Ans: OSPF is a standardized Link-State routing protocol, designed to scale efficiently to supportlarger networks.
Q2. What is the algorithm used by OSPF?
Ans: OSPF uses SPF (Shortest Path First) algorithm for calculating the best path and preparing OSPF database.
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.
Ans: Neighbor adjacencies will progress through several states, including:
Ans: A neighbor table includes the following information:
Ans: OSPF Router with Priority zero will not participate in DR/BDR election.
Ans: OSPF Routers use 224.0.0.5 multicast IP address. However, in DR/BDR it uses 224.0.0.5 and 224.0.0.6. DR/BDR will use 224.0.0.6 while replying to OSPF routers in Broadcast Multi access network.
Ans: OSPF uses the Dijkstra Shortest Path First algorithm to determine the shortest path and Link-State Advertisements (LSAs).
An example would be Frame Relay.
Ans: The OSPF process builds and maintains three separate tables:
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.
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.
Ans: No election happens in point to point network
Ans: Process id is used similarly to Autonomous system but it has local significance only. It can be different on neighbors running OSPF.
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
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.
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.
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.
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.
Ans: Different OSPF network types with their examples are given below:
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
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.