OSPF Frequently Asked Questions

Cisco Interview Questions, OSPF Frequently Asked Questions (FAQ)

  1. What is OSPF? Why and on what environment do we implement this?

Ans:- OSPF is open shortest path first. It’s a link state protocol and hierarchical in nature. It uses area to separate the network and control the excessive traffic. OSPF is generally implemented in big companies with many small subsidiaries recently merged. It’s a kind of not so easy protocol to manage.

  1. Why it’s known as Link state protocol? How it’s different than EIGRP? What algorithm does it use?

Ans:- OSPF is a link state protocol because it keeps database of the complete topology and has information about the links. OSPF uses SPF algorithm while EIGRP uses DUAL and its hybrid in nature.

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  1. What is OSPF’s metric? How is it identified in routing table? Does it support redistribution?

Ans:- It’s metric is Cost. Formula is (100000000/Bandwidth in bps). It is identified as keyword “O”. every routing protocol does support redistribution.

  1. What are the states used in OSPF when forming neighborship?

Ans:-   DOWN state – when neighborship is down initially

            INIT – Both neighbors send hello packets to each other

            2-way – Both will reply or acknowledge to each other Hello and share their router-id . Just similar to we shake handshakes and introduce ourselves. DR/BDR election also happens in  this state. Neighborship is formed now

            Exstart – Each neighbor will want to be Master so that it does not have to start first to share the information. It will be decided on the basis of Highest Router-ID.

            Exchange – Master will receive the database descriptor (DBD) information in the form of LSU ( Link state updates). Once done, Master will share its database table with neighbor.

            Loading – If there is better information, either side will make request using LSR ( Link state Request) and reply with come in the form of LSA( Link state advertisement). LSAck will be sent for acknowledgement.

             Full – OSPF database is synced.

  1. What are the packet types does OSPF use? Explain Hello, Dead Timers, LSA, LSU, LSR packets.

Ans:- Hello is a handshake packet tells about the alive status of OSPF interface.

Dead is the time it will wait after which it will break the neighborship and inform routing table for removing the route or connected route.

LSA- Link state advertisement.  It contains the information of the network  or update.

LSU – It contains multiple updates at one go.

LSR – It’s a request packet when router running OSPF needs better and updated information.

  1. What will be Hello/Dead timers in network having link capacity of >T1 and <=T1?

Ans:- For link above T1 i.e. 1.544Mbps Hello = 10 seconds, Dead is 40 seconds

For link = or below Hello = 30 seconds, Dead = 120 Seconds.

Dead interval is exact four times than Hello and its value can’t be set manually.

  1. What is the algorithm used by OSPF?

Ans:- OSPF uses SPF (Shortest Path First) algorithm for calculating the best path and preparing OSPF database.

  1. What is OSPF Router ID and how is it elected?

Ans:-  Router id is 32 bit number similar to IP address. It is elected as highest IP Address of Loopback interfaces or Highest IP of the active physical interfaces if loopback is not present. It can also be set manually but it will take effect when OSPF process is reset completely or device is reloaded. There should be only one router-id per device running ospf to avoid database issues.

  1. What is the DR/BDR in OSPF? How these are elected and in what scenario?

Ans:- DR/BDR election happens in Shared Multi access network to avoid full mesh scenario. In shared environment, every OSPF router will try to form neighborship with other router, so DR/BDR comes in to action and act as single point of contact. Neighborship will only form with DR and Backup DR. DR router will be the highest priority router and BDR will be second highest priority. If there is a tie, then Highest router-id will be used to make the decision. Router ID is elected as the highest numerical Loopback IP or the highest Physical Active IP address or the Interface IP which comes up first.

  1. What will happen if we make priority of a OSPF router as “zero”?

Ans:- OSPF Router with Priority zero will not participate in DR/BDR election.

  1. What are the multicast IPs used by OSPF? What multicast IP does DR/BDR router and non-DR use?

Ans:- OSPF Routers use multicast IP address. However, in DR/BDR it uses and DR/BDR will use while replying to OSPF routers in Broadcast Multi access network.

  1. Does DR/BDR election happen in OSPF Point to Point Network

Ans:- No election happens in point to point network

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  1. What problems can cause OSPF neighborship in “down” state?

