OSPF FAQ - Frequently Asked Questions

In this post, some of the frequently asked OSPF questions will be answered. Some of the answers will be from a design point of view and we will try to provide enough justification for the answer. Questions are selected randomly, not based on the order of importance.


How many routers should be placed in one OSPF area?

We don't give any numerical number as an answer to this question. Depending on the number of routers, links, prefixes, and the topology of the network also depends on the hardware capabilities and the performance of the routers, the number change from Network to network. In some networks, you can place only a couple of hundreds, and in some networks, you can place thousands of Routers in a single OSPF Area.


What command would you use to only see the OSPF routes in the routing table?

On Cisco devices, sh ip route ospf is used to see only the OSPF routes in the routing table.


What will an OSPF router prefer to use first as a router-id?

The first preference for an OSPF router ID is an explicitly configured 32-bit address. This address is not included in the routing table and is not defined by the network command. If it is not configured manually, the highest loopback IP address is used.


In which OSI model layer will you find the OSPF protocol?

OSPF is an IP-based protocol. Not like IS-IS or BGP. IS-IS is Layer protocol and BGP runs on top of Layer 4, TCP protocol. OSPF is Layer 3, IP Based protocol based on OSI Layer.


What is one reason to use the ip ospf priority command when the OSPF routing protocol is in use?

In DR and BDR selection in OSPF, a priority command is used for the election process. A higher priority router is selected as a DR. If the purpose is not to allow one of the routers in the election process, then ip ospf priority command is used.


With multiarea OSPF, which OSPF router type has all interfaces in the same area?

In Multi-area OSPF, which is also commonly known as Hierarchical OSPF design, Internal routers have all interfaces in the same area. ABR Routers need to have more than one Area, and one of the OSPF Areas, has to be OSPF Area 0, OSPF Backbone Area.


Why would you want to change the OSPF default reference bandwidth?

By default, the reference bandwidth is 100 Mbps. However, if you have faster links in your network, such as gigabit Ethernet or higher speed, OSPF can't give these links a better cost than 1. So you should set the reference bandwidth to at least as high as the fastest link in your network. Overall you can manipulate the cost, so you can do better Traffic Engineering by using OSPF cost.


What is the format of the router id on an OSPF-enabled router?

OSPF router ID is a 32-bit number formatted like an IPv4 address and assigned in order to uniquely identify a router.


What is a benefit of multiarea OSPF routing?

Multi-Area OSPF routing is done for scalability purposes. When routers are in place in different OSPF Areas, the purpose is to be able to send only summary routes or default route into the OSPF Non-Bachone Areas. Multiarea OSPF Routing is known as Hierarchical OSPF Routing and it is done to grow a number of routers and the prefixes in the OSPF network.


What function is performed by the OSPF designated router?

OSPF Designated Router also provides scalability. When it is used, routers don't have a full-mesh adjacency with each other in OSPF networks. Thus it reduces the number of control plane exchanges and LSA advertisements. Reducing flooding in case of newly added networks or in case of failure allows scalability. If there is a p2p network, we should avoid having OSPF DR.


Which command will provide information specific to ospfv3 routes in the routing table?

show ipv6 route ospf command provides information specific to ospfv3 routes only.


Which command is used to display the collection of OSPF link states?

show ip ospf database provides all the entries in the OSPF Link state database.


What is a characteristic of a single-area OSPF network?

Single Area OSPF network is by default placing routers in a single OSPF Area. It can be OSPF Backbone Area, which is OSPF Area 0. It is the most common deployment model. Or, if all the routers are placed in an OSPF Non-backbone area, which is any number other than Area 0, it is also called Single Area OSPF Routing design. Single-Area OSPF is known as a Flat OSPF network as well.


What does an OSPF area contain?

The OSPF area contains OSPF internal routers. Their connections, which mean links, and through these links, OSPF neighborship allows OSPF Routers to exchange IP prefixes. In a given OSPF Area, every router has the same Link state database.


What command will display all connected ospfv2 routers?

With show ip ospf neighbor command you can view the connected OSPFv2 routes.


How to configure OSPF on the Cisco router?

To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:
  1. Use the command router ospf process ID to start OSPF.
  2. Use the network command to enable the interfaces.
  3. Identify OSPF area assignments.
  4. (Optional) Assign the router ID.


Which type of OSPF router will generate type 3 lsas?

OSPF Type3 LSA is generated by the OSPF ABR to summarize OSPF Type 1 and OSPF Type2 LSA. Because between the OSPF Areas, Type 1 and Type 2 LSAs cannot be sent, Type 3 LSA is generated by the ABR to teach a given OSPF Area prefixes to another.


What is the easiest and preferred method of manipulating OSPF route costs?

