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Open Shortest Path First – OSPF Fundamentals – Scenario

Published

on June 29, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, DR and OSPF

. 0 Comments

GIVEN: For a Given Network, all routers share a common multi-access segment. One router is elected the Designated Router (DR) and another is elected the Backup Designated Router (BDR).

1. Which Parameter determines the router that will be selected as the DR in an OSPF network?

The router with the Highest OSPF Priority on a particular network segment will become the designated router (more info). The DR default OSPF priority for a Cisco Router is 1 but can be manually changed. If there are multiple routers with default OSPF Priority then the Routers elect a Designated Router using the Router ID (more info) (RID) which is basically the highest IP Address with loopbacks being preferred (if configured)

2. Could a router with a priority value of zero assume the role of a DR or a BDR in the OSPF?

An OSPF Priority of 0 (zero) means that the Router is not eligible to be elected for either DR or BDR. The zero interface state is DROTHER.

3. How is the OSPF router ID determined on a Cisco Router?

The OSPF Router ID (RID) on a Cisco Router is determined by the highest IP address configured on an interface (if not manually configured), or IP Address on a loopback interface.

4. What is the role of the DR and BDR in the OSPF network?

The role of the DR and BDR in an OSPF Network is to handle and monitor adjacencies with other routers in the network. This is because in an OSPF network adjacencies grow at a quadratic rate. This means that instead of exchanging routing information with all other routers the routers exchange information with the DR and BDR. Then in turn the DR and BDR relay the information to other routers.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Notes and Notices: This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I cannot lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 13

Published

Deon Botha

on June 24, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, OSPF and Show

. 0 Comments

Working from the my last couple of OSPF posts I am going to try and crystallize some of the material found by working through questions found in Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press book.

13. What is the advantage of the command show ip ospf interface in troubleshooting?

The advantage to use the show ip ospf interface in troubleshooting is that it allows you to verify that the interface is operating in OSPF (operational and configured as required) and see information like the, Area, DR, and BDR, list neighbours and see the network type. This command also allows drill down per interface for more informaiton.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 12

Published

Deon Botha

on June 24, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, Link State Advertisements and OSPF

. 0 Comments

12. In the show ip ospf command, there is a field called the SPF schedule delay. What is the purpose of this field, and what is the default time?

The purpose of the SPF schedule delay field specifies how long to wait to start SPF after receiving an LSA to prevent running SPF too often.

The SPF schedule default time is 5000 msec

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 11

Published

Deon Botha

on June 24, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs and Link State Advertisements

. 0 Comments

11. what is the sequence number and where is it held?

The sequence number is a field with numbers so that older and/or newer versions of LSA advertisements can be recognized from each other (start 0×8000 0001 end 0xFFFF FFFF before rolling to the start again). The eventual goal is that all routers in the same AS has the same LSDB which is then processed using SPF from which the best routes are selected and a routing table created.

The sequence number is held within the Topology Database.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 10

Published

Deon Botha

on June 24, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, OSPF and Show

. 2 Comments

10. How would you show the OSPF process ID of the router?

The OSPF process ID is the ID of the OSPF process to which the interface belongs. Defined by the below command

Router_1(config)#router ospf [process id]

The process ID is local to the router, and two OSPF neighboring routers can have different OSPF process IDs. (This is not true of Enhanced Interior Gateway Routing Protocol [EIGRP], in which the routers need to be in the same autonomous system). IOS can run multiple OSPF processes on the same router, and the process ID merely distinguishes one process from the another. The process ID should be a positive integer.

To show the OSPF Process ID use the following commands:

Router_1#show ip ospf

In the first line of output Routing Process ospf xxx with ID xxx.xxx.xxx.xxx

Router_1#show ip ospf database

In the first line of output OSPF Router with ID (xxx.xxx.xxx.xxx) (Process ID xxx)

Router_1#show ip ospf interface

In the third (plus or minus) line of output Process ID xxx, Router ID xxx.xxx.xxx.xxx, network type xxxxxxxxx, Cost: x

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 9

Published

Deon Botha

on June 22, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems and Debug

. 0 Comments

9. Which command is used to show how OSPF packets are sent and received in real time?

The command to show packets being sent and received in real time is debug ip packets

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 8

Published

Deon Botha

on June 22, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, OSPF and Show

. 0 Comments

8. What command is used to show the state of adjacencies?

The command to show adjancencies is Show ip ospf neighbor

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 7

Published

Deon Botha

on June 22, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, OSPF and Show

. 0 Comments

7. Explain Briefly what the show ip ospf database will reveal?

The show ip ospf database command displays the routers topological database with the different Link State Advertisements (LSAs) that have populated the database. Internal Routers will only display router and network LSAs.

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 6

Published

Deon Botha

on June 18, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, OSPF and Show

. 0 Comments

6. What command shows which router on a Local Area Network (LAN) is the Backup Designated Router (BDR)?

Displays neighbour information. Used to verify that all neighbours are present.

Router_2#show ip ospf neighbor

Gives information about how OSPF has been configured on each interface.

Router_2#show ip ospf interface

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 5

Published

Deon Botha

on June 17, 2009

in BSCI, BSCI Questions, Certification, Cisco Systems, Concepts and Constructs, Cost and OSPF

. 0 Comments

5. The Metric used by OSPF is cost. How would you change the metric on an interface?

OSPF cost is a 16-bit variable (0 – 65,535) where lower variables are preferred. There are two methods one preferred (at least by me) and one easier.

I prefer the “automatic” method because it is not hard coded into IOS (SET) that means when dynamic changes occur on the network a change is needed inside IOS to change the costs again.

This method is the one where you change the automatic cost calculation equation to take into account faster links by fiddling with the numerator (add a couple of zeros).

Router_2(config-if)#up ospf auto-cost reference-bandwidth [reference-bandwidth]

Second method is more straightforward and directly onto interfaces (hard code a value)

Router_2(config-if)#ip ospf cost [cost]

Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

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Tag Archive for 'Certification'

Open Shortest Path First – OSPF Fundamentals – Scenario

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 13

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 12

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 11

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 10

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 9

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 8

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 7

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 6

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 5

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