Open Shortest Path First (OSPF)

Definition

Open Shortest Path First (OSPF) is a Link-State Interior Gateway Protocol (IGP) used to dynamically exchange routing information within an Autonomous System (AS). OSPF calculates the shortest path using the Dijkstra Shortest Path First (SPF) algorithm and is widely used in medium and large enterprise networks.

OSPF Characteristics

Definition

OSPF is an open-standard dynamic routing protocol designed for fast, scalable, and efficient routing.

Key Points

Open standard (works with Cisco and non-Cisco devices)
Link-State routing protocol
Uses the Dijkstra SPF (Shortest Path First) algorithm
Fast convergence
Administrative Distance (AD): 110
Supports IPv4 and IPv6
Supports VLSM and CIDR
Uses Cost as its routing metric
Supports Equal-Cost Load Balancing (ECMP)
Has no hop-count limit
Divides large networks into Areas
Requires Area 0 (Backbone Area)

OSPF Metric (Cost)

Definition

OSPF selects the best path based on Cost.

Lower cost = Better path.

Cost Formula

Cost = Reference Bandwidth / Interface Bandwidth

Cisco traditionally uses:

Cost = 100 Mbps / Bandwidth (Mbps)

Note: Modern Cisco IOS versions allow the reference bandwidth to be changed so high-speed links (1 Gbps, 10 Gbps, etc.) can have different costs.

Common Default Costs

Interface	Cost
Serial T1	64–65 (approximately)
Ethernet (10 Mbps)	10
Fast Ethernet (100 Mbps)	1
Gigabit Ethernet	1*
10 Gigabit Ethernet	1*
40 Gigabit Ethernet	1*
100 Gigabit Ethernet	1*

*With the default 100 Mbps reference bandwidth, all links of 100 Mbps or faster have a cost of 1.

Example

Route	Total Cost
Path A	5
Path B	12

OSPF chooses Path A because it has the lower total cost.

OSPF Routing Tables

Every OSPF router maintains three databases.

Table	Purpose
Neighbor Table	Stores information about neighboring OSPF routers
Topology (Link-State) Database	Stores the complete network topology for the area
Routing Table	Stores the best routes calculated by the SPF algorithm

Router ID (RID)

Definition

A Router ID (RID) is a 32-bit unique identifier used to identify an OSPF router.

Although it looks like an IPv4 address, it is used only for identification.

Router ID Selection Order

OSPF selects the Router ID in this order:

Router ID configured manually (router-id)
Highest IP address on any Loopback Interface
Highest IP address on any active Physical Interface

Configure Router ID

R(config)# router ospf 10
R(config-router)# router-id 172.168.10.10

Verify Router ID

show ip ospf

Topology Database

Definition

The Topology Database contains all Link-State Advertisements (LSAs) received from routers within the same OSPF area.

Routers use this database to calculate the shortest path.

Link-State Advertisement (LSA)

Definition

An LSA (Link-State Advertisement) is a packet containing information about a router’s links and connected networks.

Routers flood LSAs throughout the OSPF area to maintain a consistent topology database.

DR (Designated Router)

Definition

A Designated Router (DR) is elected on a broadcast multi-access network (such as Ethernet) to reduce unnecessary routing traffic.

Instead of every router exchanging LSAs with every other router, routers exchange LSAs through the DR.

DR Election

Selection order:

Highest OSPF Priority
Highest Router ID

Configure Interface Priority

R(config-if)# ip ospf priority 100

Priority Range:

0 - 255

Higher value = Higher chance of becoming DR.
Priority 0 means the router can never become DR.

Multicast Addresses

Address	Purpose
224.0.0.5	All OSPF Routers
224.0.0.6	All Designated Routers (DR/BDR)

Non-DR routers send updates to 224.0.0.6, while OSPF Hello packets are sent to 224.0.0.5.

BDR (Backup Designated Router)

Definition

The Backup Designated Router (BDR) is elected alongside the DR.

It listens to all routing information but does not actively distribute LSAs unless the DR fails.

Characteristics

Backup for the DR
Automatically becomes DR if the DR fails
One DR and one BDR per broadcast network

OSPF Areas

Definition

An OSPF Area is a logical grouping of routers and networks.

Using areas reduces routing overhead and improves scalability.

Key Points

Every router in the same area shares the same topology database.
Every OSPF network must connect to Area 0 (Backbone Area).
A router may belong to multiple areas.
Area IDs are assigned to interfaces.
Areas reduce:
- Routing updates
- Topology database size
- Routing table size

Area Types

Backbone Area (Area 0)

The central area to which all other areas must connect.

Non-Backbone Areas

Contain routers that communicate with Area 0 through an Area Border Router (ABR).

ABR (Area Border Router)

Definition

An ABR connects multiple OSPF areas.

Responsibilities

Connects Area 0 with other areas.
Summarizes routes between areas.
Maintains separate topology databases for each connected area.

ASBR (Autonomous System Boundary Router)

Definition

An ASBR connects an OSPF network to another routing domain or external network.

