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OSPF Interview Questions - Part 4

How would you speed up OSPF convergence?

  • Configure lower hello and dead interval timers
  • Configure ISPF - incremental SPF --> The principle of ISPF is to update the affected nodes only without rebuilding the whole tree, thereby, resulting in a faster convergence and saving CPU intensive activity of processing.
  • Configure OSPF's SPF throttling - The SPF throttling feature makes it possible to configure SPF scheduling in millisecond intervals and to potentially delay SPF calculations during network instability. Command : "timers throttle spf [spf-start] [spf-hold] [spf-max-wait]

How do you modify the cost of a default route propogated into an area?

Use the command "default-information originate [always] [metric value] [metric-type value] [route-map name]"
where: metric value = numerical cost
metric-type value = Type 1 / Type 2 etc.
route-map name = name of the route-map

Command to create a stub area

area xx stub

Command to create a totally stub area

area xx stub no-summary

How do you modify the cost of the default route generated by the ABR into stub?

The OSPF command "area default-cost" command is configured on an ABR attached to a stub or NSSA

Which OSPF network types do not necessitate the DR/BDR election?

OSPF doesn't need DR/BDR election for point-to-point and point-to-multipoint networks

OSPF matching parameters to form adjacency?

Primary subnet
Hello and dead timers
stub area flag
area ID

Where can you configure summarization in OSPF?

ABR and ASBR routers

Command to perform summarization on ABR?

area [area-id] range [ip-address] [mask] [advertise | not-advertise | cost <>]

Command to perform summarization on ASBR?

summary-address [not-advertise] [tag]

What are virtual links?

A virtual link is used to connect an area to a backbone area in OSPF if these two areas aren't directly connected. The aim is to ensure that every OSPF area is connected to the backbone area either physically or logically (virtual link).

[Alternative of a virtual link : Configure a GRE tunnel between the ABR and the "non-backbone" ABR, and form OSPF peering over the virtual link.]

Troubleshooting based on OSPF neighborship status

  • OSPF neighbors are stuck in 2-way state - They are not able to elect a DR
  • OSPF neighbors are stuck in EXSTART state - They is an MTU mismatch
  • OSPF neighbors are stuck in Init state - There is an issue receiving hello packets from the neighbor
  • OSPF neighbors are stuck in Loading state - A packet might be corrupted, or a router might have a memory issue. Sometimes the neighbors with MTU mismatch might cross the EXSTART state to Loading and then come back to EXSTART state

How can you influence OSPF DR/BDR election?

The interface command " ip ospf priority <> " influences the DR/BDR election. Higher value ensures that the router would be elected as a DR. A value of "0" implies the router wouldn't participate in DR/BDR election process on that broadcast network

Which LSAs are propogated in stub area?

External routes can be reached by a default route

Which LSAs are propogated in totally stub area?

Default route helps to reach other networks outside the area

Which LSAs are propogated in NSSA area?


Can we have an OSPF domain without a backbone area?

In case there is only one area in the OSPF domain, there should be no issues with communication. Issues would arise only if there is an inter area communication required.

The summary of LSA restrictions in different OSPF area types..

StubNo Type 5 External LSA allowed
Totally StubNo Type 3, 4 or 5 LSAs allowed except the default summary route
NSSANo Type 5 External LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed
Totally NSSANo Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed


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