Ans:-  Steps in forming neighborship in OSPF is almost similar to EIGRP’s. Two ospf devices should be connected back to back with cables connected on correct interfaces. IP and mask should be correct. Then run the ospf process if not run already and advertise the interface segments on both routers. Hello, Dead timers and MTU size should be same at both ends. There should not be any access-list or transparent firewall blocking the OSPF Packets or IP addresses. Neighborship should establish now. Area number and authentication parameters must match. If issues are still there, then check the router logs and try to analyze the problem.

  1. What is “Process id”? Does it have local or global significance? Does it need to be same on OSPF neighbor routers?

Ans:- Process id is used similarly to Autonomous system but it has local significance only. It can be different on neighbors running OSPF.

  1. What is the concept of “Area” in OSPF? What is Backbone area, stub area, Totally Stubby Area, Not So Stuby area (NSSA)?

Ans:- OSPF is hierarchical in nature and therefor it uses areas to control and manage its LSA or database traffic. The concept of area brings stability in the environment and suppresses the unnecessary convergence updates within the area unless meant for other areas. Backbone area is “area 0” – this backbone area is must be physically or logically connected to other areas to communicate. No two areas can share information if one of the areas is not backbone area. Stub area blocks the LSA 4 and 5 i.e. External information which is sent by ABR ( Area Border Router)- It does so to reduce the external traffic. However, connectivity can happen using Default route injection done by ABR as an alternate way.  Any router wants to send packets external network from stub area will have default route in its routing table.

Totally Stubby Area – It blocks external LSA and summary LSA i.e. LSA 3, 4 and 5. Default route will be injected by ABR to routers of area.

Not So Stubby area –

  1. Can area 1 and area 2 exchange or advertise networks if they are not connected to area 0 (Backbone area)? What we need to do make it happen?

Ans:- Every area should be connected to area 0 else there won’t be exchange of inter area routes. If it is not possible then use the concept of virtual link

  1. What is the concept of Virtual-link in OSPF? Can virtual link be formed thru stub area?

Ans:- Per OSPF Rule, Inter area communication will happen when atleast one area should be area 0 or backbone area. If this is not possible, then we can use virtual-link to form a logical association. Virtual link is not possible thru Stub area.

  1. Explain LSA and its all Types such as LSA 1, 2, 3,4,5,7. When do we use LSA type 6

Ans:-  LSA Type 1 :- Every OSPF router within the area speaks this LSA.

LSA Type 2 :- This LSA will be used in Shared multi-access network in DR/BDR election.

LSA Type 3 :- This LSA helps in exchange of inter area summary information. It will be used by ABR( Area Border Router)

LSA Type 4 :- This LSA is meant for external communication i.e. with other protocols. This LSA gets the information about position of ASBR.

LSA Type 5 :- This will be generated by ASBR and helps in exchange of external information of routes.

LSA Type 7:- It is used in special area i.e. NSSA Not So Stubby Area. It gets converted in to Type 5 by ABR.

OSPF Frequently Asked Questions

  1. What LSA is used for inter area communication? What LSA type is used for inter protocol communication in OSPF?

Ans:-  Type 3 LSA Is used for inter area communication. For communication with other protocols or external routes , type 4 and 5 will be used.

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  1. Explain the concept of Area Border Router (ABR) and Autonomous System Border Router (ASBR). Can router sitting between area 2 and 3 be called as ABR?

Ans:- ABR is the router which sits between Backbone area i.e area 0 and non Backbone area. ASBR is the router which sits between OSPF routing protocol and non-OSPF routing protocol. A Router in OSPF can only be called ABR when it has atleast one interface in area 0. Router between area 2 and 3 can not be known as ABR.

  1. Which area blocks external route? Which area blocks Summarization?

Ans:-  Totally Stubby area blocks external LSAs i.e. Type 4 and 5 and summarization i.e. type 3. Stub area also blocks Type 4 and 5 LSA

  1. What is the concept of “auto cost reference” and in what scenarios we should use it?
  2. How OSPF inter area, external routes are denoted or represented in routing table?

Ans:-  Inter area as O IA , External as O E1 or E2.

  1. What is O E1, O E2 letters in OSPF? Which is default external type 1 or external type 2?
  2. Where does the summarization happen in OSPF? Which command is used for summarization at ABR and at ASBR?

Ans:- Summarization happens at Boundaries in OSPF i.e. either at ABR ( Boundary of two areas with one area as Backbone) OR at ASBR ( Autonomous System Border Router). ABR uses “ area range” command while ASBR uses “summary-address” command.

OSPF Frequently Asked Questions


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