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OSPF Interview Questions - Part 3

What are the tables maintained by OSPF?

  • Neighbor Table : This table stores information about all the OSPF neighbors. Can be displayed using the command : show ip ospf neighbor
  • Database / Topology Table : This table stores link state database of all the OSPF learned routes. Can be displayed using the command : show ip ospf database
  • Routing table : After running the SPF algorithm on all the routes in the database table, the best route is selected and promoted to the Routing table. Can be displayed using the command : show ip route ospf

What are the benefits of dividing an OSPF domain into multiple areas?

  • Decrease routing overhead per router (this especially makes sense for internal non-backbone routers i.e. routers inside a single non-backbone area
  • Ensure speedy convergence.. less routes implies faster processing
  • Confine network instability into single area.. the inter-area summary routes and the routes from special areas such as stub, totally stub, NSSA, totally NSSA are usually "hidden" from other areas which implies the disruptions in a single area doesn't translate to the disruptions in other areas.

For cost manipulation in OSPF, can you use offset-list command?

Offset-list doesn't work in case of OSPF. It works only for EIGRP and RIP

What is the OSPF preference order w.r.t. the areas?

  • Intra-area (O) routes are preferred over
  • Inter-area (O IA) routes are preferred over
  • External E1 (O E1) routes are preferred over
  • External E2 (O E2) routes are preferred over
  • NSSA N1 (O N1) routes are preferred over
  • NSSA N2 (O N2) routes

Something about Stub Areas

Type-5 LSA's (External routes) are not allowed. The ABR advertises a default route instead (as a Type 5 LSA). The default route cost from ABR is set using "area x default-cost" command. The only Type 5 LSA allowed in this area is the default route from ABR.

How do you flush Type-5 and Type-7 LSAs?

The "clear ip ospf redistribution" command flushes Type-5 and Type-7 LSAs and checks the routing table for the redistributed routes. The SPF calculation runs on all the OSPF routers in the network which receive these flushed LSAs.

What is the frequency of OSPF Link-State Advertisements?

Link-State Advertisements are sent every 30 minutes by default

How do you prevent an OSPF interface from forming an OSPF adjacency with the OSPF capable neighbor router?

Use "passive-interface " router sub-command to ensure that Hello packets are not sent on that particular interface thereby avoiding OSPF adjacency formation.

In case of multiple Type-5 LSAs to an external route, which route would be preferred?

The LSA with the shortest path (cost) to the ASBR would always be preferred

Can routers with different "process-id"s form adjacency?

Yes. OSPF process-id is the local property of the router and is not a parameter exchanged during the formation of OSPF adjacency

Can you run OSPF over GRE tunnel?

Yes. OSPF can be run over GRE tunnel


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