Link State Database

The Link State Database (LSDB) is the heart of OSPF operations. It contains all topology information within an area and is synchronized across all routers in that area.

LSDB Key Characteristics

Area-specific: Each area maintains its own LSDB
Synchronized: Identical across all routers in the area
Dynamic: Updated as network topology changes
Reliable: Uses flooding and acknowledgments
Aged: LSAs expire after MaxAge (3600 seconds)

LSDB Structure

The LSDB is organized by LSA type and contains multiple data structures:

LSA Storage

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

LSA Identification

LS Type: LSA category
Link State ID: Content identifier
Advertising Router: Originator
Sequence Number: Freshness
Checksum: Integrity
Age: Time since origination

LSA Aging Process

LSA Generated

→

Age = 0

→

Age Increments

→

Refresh @ 1800s

→

MaxAge @ 3600s

LSA Timers

Timer	Default Value	Purpose	Action
LSRefreshTime	1800 seconds	Refresh interval	Re-originate LSA
MaxAge	3600 seconds	LSA expiration	Flush from database
MinLSInterval	5 seconds	Rate limiting	Minimum time between updates

LSDB Verification

Cisco LSDB Commands

# View complete LSDB
show ip ospf database

# View specific LSA types
show ip ospf database router
show ip ospf database network
show ip ospf database summary
show ip ospf database external

# View specific LSA
show ip ospf database router 192.168.1.1

# Database statistics
show ip ospf database database-summary

Juniper LSDB Commands

# View complete LSDB
show ospf database

# View specific LSA types
show ospf database router
show ospf database network
show ospf database summary
show ospf database external

# View detailed LSA
show ospf database router 192.168.1.1 detail

# Database statistics
show ospf database summary

Sample LSDB Output

OSPF Router with ID (192.168.1.1) (Process ID 1)

                Router Link States (Area 0)
Link ID         ADV Router      Age  Seq#       Checksum Link count
192.168.1.1     192.168.1.1     234  0x80000002 0x1234   2
192.168.1.2     192.168.1.2     156  0x80000001 0x5678   3

                Network Link States (Area 0)
Link ID         ADV Router      Age  Seq#       Checksum
192.168.1.1     192.168.1.1     234  0x80000001 0x9ABC

                Summary Net Link States (Area 0)
Link ID         ADV Router      Age  Seq#       Checksum
10.0.0.0        192.168.1.2     567  0x80000001 0xDEF0

Dijkstra's SPF Algorithm

The Shortest Path First (SPF) algorithm, developed by Edsger Dijkstra, calculates the shortest paths from a source router to all other routers in the network.

SPF Algorithm Key Concepts

Graph Theory: Network represented as weighted graph
Cost-based: Uses interface costs as edge weights
Loop-free: Guarantees shortest path tree
Efficient: O(n²) complexity for dense graphs
Deterministic: Same input produces same output

Cost Calculation

OSPF uses interface costs to determine the best path. The default cost calculation is:

Default Cost Formula

Cost = Reference Bandwidth / Interface Bandwidth

Reference Bandwidth: 100 Mbps (default)
Interface Bandwidth: Physical interface speed
Minimum Cost: 1 (for interfaces ≥ reference bandwidth)

Interface Cost Examples

Interface Type	Bandwidth	Default Cost	Adjusted Cost (1Gbps Ref)
Serial (T1)	1.544 Mbps	64	647
Ethernet	10 Mbps	10	100
Fast Ethernet	100 Mbps	1	10
Gigabit Ethernet	1000 Mbps	1	1
10 Gigabit Ethernet	10000 Mbps	1	1

SPF Algorithm Steps

Candidate List: All routers with tentative distances
SPF Tree: Routers with confirmed shortest paths
Distance Array: Shortest distance to each router
Parent Array: Previous hop for path reconstruction

Set root distance to 0: Starting point
Add root to SPF tree: First confirmed node
Examine root's LSA: Find directly connected routers
Add neighbors to candidate list: With calculated distances

Select closest candidate: Router with minimum distance
Move to SPF tree: Confirm shortest path
Process router's LSA: Examine all links
Update distances: Relax adjacent nodes
Repeat until candidate list empty

SPF Calculation Example

Network Topology

    R1 ----10---- R2
    |             |
    5             15
    |             |
    R3 ----20---- R4

Step	Current Router	Candidate List	SPF Tree	Best Path to R4
1	R1 (Root)	R2(10), R3(5)	R1(0)	∞
2	R3	R2(10), R4(25)	R1(0), R3(5)	25
3	R2	R4(25)	R1(0), R3(5), R2(10)	25
4	R4	Empty	R1(0), R3(5), R2(10), R4(25)	25 via R3

SPF Triggers

When SPF Runs

Topology Change: New or changed LSA received
Interface State Change: Link up/down events
Area Configuration Change: Area type modifications
Router ID Change: Requires process restart

SPF Timers Configuration

Cisco SPF Timers

# Configure SPF timers
router ospf 1
 timers spf 5 10 10000

# Throttle LSA generation
 timers lsa-group-pacing 240

# Show SPF statistics
show ip ospf statistics

Juniper SPF Timers

# Configure SPF timers
protocols {
    ospf {
        spf-options {
            delay 5;
            holddown 10;
            rapid-runs 3;
        }
    }
}

# Show SPF statistics
show ospf statistics

Route Calculation

After SPF calculation completes, OSPF translates the shortest path tree into routing table entries. Different route types have different calculation methods and preferences.

