OSPF

Question 1

Administrative Distance

Answer 1

110

Question 2

OSPF Process stages

Answer 2

1. Down - no hello packet recieved
2. INIT - recieved hello packet but it’s router-id was not included in hello packet neighbor field.
3. 2Way - We have bi-directional communication. both routers see their router-ids in hello packets

• DR/BDR election will occur.
• In NBMA and broadcast networks, the router will only become FULL with the DR and BDR. it will become DROTHER with the other routers.
• In point2point or point2multipoint networks, the routers will become FULL with other router.

4. Exstart - The start of the exchange process. The routers and their DR and BDR establish a master-slave relationship and choose the initial sequence number for adjacency formation.
5. Exchange - OSPF routers will exchange database descriptors (DBD) packets. DBD contains link-state advertisement (LSA) Headers only and describe the contents of the entire link-state database.
6. Loading
7. Full
   https: //www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13685-13.html

Question 3

Tables

Answer 3

Neighbor Table | Database Table | Routing Table

Question 4

OSPF Packet types

Answer 4

Hello
Database Description
Link-State Request
Link-State Update
Link-State Acknowledge

Question 5

Hello Packets

Answer 5

Every 10 seconds

Question 6

Dead Timer

Answer 6

Every 40 Seconds

Question 7

Multicast addresses

Answer 7

224. 0.0.5 - DR will send updates to DROTHERS using this multicast address.
225. 0.0.6 - DROTHERS will send updates to DR/BDR routers using this address.

Question 8

Understanding OSPF Packet types in detail

Answer 8

https://sites.google.com/site/amitsciscozone/home/important-tips/ospf/ospf-packet-types

Question 9

Metric

Answer 9

Open Shortest Path First (OSPF) uses “Cost” as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation.

The formula to calculate the cost is Reference Bandwidth divided by interface bandwidth. For example, in the case of 10 Mbps Ethernet , OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10.

The default Reference Bandwidth of OSPF is 100 Mbps and the default OSPF cost formula doesn’t differentiate between interfaces with bandwidth faster than 100 Mbps. These days, 1 Gbps and 10 Gbps links are also common.

Question 10

Virtual Links

Answer 10

• Virtual links are used to connect a dis-contiguous area to area 0.
• A logical connection built between routers.
• Virtual links are recommended for backup or temp connections.

Question 11

configuring Virtual Links

Answer 11

Virtual links command will need to be configured on both ABRs that bound the area that we’re trying to build the link through.

Router(config)#router ospf
Router(config-router)#Area 20 virtual-link 3.3.3.3

Verify Virtual links
sh ip ospf virtual-links

Question 12

DR / BDR

Answer 12

The designated router (DR) is responsible for updating all other routers about changes in the network. The Backup designated router (BDR) is the back up for the DR and will only assume the position or DR when the DR goes down.
Selecting the DR
1. highest priority [0-255]
2.second highest priority will be the BDR
3.default priority is 1.
4. in case of a tie, the router with the highest router-id will be the DR and the second highest router-id will be the BDR.
5. routers with priority value of 0 will never be DR or BDR
. These routers will be DROther

Question 13

changing the priority value

Answer 13

router(config)interface x

router(config-if)ip ospf priority [0-255]

Question 14

OSPF router relationships

Answer 14

DR/BDR —> DROTHER = FULL

DROTHER – > DROTHER = 2WAY

Question 15

Link State Advertisements Types

Answer 15

LSA Type 1 - Router LSAs
LSA Type 2 - Network LSAs
LSA Type 3 or 4 - Summary LSAs
LSA Type 5 - Autonomous Systems External LSAs
LSA Type 6 - Multicast OSPF LSA
LSA Type 7 - Defined for Not-so-stubby areas
LSA Type 8 - External Attributes LSA for BGP
LSA Type 9,10,11 - Opaque LSAs

Question 16

LSA Type 1

Answer 16

Router LSA

• generated by the internal router
• “o” routes in routing table
• Floods within it’s router only; does not cross ABR

Question 17

LSA Type 2

Answer 17

Network LSA

• happens in broadcast networks
• advertised by DR of transit network
• floods within its area only; does not cross ABR
• “o” routes

Question 18

LSA Type 3

Answer 18

Summary LSA

• used to flood network information to areas outside the originating area (inter area)
• “oIA” routes in routing table.
• Advertised by ABR of originating area.
• regenerated by subsequent ABR to flood throughout the AS

Question 19

LSA Type 4

Answer 19

LSAs are used to advertise the address of the ASBR to all other areas.

• they are generated by the ABR of the originating router
• -regenerated by subsequent ABR to flood throughout the AS

Question 20

LSA Type 5

Answer 20

are used to advertise routes from another autonomous system.
they are advertised and owned by the ASBR of the originated routers.

Question 21

Purpose of Ospf stubs

Answer 21

1. Reduce the processing overheads from routers.

2. Reduce the size of the routing table.

Question 22

What does it mean to configure a stub area?

Answer 22

When an Area is configured as a stub.. it means that the ABR will no longer send LSA Type 5 advertisments to the internal routers but just a default route. External routes are replaced by a default route (o*IA)

Question 23

E1/E2 routes

Answer 23

E1 - internal cost to ASBR added to external cost
E2 - internal cost. It is same as external cost.

Https://ipwithease.com/ospf-external-e1-and-e2-routes/

Question 24

Areas which cannot be stub

Answer 24

1. Area 0
2. Area with ASBR
3. Virtual Area (Area that the virtual link passes through)

Question 25

How to configure stub area

Answer 25

Router ospf 1 | Area 10 stub

Question 26

What does it mean to be a Totally Stubby Area?

Answer 26

When an Area is configured as a totllly stubby area.. it means that the ABR will no longer send LSA Type 5 & Type 3 advertisments to the internal routers but just a default route. External routes and inter-Area routes are replaced by a default route (o*IA)

Question 27

How to configure Totally Stubby Area

Answer 27

On All Internal Routers: > router ospf 1 > area 10 stub On the ABR > routers ospf 1 >Area 10 stub no summary

Question 28

NBMA

Answer 28

NBMA is mainly employed on networks that do not have broadcast or multicast capabilities. Frame Relay, Asynchronous Transfer Mode (ATM), home power line networking, and X.25 are some common examples of NBMA network technologies.