OSPF uses cost to calculate the metric, based on the bandwidth of the link. Higher bandwidth means lower cost, and it is preferred over higher cost. So increasing bandwidth is one way to influence/manipulate the path in OSPF.


What does the Cisco IOS use to calculate the OSPF cost to a destination network?

Cisco and many other vendor Routers use Bandwidth to calculate OSPF cost to a destination network. Although cost can be manipulated manually and manual cost can be assigned based on other criteria such as delay/latency, fiber route mile, monetary cost, and so on, many networks in real-life OSPF design, keep the default cost parameter, which is bandwidth.


In OSPF, hellos are sent to what IP address?

OSPF neighbors are dynamically discovered by sending Hello packets out to each OSPF-enabled interface on a router. Hello packets are sent to the multicast IP address of 224.0.0.5.


Which command can be used to verify the contents of the lsdb in an OSPF area?

show ip ospf database is used to verify all the contents of the OSPF link-state database. We have two different databases in OSPF. One of them is LSDB, a link-state database and another is TED, a Traffic Engineering Database.


Which type of OSPF LSA is generated by an ASBR and includes external routes?

Type5 LSA is generated in OSPF by the ASBR and sent into the network. Single-Area OSPF and Multi-Area OSPF networks Type5 LSA is sent everywhere. Except, if the Area is Stub, Totally Stub, NSSA, or Totally NSSA, then Type 5 LSAs are not allowed into those OSPF Area Types.


What are two reasons for creating an OSPF network with multiple areas?

The main reason we create Multiple Areas in OSPF is Scalability. Scalability means allowing for growth. So, the number of Routers, links, and prefixes, in time, can grow and our network design shouldn't stop growing. When Multi-area OSPF is designed, into the Non-backbone areas topology information is not sent, thus it provides more stability in those areas. Also, convergence time gets faster and troubleshooting becomes easier in Multi-area network design.


What default length is used for the OSPF dead interval?

The default values are 10 seconds for the hello time, and 40 seconds for the dead time. The usual rule of thumb with OSPF is to keep the dead time value four times the hello interval. If OSPF fast convergence is the requirement, then these values can be reduced to provide fast failure detection. BFD is another method for fast failure detection.


What are the enhancements that ospfv3 supports over ospfv2?

In OSPFv3, similar to IS-IS protocol, reachability information and topology information are separated. So. when we add a new loopback interface (not topology change), in OSPFv2, it triggers full-SPF calculation, which is a resource-intensive task. Also, OSPFv3 supports both IPv6 and IPv4, but OSPFv2 only supports IPv4.
Created by
Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

Today, with his carefully selected instructors, OrhanErgun.Net is providing IT courses to tens of thousands of IT engineers. 

View profile

Daniel Lardeux
Daniel Lardeux Senior Network Consultant at Post Telecom

I passed the CCDE Practical exam and Orhan’s CCDE course was very important contributor to my success. I attended the CCDE course of Orhan Ergun in July and it was exactly what I needed, Orhan is taking the pain to break down the different technologies.

Roy Lexmond
Roy Lexmond Senior Network Designer at Routz CCDE #20150017 & CCIE R&S; #26557

After I attended Orhan Ergun’s CCDE course I passed the CCDE practical exam.I really enjoyed the course a lot ...

Nicholas Russo
Nicholas Russo Network Consulting Engineer (CCDE/CCIEx2), Cisc

I signed up for Orhan’s CCDE training. This training is very technically detailed and the use-cases, quizzes, scenarios, and mind maps are all great resources in the overall training program. Orhan teaches his students to think like a network designer ...

Slide Heading
Slide Heading Network Systems Engineer at Conscia A/S CCIE #42544 (SP) & CCDE #20160015

Orhan is forcing you to take off the implementation hat that most of us have been wearing for many years, instead he is providing a new fancy design hat, which makes you see and deal with the issues presented ...

Kim Pedersen
Kim Pedersen CCIE in RS and SP (#29189) CCDE#20170021

I’ve used Orhan’s self-paced CCDE training material. If you are interested in knowing how all the technologies go together in a coherent design i can highly recommend it.I also enjoyed the Quizzes which helped pick out my weak spots in selecting ...

Laurent Metzger
Laurent Metzger 3xCCIE/CCDE Senior Network Architect

Hi Orhan. I passed the CCDE exam on February 22. I read everything that you put on your Self Paced CCDE Training course and it was very helpful in my success. Thank you very much.

Martin J. Duggan
Martin J. Duggan Network Architect at AT&T;, Ciscopress Author CCDE #20160006 & CCIE#7942

I attended Orhan’s April 201610 days CCDE Bootcamp. I am CCDE now !

You can tell Orhan has a great deal of experience, it really comes through when he presents his design case studies and the CCDE Practical scenarios.

