CCNA OSPF Configuration: Free Practice Questions + Study Guide

What is OSPF?

Open Shortest Path First (OSPF) is a link-state routing protocol used in large enterprise networks. Unlike distance-vector protocols like RIP, OSPF builds a complete map of the network topology (the Link-State Database or LSDB) and uses Dijkstra's Shortest Path First (SPF) algorithm to calculate the best path to every destination.

OSPF is an interior gateway protocol (IGP) defined in RFC 2328. It's the most widely deployed IGP in enterprise networks and a critical topic on the CCNA 200-301 exam.

Key characteristics of OSPF:

• Link-state protocol - routers share link-state advertisements (LSAs) to build a complete topology map
• Uses SPF algorithm - calculates the shortest path tree from the LSDB
• Hierarchical design - uses areas to reduce LSDB size and SPF computation
• Fast convergence - detects changes quickly and recalculates routes
• Classless - supports VLSM and CIDR
• Administrative distance of 110 - lower than IS-IS (115) but higher than EIGRP (90)

OSPF Area Types

OSPF uses a hierarchical design with areas to scale efficiently. All areas must connect to the backbone area (Area 0). Here are the key area types you need to know:

OSPF Neighbor Adjacency & States

Before OSPF routers can exchange routing information, they must form a neighbor adjacency. OSPF neighbors go through several states before becoming fully adjacent:

1. Down - No Hello packets received from neighbor
2. Init - Hello received but own Router ID not seen in neighbor's Hello
3. 2-Way - Bidirectional communication established; DR/BDR election occurs here
4. ExStart - Master/Slave relationship established for DBD exchange
5. Exchange - DBD (Database Description) packets exchanged
6. Loading - LSR/LSU packets exchanged for missing LSAs
7. Full - LSDBs synchronized; routers are fully adjacent

Use show ip ospf neighbor to verify neighbor states. The most common states you'll see in a healthy network are FULL (on point-to-point and DR/BDR relationships) and 2-WAY (between DROther routers on multi-access networks).

DR/BDR Election Process

On multi-access networks (like Ethernet), OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) to reduce the number of adjacencies and LSA flooding.

The election process works as follows:

1. Router with the highest OSPF priority becomes DR (default priority = 1)
2. If priorities are equal, the router with the highest Router ID wins
3. Second highest becomes BDR
4. All other routers become DROther
5. Setting priority to 0 prevents a router from participating in the election

Configure OSPF priority on an interface:

interface GigabitEthernet0/0
 ip ospf priority 200

OSPF Configuration Commands

Here's how to configure basic single-area OSPF on a Cisco router:

! Enable OSPF process (process ID is locally significant)
router ospf 1

! Advertise networks into OSPF Area 0
network 10.1.1.0 0.0.0.255 area 0
network 10.1.2.0 0.0.0.255 area 0

! Set Router ID explicitly (recommended)
router-id 1.1.1.1

! Prevent OSPF on interfaces facing end devices
passive-interface GigabitEthernet0/2

! Set reference bandwidth to match modern links
auto-cost reference-bandwidth 10000

Key points about OSPF configuration:

• The process ID (e.g., router ospf 1) is locally significant - it doesn't need to match between neighbors
• The network command uses a wildcard mask (inverse of subnet mask)
• passive-interface suppresses OSPF Hello packets on that interface but still advertises the network
• router-id should be set explicitly; otherwise OSPF uses the highest loopback IP, then highest physical IP

Useful verification commands:

show ip ospf neighbor          ! View neighbor table and states
show ip route ospf             ! View OSPF-learned routes
show ip ospf interface brief   ! View OSPF-enabled interfaces
show ip ospf database          ! View the LSDB

OSPF Cost & Path Selection

OSPF uses cost as its metric. The cost is calculated as:

OSPF selects the path with the lowest total cost from source to destination. The total cost is the sum of all outgoing interface costs along the path.

CCNA OSPF Practice Questions

Test your understanding with these 5 expert-created questions. Each includes a detailed explanation to reinforce your learning.