Ch.8: OSPF Foundations (1)

Question 1

What type of protocol is OSPF?

Answer 1

A link-state Interior Gateway Protocol (IGP)

Question 2

What does OSPF advertise to show link states and metrics?

Answer 2

Link State Advertisements (LSAs)

Question 3

What does every OSPF router have an identical copy of?

Answer 3

Routers in the same area will have an identical copy of LSAs and an identical Link State Database (LSDB).

Question 4

What algorithm does OSPF use to calculate loop-free paths in a network?

Answer 4

Dijkstra’s SPF algorithm (shortest-path first)

Question 5

Every router will have an identical _____, but a different ____. Why?

Answer 5

Each router will have an identical LSDB but the SPF tree (SPT) will differ on each router. This is because each router’s SPT is made from each router’s perspective, even though each router has the same link state / metric information.

Question 6

What provides scalability with OSPF?

Answer 6

The ability to create different areas; logically connected rotuers/subinterfaces that reduce the size and amount of LSAs being flooded to every router

Question 7

What is the purpose of Area 0? What is it also known as?

Answer 7

Area 0, AKA the “backbone,” provids transit connectivity between all areas. All Area Boundary Routers (ABRs) have to connect to the backbone, and type 1 LSAs from one area is advertised as a type 3 (Summary) LSA when being advertised to or from Area 0 by an ABR.

Question 8

Where does the backbone get its routes from? How do non-backbone areas get routes from other areas?

Answer 8

Routes get advertised into the backbone from non-backbone areas. The advertising router is the ABR.

Routes in Area 0 get advertised into non-backbone areas

Question 9

What IP protocol does OSPF use for inter-router communication?

Answer 9

IP Protocl 89

Question 10

What multicast addresses and MAC addresses do OSPF routers advertise to?

Answer 10

AllSPFRouters: 224.0.0.5 or MAC 01:00:5E:00:00:05
AllDRouters: 224.0.0.6 or MAC 01:00:5E:00:00:06

Question 11

What are the 5 types of OSPF packets?

Answer 11

1 - Hello - discovering & maintaining neighbors
2 - Database Description (DBD/DDP) - summarizing DB contents
3 - Link-state request (LSR) - for DB downloads. Requested if part of the LSDB may be stale
4 - Link-state update (LSU) - for DB updates. Normally set as a response to an LSR
5 - Link-state acknowledgement - for flooding acknowledgements, resposne to LSA flooding

Question 12

What is OSPF packet type 1?

Answer 12

1 - Hello - discovering & maintaining neighbors

Question 13

What is OSPF packet type 2?

Answer 13

Database description (DBD/DDP) - summarizing DB contents

Question 14

What is OSPF packet type 3?

Answer 14

3 - Link-state Request (LSR) - for DB downloads. Requested if part of the LSDB may be stale

Question 15

What is OSPF packet type 4?

Answer 15

4 - Link-state update (LSU) - for DB updartes. Normally sent as a response to an LSR

Question 16

What is OSPF packet type 5?

Answer 16

5 - Link-state acknowledgement - for flooding acknowledgemenets, response to LSA flooding

Question 17

What is a Router ID?

Answer 17

A 32-bit number, unique ID for a router.
Must be unique for each AND between OSPF processes in an OSPF domain

Question 18

What is an OSPF neighbor?

Answer 18

A router that shares a common OSPF-enabled link

Question 19

What is the difference between a non-adjacenet and adjacent OSPF neighbor?

Answer 19

Adjacent neighbors are 2 OSPF routers that share a synchronized OSPF DB. Note that this full adjacency does not happen between all routers in an OSPF domain.

Question 20

In what situation does an OSPF router form a full adjacency with another router?

Answer 20

An OSPF router will only form a full adjacency with the Designated Router (DR) and Backup DR (BDR, if applicable).

Question 21

What is the purpose of the DR/BDR in an OSPF domain?

Answer 21

The DR/BDR is responsible for managing adjacency with non-DR/BDR routers in that broadcast segment. This ultimately saves resources on each router.

Question 22

What does the DR create to manage adjacency with other routers, and allow other routers to form an adjacency with the DR?

