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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 10

Published

Deon Botha

on June 24, 2009

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

. 2 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.

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 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.

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 4

Published

Deon Botha

on June 17, 2009

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

. 2 Comments

4. The address 192.100.56.10/21 has been allocated to an interface on the router. This interface alone is to be included in the OSPF process. State the command that would start the process on this interface.

To include this interface alone into the OSPF process

Router_2(config-network)#network 192.168.56.10 0.0.0.0 area number

According to the book (I don’t understand)

Router_2(config-network)#network 192.168.56.8 0.0.0.7 area number

I would have done it like below if I was going to include the entire range, because you are using the /21 which has 2048 nodes/hosts per network ( I hate it when I start doubting myself?!?!)

Router_2(config-network)#network 192.168.56.0 0.0.7.255 area number

Which coincidentally is also in the book (I’m totally confused but moving swiftly along).

The last method is allowing the /24 mask??? why I don’t know (in the book).

Router_2(config-network)#network 192.168.56.0 0.0.0.255 area number

I suppose all of the answers including the first one allows the address which was the goal. Not that allowing the /24 mask or the weird small portion of hosts make sense to me. (why? and where the logic comes from is making me scratch my head).

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 3

Published

Deon Botha

on June 17, 2009

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

. 0 Comments

3. Is it possible to have more than one OSPF process on a router. How would you do this?

Yes it is possible to have more than one OSPF process on a router.

You may want to do this to create two different OSPF domains / areas with separate routing policies.

To accomplish this simply specify two (or more) OSPF processes in the following way:

Router_2(config)#router ospf 1 Router_2(config-router)#network [ip address] [mask] area [number] Router_2(config-router)#exit Router_2(config)#router ospf 2 Router_2(config-router)#network [ip address] [mask] area [number]

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 2

Published

Deon Botha

on June 16, 2009

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

. 0 Comments

2. What Parameter is used to calculate the metric of a link in OSPF on a Cisco Router?

The OSPF metric used to calculate link speed is 100,000,000 divided by the bandwidth of the interface in bits per second.

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 1

Published

Deon Botha

on June 15, 2009

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

. 0 Comments

1. What command is used to manually determine which router on a Local Area Network (LAN) will become the Designated Router (DR)?

The hello message includes a priority field which provides a mechanism to elect a Designated router (DR) and Backup Designated Router (BDR). To be eligible for election the value must be a positive integer between 1 and 255. A priority of 0 (zero) means the router cannot participate in the election process.

The highest priority wins the election process. All Cisco routers have a default priority of 1 (one), the highest Router ID is used as the tiebreaker when no manual adjustment is made.

The command to adjust priority on an interface-by-interface method is:

Router_2(config-if)#ip ospf priority number

In summation the designated router can be determined using the priority command.

Resources:

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

Open Shortest Path First – OSPF Fundamentals – Checking or Troubleshooting OSPF Troubleshooting

Published

Deon Botha

on June 12, 2009

in BSCI, BSCI Notes, Concepts and Constructs, Debug and Show

. 0 Comments

I’m using a Simulator and sometimes output differs from what it should be (which is happening more and more often). Make the best out of the explanations when things differ (I’ve used verbatim examples where the simulator just came up blank with output).

All the below is to test if OSPF is functioning properly and has been configured correctly.

Base the output (for examples that worked) on the show commands on the below OSPF Configuration.

Working from a functional configuration on a single router, Router_1 will be the DR on Fe1/0 unless another device on that segment has a priority greater than 100. The link attached to Fe1/0 has a cost of 1. The cost on Fe2/0 has been changed to 10.