Muhammad Abubakar
Muhammad Abubakar Lead Network Architect – CCDE #20160016 2xCCIE #26693 2xJNCIE VCIX

Your excellent CCDE materials and amazing Bootcamp helped me tremendously through my learning journey.Also thank you very much for being available whenever I have a design question or a complex design topics. I can’t compare your design skills ...

Jennifer Pai
Jennifer Pai Network/Security Engineer at KNET Technology

Thanks Orhan very much for this course. It helped strengthen my “Network design mind”.

Ruslan Silyayev
Ruslan Silyayev Solution Architect at R.I.S.K Company

Training by Orhan is not a CCDE preperation training only. It will be useful for engineers which are dealing with design. You want to pass CCDE exam or learn network design, then don’t look at anywhere else!

Sameer Meher
Sameer Meher Solutions Architect at 23 Wards/Japan

Orhan Ergun’s CCDE course was really very good. CCDE Level Intelligence was delivered very well and with very useful case studies and the scenarios, I am thankful to Orhan for all his help!

Ken Young
Ken Young Senior Technical Architect Province of Nova Scotia, 2xCCIE #41597 | CCDE #20170047

If anyone wants to understand network design and architecture, also pass CCDE exam , I recommend you to attend Orhan’s online courses! I am a CCDE now but learning is a journey, we will be together in your other courses too Orhan!

Matt Cross
Matt Cross Technical Architect at Heartland – CCDE #2019::7

Orhan did an excellent job of filling in the gaps of knowledge that I had that took me to the finish line of the practical exam CCDE. The community of people that Orhan facilitates are both engaging and supportive of the journey to CCDE. Orhan ...

Shiling Ding
Shiling Ding Sentinel Technologies – CCDE #2019::12

Just passed the CCDE Practical exam! I attended Orhan Ergun’s CCDE training program , used Orhan’s Instructor Led and Self-Paced CCDE training and Online CCDE Practical Scenarios during my CCDE journey. Orhan’s CCDE In Depth book is an excellent summary ...

Abelardo Basurto
Abelardo Basurto Solutions Architect at Cisco Systems – CCDE 2018::6

Hi everyone, I’ve just passed the CCDE Exam. My Number is CCDE 2018::6 I attended to Online CCDE Bootcamp of Orhan. I want to thank Orhan not only for the great book and bootcamp, but also for his commitment, availability and willingness to assist the ...

Hady Mohamed Abdellah
Hady Mohamed Abdellah Network Architect Hamad International Airport Qatar – CCDE 2018::1

Hi guys, I’m so happy that I passed the exam. I’ve already got my number CCDE 2018::1. Thanks to Orhan for being the best CCDE instructor in the world. I highly recomend Orhan’s CCDE Training and In-Depth-CCDE ...

Bryan Bartik
Bryan Bartik Sr. Systems Engineer at CompuNet – CCDE 20170059

Hi Orhan I passed CCDE Practical exam on November 2017 ! I really enjoyed your materials and quizzes and use cases. They were definitely helpful in my preparation. Thanks a lot !

Giedrius Trapkauskas
Giedrius Trapkauskas Network Solutions Architect at Liberty Global – CCDE 20180004

I attended Orhan’s CCDE Training in Istanbul and it was very helpful in my preparation. I passed the exam recently and I want to say Thank you Orhan! For those who want to pass the CCDE exam, definitely start with ...

Alaa Issa
Alaa Issa Sr.Solutions Architect – CCDE#20180033 3xCCIE ( Collab|DC|Security )#27146

I registered to Orhan’s training in Feb 2017. From that time, I attended Orhan’s training several times. The depth of knowledge which Orhan has is amazing, and how to present such consistent knowledge to the ...

Mazin Ahsan Design Lead Engineer | Solutions Engineer | CCDE License # 20160030 | CCIE Licence # 23892

I passed the CCDE Practical Lab exam on November 17,2016 from supplications of elders and dedication from my Sensei Mr. Orhan Ergun I took different CCDE bootcamps in the past. Orhan has the most depth and expertise ...

Jeff Patterson CCDE# 2018::11

Hi Orhan I wanted to pass along my appreciation for the outstanding training material. I used the online CCDE training provided by Orhan as well as the In-Depth-CCDE book and passed the exam in February 2018. Thank you Orhan!

Mehdi Sfar
Mehdi Sfar Network and Security Architect / CCDE #20210003 | CCIE R&S; #51583

I signed up for Orhan’s CCDE Self paced Course. This course, along with the CCDE In Depth book, helped me for my CCDE Practical as well as Written exams. It pushed me to ask the "WHY" questions and allowed ...

Related courses

OSPF Training

06:18:51 Hours
26 Lectures
Intermediate

$50

CCIE Enterprise Infrastructure Training

117:12:56 Hours
287 Lectures
Expert

$995

Routing Protocols Design and Deployment Course

47:00:55 Hours
51 Lectures
Intermediate

$200