Route Types in OSPF

Intra-area routes: Destinations within the same area
Inter-area routes: Destinations in other OSPF areas
External Type 1 (E1): External routes with internal cost
External Type 2 (E2): External routes with external cost only

Route Preference Order

Preference	Route Type	Description	Cost Calculation
1 (Best)	Intra-area	Routes within same area	SPF tree cost
2	Inter-area	Routes to other areas	Cost to ABR + Type 3 LSA cost
3	External Type 1	External with internal cost	Cost to ASBR + external cost
4 (Worst)	External Type 2	External with external cost only	External cost (+ ASBR cost for tiebreak)

Intra-area Route Calculation

Process

SPF tree traversal: Follow shortest path tree
Leaf network identification: Find stub networks
Cost assignment: Use SPF calculated cost
Next-hop determination: First hop from root

Inter-area Route Calculation

Type 3 LSA

→

Find ABR

→

Cost to ABR

→

Add LSA Cost

→

Install Route

Inter-area Calculation Steps

Examine all Type 3 LSAs from ABRs
Verify ABR is reachable via intra-area route
Calculate total cost: Cost to ABR + LSA metric
Compare costs from multiple ABRs

Select path with lowest total cost
Handle equal-cost paths (ECMP)
Set next-hop to ABR address
Install in routing table

External Route Calculation

Type 1 External (E1)

Cost = Cost to ASBR + External Cost

Includes internal path cost
Best for when internal topology matters
More accurate cost representation
Higher processing overhead

Type 2 External (E2)

Cost = External Cost Only

Ignores internal path cost
Default external route type
Simpler calculation
Uses ASBR cost for tiebreaking

Equal Cost Multi-Path (ECMP)

Load Balancing

OSPF supports equal-cost load balancing across multiple paths with the same cost. This provides:

Increased bandwidth: Traffic distributed across links
Redundancy: Automatic failover if one path fails
Better utilization: More efficient use of network resources

ECMP Configuration

Cisco ECMP Configuration

# Set maximum equal-cost paths
router ospf 1
 maximum-paths 8

# Verify load balancing
show ip route 10.0.0.0
show ip cef 10.0.0.0 detail

Juniper ECMP Configuration

# Configure load balancing
routing-options {
    forwarding-table {
        export load-balance;
    }
}

# Policy for load balancing
policy-options {
    policy-statement load-balance {
        from protocol ospf;
        then load-balance per-packet;
    }
}

Route Calculation Tips

Monitor SPF frequency: Frequent runs indicate instability
Use route summarization: Reduces routing table size
Consider ECMP limits: More paths = more memory usage
Verify next-hops: Ensure reachability to forwarding addresses

LSA Flooding

LSA flooding is the mechanism by which OSPF distributes topology information throughout the network. It ensures all routers have consistent database information for accurate path calculations.

Flooding Principles

Reliable delivery: Uses acknowledgments and retransmission
Loop prevention: Tracks LSA source and avoids back-sending
Duplicate detection: Sequence numbers prevent duplicates
Scope control: Different LSA types have different flooding scopes
Optimization: Uses designated routers on multi-access networks

Flooding Algorithm

Receive LSU

→

Validate LSA

→

Install in LSDB

→

Flood to Neighbors

→

Wait for LSAck

Flooding Validation Steps

Checksum verification: Ensure LSA integrity
LS Type validation: Check if type is supported
Age check: Discard if MaxAge exceeded
Scope verification: Ensure appropriate area/domain

Lookup existing LSA: Search by Type, LSID, AdvRouter
Sequence number comparison: Determine newer LSA
Age comparison: Handle special MaxAge case
Identical LSA detection: Avoid unnecessary processing

Install in database: If newer than existing
Send LSAck: Acknowledge receipt to sender
Flood to neighbors: Except the sender
Trigger SPF: If topology change detected

Flooding Scopes

LSA Type	Flooding Scope	Boundaries	Special Handling
Type 1 (Router)	Area-local	Stops at area boundaries	Generated by each router
Type 2 (Network)	Area-local	Stops at area boundaries	Generated by DR only
Type 3 (Summary)	Area-local	Generated at area boundaries	ABR regenerates per area
Type 4 (ASBR Sum)	Area-local	Generated at area boundaries	ABR regenerates per area
Type 5 (External)	AS-wide	Blocked by stub areas	Flooded throughout AS
Type 7 (NSSA Ext)	NSSA-local	Converted to Type 5 at ABR	Special NSSA handling

Network Type Flooding Behavior

Broadcast Networks

DROther → DR/BDR: 224.0.0.6
DR → All: 224.0.0.5
Optimization: Reduces flooding
Acknowledgment: Back to DR

Point-to-Point

Direct flooding: Between neighbors
Multicast: 224.0.0.5
No DR/BDR: Simple flooding
Immediate Ack: Fast convergence

Flooding Optimization

Performance Optimizations

LSA Pacing: Controls LSA generation rate
Flooding Reduction: DR/BDR on multi-access networks
Incremental SPF: Partial recalculations
LSA Grouping: Multiple LSAs per LSU packet

Flooding Timer Configuration

Cisco Flooding Timers

# LSA pacing (default 240ms)
router ospf 1
 timers lsa-group-pacing 240

# Retransmission interval
interface GigabitEthernet0/0
 ip ospf retransmit-interval 5

# Show flooding statistics
show ip ospf statistics

Juniper Flooding Configuration

# Retransmission interval
protocols {
    ospf {
        area 0.0.0.0 {
            interface ge-0/0/0.0 {
                retransmit-interval 5;
            }
        }
    }
}

# Show flooding statistics
show ospf statistics

Troubleshooting Flooding Issues

Common Flooding Problems

Missing LSAcks: Check network connectivity
Excessive retransmissions: Adjust retransmit timers
Database inconsistencies: Verify flooding scope
High CPU during flooding: Consider LSA pacing

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