Section 2 OSPF

Question 1

What does ‘2-WAY/DROTHER’ indicate in OSPF routers?

Answer 1

Routers have seen each other’s Hello packets, but neither is the DR/BDR.

Question 2

What are the default OSPF Hello and Dead timers?

Answer 2

10 seconds for Hello and 40 seconds for Dead.

Question 3

How can OSPF timers be decreased for faster convergence?

Answer 3

Reduce Hello/Dead timers to faster values, e.g., 1/3 seconds.

Question 4

Why authenticate OSPF?

Answer 4

To prevent unauthorized updates.

Question 5

What are the two types of OSPF authentication?

Answer 5

• Simple authentication (passwords)
• MD5 authentication (more secure)

Question 6

What are the different types of LSA in Multi-Area OSPF?

Answer 6

• Type 1: Router LSAs
• Type 2: Network LSAs
• Type 3: Summary LSAs
• Type 4: ASBR Summary LSAs
• Type 5: External LSAs
• Type 7: NSSA External LSAs

Question 7

What connects all other areas in OSPF’s two-layer hierarchy?

Answer 7

Backbone Area (Area 0).

Question 8

What are the benefits of hierarchical routing?

Answer 8

• Scalability
• Faster convergence
• Reduced routing table size
• Better LSA control

Question 9

What is BGP’s primary role?

Answer 9

To exchange routing info between autonomous systems (ASes).

Question 10

What is the main difference between BGP and IGPs?

Answer 10

• BGP: Inter-domain, path vector, scalable, slower convergence.
• IGPs: Intra-domain, faster, less scalable.

Question 11

What distinguishes iBGP from eBGP?

Answer 11

• iBGP: Within same AS; requires full mesh or route reflectors.
• eBGP: Between ASes; used for Internet routing.

Question 12

What is the function of the AS-path in BGP?

Answer 12

To prevent loops and make policy-based decisions.

Question 13

What are the characteristics of IGPs and EGPs?

Answer 13

• IGPs: OSPF, EIGRP – used within an AS.
• EGPs: BGP – used between ASes.

Question 14

What are the types of OSPF routers?

Answer 14

• Internal: All interfaces in same area.
• Backbone: At least one interface in Area 0.
• Area Border Router (ABR): Connects multiple areas.
• Autonomous System Boundary Router (ASBR): Connects to other ASes.

Question 15

What could R3 and R6 be classified as in a router diagram?

Answer 15

Area Border Routers (ABRs) if linked to Area 0 and another area.

Question 16

What multicast addresses does OSPF use?

Answer 16

224.0.0.5/6 for Hello and updates.

Question 17

What are common reasons for OSPF adjacency failure?

Answer 17

• Mismatched OSPF timers
• Wrong area ID or subnet
• Authentication mismatch
• Passive interface configured
• Interface down

Question 18

What happens to the Router ID after a link failure?

Answer 18

It may be recalculated if tied to an interface IP that went down.

Question 19

What is an Autonomous System?

Answer 19

A collection of IP networks under a common administration, identified by an ASN.

Question 20

What does an O*E2 route indicate?

Answer 20

OSPF external Type 2 route; metric is not incremented across OSPF domain.

Question 21

What questions should you ask your ISP?

Answer 21

• What is the IP address range?
• What is the ASN?
• Is BGP supported?
• What are the SLAs?
• Redundancy/failover options?
• Do they support IPv6?

Question 22

When should BGP not be used?

Answer 22

• Small networks with single ISP.
• No need for policy-based routing.
• Lack of skilled admins.
• OSPF or static routing is sufficient.

Question 23

What is BGP multihoming?

Answer 23

Connecting to two or more ISPs for redundancy/load balancing.

Question 24

What factors are considered in BGP path selection?

Answer 24

• Highest weight (Cisco-specific)
• Highest local preference
• Shortest AS path
• Lowest origin type
• Lowest MED
• eBGP > iBGP
• Oldest route
• Lowest router ID

Question 25

What does BGP as path-vector entail?

Answer 25

Routes contain path attributes, including full AS path history to prevent loops and allow policy control.