OSPF Interview Questions & Answer Part-2 | OSPF LSA,s & Special Area Types

Interview Questions:

• What is the name of all LSAs in OSPF?
• Which information are sending in Router LSA?
• Which router sends the Network LSA?
• What is Summary LSA & which router sends this?
• Details, in summary, ASBR LSA? Which router sends this LSA-4?
• The routers in the same area of ASBR will receive LSA-4?
• What is an Autonomous system external LSA?
• What is the difference b/w Type-1(OE1) & Type-2(OE2) External Routes?
• By default external routes type-1 or type-2?
• What is the name of LSA-6?
• What is LSA-7? Which bit is used to determine LSA-7?
• LSA-8 is used for which purpose?
• Opaque LSAs?
• How many types of OSPF Special areas?
• What is a Stub Area? & which LSA are not allowed in this area?
• What is totally stub area & which LSAs are allowed in this?
• NSSA & Totally NSSA area?
• How eternal routes are transmitted to other areas in NSSA?
• If redistribute/import external routes in normal area ASBR, will those routes be learned in NSSA?
• If the above question’s answer is No, then how to solve this Routes issue in NSSA?
• Why ASBR not forward LSA-1 & LSA-2 to other area?

OSPF LSAs Types:

Now let me summarize the answers to the above LSA-related questions.

Router LSA-1 of OSPF contains the connected interface information & this LSA is advertising in same area only. It will not cross to other areas means No inter-area details.

Network LSA-2 is also advertising in the same area of OSPF. But this LSA is advertised by a designated router (DR). All connected link information of all routers in the same Networks are distributed by DR to all DR others.

Summary LSA-3 is used for inter-area communication. This LSA contains details of the local area & advertised to other areas by Area border router (ABR).by this way all nodes in the OSPF domain learn different area routes & reachable to each other.

Summary ASBR LSA-4 is used to find the location of ASBR. This LSA-4 is advertised by ABR not ASBR. The more interesting thing that I mentioned in the above Question as well that all routes in the same area where ASBR is not receive LSA-4 but they received LSA-1 or LSA-2 to find the location of ASBR because they all are in the same area.

External LSA-5 is used by ASBR to advertise the redistributed/imported routes to all other areas in the same domain. For example, if some other protocol i-e RIP, BGP, static, or multiple processes of OSPF in the same router running then in this case we redistribute/import those routes to OSPF, these all routes are advertised in LSA-5.

Multicast LSA-6, before OSPF was tried to use for multicast traffic like nowadays PIM & other protocols are used.in those days LSA-6 was used for multicast services but now no use.

NSSA External LSA-7 is an external LSA using in one special area type NSSA that I will explain later in this post in detail with examples.

External attribute LSA for BGP LSA-8 when we import/redistribute BGP into OSPF on ASBR router, then during this process BGP attributes/features are lost. To maintain & carry BGP attributes in OSPF Domain LSA-8 is used.

Opaque LSA-9/10/11. These LSAs carry some extra feature information in MPLS TE & the latest technology Segment Routing (SR) & SRV6.I will explain this in a later MPLS/SR topic.

Next post  I will continue to explain OSPF Special area types.

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