Answer 22

A “pseudonode” (virtual router)

Question 23

What happens when a DR fails (re-election general steps):

Answer 23

1) DR fails
2) BDR becomes the DR
3) A new BDR is elected

Question 24

What are the steps of DRs receiving and advertising an LSA?

Answer 24

1) All routers in broadcast segment form full adjacency with the DR/BDR
2) When a router learns of a new route, it sends an updated LSA to the AllDRouters multicast address
3) DR sends a unicast ack message to advertising router
4) DR floods the LSA to the AllSPFRouters multicast address

Question 25

How do you enable OSPF using the OSPF network statement?

Answer 25

> enable
configure terminal
router ospf 1 —- go to ospf process 1 on router
network [Network IP Address] [Subnet Wildcard Mask] area [Area #]

ex. network 192.168.0.0 255.255.255.0 area 0 – any up interface that matches 192.168.0.0-192.168.255.0 will have OSPF enabled in area 0

Question 26

What command(s) would you use to enable OSPF on a per-interface basis?

Answer 26

> enable
configure terminal
int [interface ID] —- go to interface-level configuration
ip ospf [process-id] area [area-id] [secondaries none]

If [secondaries none] is not specified, secondary connected networks will be added

Question 27

What does the router use for the RID? Can you set this manually?

Answer 27

The router will normally use the highest “up” loopback interface IP, or highest active & up physical interface.

To manually set, use: >router-id [rid]

Question 28

How do you restart an OSPF process?

Answer 28

> enable
clear ip ospf process

Question 29

What is an OSPF passive interface, and how do you set it?

Answer 29

Passive interfaces will not form adjacencies (process or send hello packets). Can be used to stop rogue OSPF routers

> passive [interface-id] — per interface
passive interface default — all interfaces

Question 30

What are the neighbor adjacency requirements?

Answer 30

1) RIDs must be unique bettw. 2 devices forming an adjacency. The RID must also be unique across the OSPF domain
2) Interfaces must share common subnet
The following should match on both routers:
3) MTUs on each interface
4) Area IDs
5) DR enablement setting
6) OSPF hello & dead timers
7) Auth type and any credenetials
8) Area type flags

Question 31

What is a simple link cost formula for OSPF? What is the default reference bandwidth (BW)?

Answer 31

Cost = Reference BW / Interface BW

Default is 100 Mbps

Question 32

How do you change the reference BW for all interfaces?

Answer 32

On an OSPF process:
> auto-cost reference-bandwidth [bw in mbps]

Per-interface:
> ip ospf cost [1-65535]

Question 33

What timers does OSPF use for failure detection?

Answer 33

1) Hello timer (10 by default on Ethernet or broadcast multi-access)
2) Dead interval timer (4x hello timer by default)

Neighbor considered down when dead interval timer reaches 0. Both timers can be set from 1-65535

Question 34

How do you set the hello and dead timer for OSPF?

Answer 34

On an interface level:
>ip ospf hello-interval [1-65535]
>ip ospf dead-interval [1-65535]

Question 35

Routers with _________ may encounter issues if placed as the DR.

Answer 35

low resources

Question 36

What are DR elections based off of?

Answer 36

Highest OSPF priority, then highest RID. During the last phase of 2-Way state

Question 37

How do you set the OSPF priority?

Answer 37

Per-interface:
>ip ospf priority [0-255]

Question 38

After changing the OSPF priority for an interface, what might you need to do?

Answer 38

Might need to restart the OSPF process if it is changed after DR election.

Question 39

What are the different OSPF network types (relevant to the CCNP)?

Answer 39

1) Broadcast - normal multi-access ethernet links (this is default)
- Hello 10, Wait 40, Dead 40
2) Point-to-point - advertised as a /32, even if interface prefixes are different.
- Hello 10, Wait 40, Dead 40
- NO DR election. Faster adjacency.

Question 40

What is the OSPF wait timer?

Answer 40

of seconds a router waits before a router participates in DR election.

A router will elect itself if hello packets it receives still don’t have a DR or BDR present. Once there is a DR/BDR it uses those RIDs in its hello packets for DR/BDR.