OSPF-1

Question 1

What is the OSPF Routing Protocol?

• Is OSPF a Link State or path-vector routing protocol?
• How does it calculate the shortest paths?
• What is the AD?
• What is the metric?
• What is the Transport?
• What are the IPs for OSPF multicast?

Answer 1

• OSPF is an Link State routing protocol.
• OSPF uses the Dijkstra algorithm to construct the shortest paths.
• OSPF AD: 110
• OSPF Metric: Cost (Bandwidth)
• OSPF Transport: IP/89
• OSPF Multicast: 224.0.0.5 & 224.0.0.6

Question 2

What are the steps required to change OSPF neighbors into full adjacency?

Answer 2

Below are the steps required to ensure that an OSPF neighbor transitions into a full adjacency.

• Physical & IP Connectivity: Interfaces must be up, in the same subnet.
• OSPF Process is Enabled: Both routers must run OSPF on the interface.
• Hello Packets are Exchanged: Hello messages are sent every 10s (broadcast) or 30s (NBMA).
• Elect a DR/BDR: Designated Router (DR) and Backup DR (BDR) are selected on broadcast networks.
• Exchange LSAs: Routers exchange LSAs to synchronize databases.
• OSPF Adjacency Forms: OSPF reaches the FULL state.

Question 3

Explain LSA (Link-State Advertisement), LSU (Link State Update), and LSR (Link State Request).

Answer 3

OSPF routers communicate using LSAs, LSUs, and LSRs to synchronize their Link-State Databases (LSDBs).

• LSA: Contain topology information about a router’s links and network.
• LSU: Used to synchronize the LSDB among OSPF routers.
• LSR: Sent when a router detects missing or outdated information in its LSDB.

Question 4

Explain OSPF Router ID?

Answer 4

The Router ID is used to identify the Router.

• The OSPF Router ID can be manually set.
• The OSPF Router ID can be The highest IP address of the router’s loopback interfaces.
• If no loopback is present, then the highest IP address of the router’s physical interfaces will be chosen as the Router ID.

R1(config)# router ospf 5
R1(config-router)# router-id 5.5.5.5

Question 5

What is the difference between an OSPF neighbor and an adjacent neighbor?

Answer 5

LSAs are exchanged only among adjacent routers, not among neighbor routers.