Examples:

Internet
EIGRP
RIP
Static routes

OSPF Neighbor Relationship

Definition

Before exchanging routing information, OSPF routers must first become Neighbors.

Neighbors are discovered automatically using Hello Packets.

Hello Packets

Sent every 10 seconds (on broadcast and point-to-point networks by default)
Destination Multicast Address: 224.0.0.5

Dead Timer

Default:

Dead Interval = Hello Interval × 4

Example:

10 × 4 = 40 Seconds

If no Hello packets are received within the Dead Interval, the neighbor is considered down.

Requirements to Become Neighbors

For two routers to establish an OSPF neighbor relationship, the following must match:

Subnet
Area ID
Hello Interval
Dead Interval
Authentication
MTU size (typically 1500 bytes)

Neighbor vs Adjacent

Neighbor

Routers that exchange Hello Packets.

Adjacent

Routers that exchange LSAs and synchronize their databases.

A router must first become a Neighbor before becoming Adjacent.

Comparison

Neighbor	Adjacent
Exchanges Hello packets	Exchanges LSAs
Discovers routers	Synchronizes databases
First stage	Second stage

OSPF Configuration

Step 1: Enable OSPF

R(config)# router ospf 10

Explanation

Starts the OSPF process.
10 is the Process ID.
Process ID is locally significant and does not have to match between routers.

Step 2: Advertise Networks

R(config-router)# network 10.1.1.0 0.255.255.255 area 0

Explanation

10.1.1.0 → Network address
0.255.255.255 → Wildcard mask
area 0 → Places matching interfaces into Area 0

Wildcard Mask

Definition

OSPF uses Wildcard Masks, which are the inverse of subnet masks.

Examples

Subnet Mask	Wildcard Mask
255.255.255.0	0.0.0.255
255.255.0.0	0.0.255.255
255.0.0.0	0.255.255.255

Correction: The lecture slide shows 255.255.255.0 → 0.255.255.255, which is incorrect. The correct wildcard mask is 0.0.0.255.

OSPF Verification Commands

show ip ospf

Displays OSPF process information.

show ip route

Displays the routing table.

show ip ospf database

Displays the Link-State Database.

show ip ospf interface FastEthernet0/1

Displays OSPF information for a specific interface.

show ip ospf neighbor

Displays OSPF neighbors.

show ip protocols

Displays routing protocol information.

OSPF Debug Commands

debug ip ospf packet

Displays OSPF packets.

debug ip ospf hello

Displays Hello packets.

debug ip ospf adj

Displays adjacency events.

debug ip packet

Displays IP packet processing.

Loopback Interface

Definition

A Loopback Interface is a virtual interface that remains active as long as the router is operational.

It is commonly used as the Router ID because it is stable.

Configuration

R(config)# interface loopback 0
R(config-if)# ip address 192.168.10.1 255.255.255.255

A /32 subnet mask (255.255.255.255) is typically used for loopback interfaces.

Example / Code

Basic OSPF Configuration

R1(config)# router ospf 10
R1(config-router)# router-id 1.1.1.1
R1(config-router)# network 192.168.1.0 0.0.0.255 area 0

Line-by-Line Explanation

Command	Purpose
`router ospf 10`	Starts the OSPF process.
`router-id 1.1.1.1`	Manually sets the Router ID.
`network 192.168.1.0 0.0.0.255 area 0`	Enables OSPF on matching interfaces in Area 0.

Common Mistakes

Confusing Router ID with an interface IP address.
Assuming the OSPF Process ID must match between routers; it is locally significant.
Using the wrong wildcard mask (it is the inverse of the subnet mask).
Forgetting to connect all non-backbone areas to Area 0.
Assuming OSPF uses Hop Count; it uses Cost.
Forgetting that neighbor routers must have matching Hello/Dead timers, Area IDs, authentication, subnet, and MTU.
Believing every router exchanges LSAs with every other router on broadcast networks; DR and BDR reduce this overhead.

Short Exam Notes

OSPF: Open-standard Link-State IGP.
Algorithm: Dijkstra Shortest Path First (SPF).
Administrative Distance: 110.
Metric: Cost (lower is better).
Hop Count: Unlimited.
Supports: IPv4, IPv6, VLSM, CIDR, Equal-Cost Load Balancing.
Multicast Addresses: 224.0.0.5 (All OSPF Routers), 224.0.0.6 (All DR/BDR).
Router ID Priority: Manual → Highest Loopback IP → Highest Physical IP.
OSPF Tables: Neighbor Table, Topology Database, Routing Table.
DR Election: Highest Priority, then highest Router ID.
BDR: Takes over if the DR fails.
Area 0: Backbone Area; all other areas should connect to it.
ABR: Connects OSPF areas and summarizes routes.
ASBR: Connects OSPF to external routing domains.
Hello Interval: 10 seconds (default on Ethernet).
Dead Interval: 40 seconds (default on Ethernet).
Neighbor: Exchanges Hello packets.
Adjacent: Exchanges LSAs and synchronizes databases.
Wildcard Mask: Inverse of the subnet mask (e.g., 255.255.255.0 → 0.0.0.255).