OSPF-3

Question 1

What is the default redistribution OSPF cost?

Answer 1

Redistribution into OSPF uses the following defaults:

1. BGP, use a default metric of 1.
2. All other IGP sources use a default metric of 20.

Question 2

Explain OSPF Virtual Link?

Answer 2

OSPF requires using a backbone area (area 0), with each area connecting to area 0 through an ABR. However, in some cases, the regular area might not have a convenient point of connection to the backbone area. In this case, OSPF uses the virtual link to connect that regular area to the backbone area virtually.

Question 3

Explain OSPF Stub Area and different types of Stub Areas.

Answer 3

• Stub Area: Filter external routes (EIGRP/BGP) and a default route is injected into a stub area by ABR.
• Totally Stubby Area: Routers within the Totally Stubby Area receive only a single default route to reach external destinations, filter all other updates.
• Not-So-Stubby Areas: NSSA permits the injection of external routes, such as routes redistributed from other routing protocols or static routes, into the area while still summarizing them into a default route.
• Totally NSSA: In a Totally NSSA, external routes, including those redistributed from other routing protocols or static routes, are allowed to enter the area, but they are summarized into a default route.

Question 4

How does OSPF calculate its metric or cost?

Answer 4

OSPF uses Cost as its metric. The formula to calculate the OSPF cost is:

reference bandwidth / interface bandwidth.