.