The Config

Router_1(config)#router ospf 100 Router_1(config-router)#network 192.168.0.0 0.0.255.255 area 3 Router_1(config-router)#exit Router_1(config)#interface Ethernet 0/0 Router_1(config-if)#ip address 192.168.16.1 255.255.255.240 Router_1(config-if)#ip ospf priority 100 Router_1(config-if)#exit Router_1(config)#interface Ethernet 0/1 Router_1(config-if)#ip address 192.168.16.15 255.255.255.240 Router_1(config-if)#ip ospf priority 20 Router_1(config-if)#exit Router_1(config)#interface Ethernet 0/2 Router_1(config-if)#ip address 192.168.16.30 255.255.255.240 Router_1(config-if)#ip ospf priority 15 Router_1(config-if)#exit Router_1(config)#interface Ethernet 0/3 Router_1(config-if)#ip address 192.168.16.17 255.255.255.240 Router_1(config-if)#ip ospf cost 10

Checking the Configuration

As we’ve been through before in previous posts, the show commands are detailed and comprehensive views on the health and status of the network and hardware (there are a gazillion of them and the detail can be overwhelming same with debug commands). To understand the output from the show commands read-on.

The show-ip ospf Command

The show-ip ospf command shows how OSPF is running on a given router. Output includes the number of times that the SPF routing algorithm has run (indicates the stability of the network). From the previous posts the SPF routing Algorithm runs when there is “instability” on the network (the higher the number of “recalculations” the less stable the network).

Router_2#show ip ospf [process-id]

Output

Explanation

The show ip ospf Database Command

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

Router_2#show ip ospf database

Output

Explanation

Show ip ospf interface Command

The show ip ospf interface command shows how OSPF has been configured and how it is working on an interface. This level of detail is excellent to troubleshoot config errors.

Router_2#show ip ospf interface [type number]

The command shows information such as the Designated Router (DR) and Backup Designated Router (BDR), a list of neighbours, and the network type.

Output

Explanation

There are some things that don’t come up once again on my output. This would be because I am using a simulator when studying and not “real” kit. You can’t really expect the lab to do “Everything” you want but it does a good job of giving you the basic ideas. Use your imagination.

From here on in things are verbatim from the book. The deviations on the Simulator is so GREAT from what it should be (a.k.a a blank output) that I really can’t study from that).

Show ip ospf neighbour Command

The show ip ospf neighbour command shows OSPF neighbours (known neighbours can be viewed using this command).

Router_2#show ip ospf neighbor

Output

The command can be made more granular and the neighbours can be viewed in a per-interface method

The command can once again be expanded further to show a deep-dive per interface view in as much detail as possible. Use the command displayed below.

Router_2#show ip ospf neighbor {type number} {neighbour id} [detail]

Output

Explanation

Show ip protocols Command

The show ip protocols command shows the configuration of IP routing protocols configured on the router. The command brings up how protocols were configured and how they interact with one another (updates, interactions, etc). Great for troubleshooting configuration errors and understanding how the network is communicating about routes

Router_2#show ip protocols

Output

Explanation

Show ip route Command

The show ip route command shows the IP routing table on the router. This particular command shows how the network is known to the router and how the router discovered routes. Most of us know about this one and would have used it many times before. I know I have.

Router_2#show ip route

Debug Commands

A rather dangerous command is debug (this is because it can make a router totally freak out). This is due to the fact that the debug command has the highest process priority and can consume all resources on the router causing the router to freeze up and need a power cycle.

Good practice would be to turn on debug commands for a specific function and then turn off that debug command as soon as the needed information has been gathered.

To turn of all debug commands that could be active on the router:

Router_2#no debug all

The particular commands with relevance to OSPF:

Router_2#debug ip ospf events

This command displays information about OSPF-related events, such as adjacency, flooding information, designated router selection, and SPF calculation.

Router_2#debug ip packet

This command is IP debugging and includes packets received, generated, and forwarded. Fast-Switched packets do not generate messages.

If I added some value to your Cisco Experience with this post please add some value to my studies and leave a comment, question, suggestion, note of thanks or encouragement for me to hurry up and complete my certifications. My reasoning for wanting some interaction is that the last Recruiter said I need CCNP, Juniper and a Specialization track. The LOOOOONG Road to Cisco Indeed. Thanks Deon

Network Ninja

Tag Archive for 'Router'

Open Shortest Path First – OSPF Fundamentals – Scenario

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

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

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 4

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 3

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 2

Open Shortest Path First – OSPF Fundamentals – Questions and Answers – Question 1

Open Shortest Path First – OSPF Fundamentals – Checking or Troubleshooting OSPF Troubleshooting

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