Design Principles

Learning Objectives

  • Understand fundamental OSPF network design principles
  • Learn hierarchical area design best practices
  • Master router placement and role assignment
  • Implement proper addressing and summarization schemes
  • Design for redundancy and fault tolerance

Hierarchical Design Principles

1. Area Design Guidelines

Area 0 (Backbone) Requirements

  • Single Backbone: All non-backbone areas must connect to Area 0
  • Contiguous Design: Area 0 must be contiguous (no partitions)
  • Minimal Complexity: Keep Area 0 simple with few routers
  • High Availability: Design with redundant paths and routers

Non-Backbone Area Design

  • Size Limitations: 50-100 routers per area maximum
  • Single ABR: Avoid multiple ABRs per area when possible
  • Logical Boundaries: Align areas with administrative boundaries
  • Growth Planning: Plan for future expansion

Recommended Area Structure

                    Area 0 (Backbone)
                    R1 ---- R2 ---- R3
                    |               |
                 Area 1          Area 2
                    |               |
                [Access]        [Access]
                Networks        Networks
                
# Benefits:
- Clear hierarchy
- Scalable design  
- Efficient routing
- Easy troubleshooting

2. Router Role Assignment

Area Border Routers (ABRs)

  • High-Performance Hardware: ABRs handle inter-area routing
  • Redundancy: Deploy dual ABRs for critical areas
  • Placement: Position ABRs at network aggregation points
  • Memory Requirements: ABRs maintain multiple area LSDBs

Autonomous System Boundary Routers (ASBRs)

  • Edge Placement: Position at network boundaries
  • Route Filtering: Implement strict route filtering
  • Redistribution Control: Careful route redistribution
  • Security Focus: Enhanced security configurations

Internal Routers

  • Single Area: Belong to only one area
  • Stable Configuration: Minimal configuration changes
  • Cost-Effective: Can use lower-end hardware
  • Access Layer: Typically serve end-user networks

3. Addressing and Summarization

IP Address Planning

# Example addressing scheme
Area 0 (Backbone):     10.0.0.0/16
Area 1 (Branch East):  10.1.0.0/16  
Area 2 (Branch West):  10.2.0.0/16
Area 3 (DMZ):          10.3.0.0/16

# Point-to-point links
Area 0 P2P:            10.0.255.0/24
Area 1 P2P:            10.1.255.0/24
Area 2 P2P:            10.2.255.0/24

# Router IDs
Area 0 Routers:        10.0.0.x
Area 1 Routers:        10.1.0.x
Area 2 Routers:        10.2.0.x

Route Summarization Strategy

# ABR summarization
router ospf 1
 area 1 range 10.1.0.0 255.255.0.0
 area 2 range 10.2.0.0 255.255.0.0
 area 3 range 10.3.0.0 255.255.0.0

# ASBR summarization
router ospf 1
 summary-address 192.168.0.0 255.255.0.0

# Benefits:
- Reduced routing table size
- Faster convergence
- Improved scalability
- Better network stability

Network Topology Design

1. Redundancy Planning

Backbone Redundancy

           Area 0 (Backbone)
    R1 ---- R2 ---- R3 ---- R4
    |                       |
    +-------R5------R6------+
    
# Dual backbone paths provide:
- Link failure protection
- Router failure protection  
- Load distribution
- Maintenance flexibility

ABR Redundancy

         Area 0
    R1 ---- R2 ---- R3
    |               |
    ABR1          ABR2
    |               |
    +---- Area 1 ---+
    
# Dual ABRs provide:
- ABR failure protection
- Load sharing
- Maintenance capability
- Improved convergence

Redundancy Guidelines

  • No Single Points of Failure: Eliminate critical single points
  • Diverse Paths: Use physically diverse paths when possible
  • Proper Costs: Configure appropriate interface costs
  • BFD Integration: Enable BFD for fast failure detection

2. Link Cost Assignment

Cost Calculation Methods

# Method 1: Bandwidth-based (default)
auto-cost reference-bandwidth 10000  # 10 Gbps reference

# Method 2: Manual cost assignment
interface gigabitethernet0/0
 ip ospf cost 10

interface serial0/0
 ip ospf cost 64

# Method 3: Delay-based
interface gigabitethernet0/0
 ip ospf cost 1   # Primary path
 
interface gigabitethernet0/1  
 ip ospf cost 5   # Backup path

Cost Design Principles

  • Consistent Methodology: Use same cost calculation method
  • Primary/Backup Differentiation: Clear cost differences
  • Avoid Equal Cost: Prevent unnecessary load balancing
  • Future-Proof: Plan for bandwidth upgrades

Scalability

Learning Objectives

  • Understand OSPF scalability limitations and solutions
  • Learn techniques for scaling large OSPF deployments
  • Master area design for optimal scalability
  • Implement route summarization for scale
  • Optimize performance in large networks

OSPF Scalability Fundamentals

1. Scalability Limitations

Key Limiting Factors

  • LSA Flooding: All routers must process every LSA
  • SPF Calculation: Complexity increases with network size
  • Memory Usage: LSDB size grows with network
  • CPU Utilization: Processing overhead increases
  • Convergence Time: Larger networks take longer to converge

Single Area Limitations

Parameter Recommended Max Hard Limit Impact
Routers per Area 50-100 ~200 SPF calculation time
LSAs per Area 1000-2000 ~10000 Memory usage
Neighbors per Router 60-80 ~200 Hello processing
Areas per Router 3-5 ~30 Memory per LSDB

2. Route Summarization for Scale

ABR Summarization

# Before summarization (Area 1 -> Area 0)
10.1.1.0/24    # Network 1
10.1.2.0/24    # Network 2  
10.1.3.0/24    # Network 3
10.1.4.0/24    # Network 4
# 4 Summary LSAs generated

# After summarization
router ospf 1
 area 1 range 10.1.0.0 255.255.0.0
# Only 1 Summary LSA generated

Summarization Benefits

Metric Before Summarization After Summarization Improvement
LSA Count 1000 200 80% reduction
Memory Usage 100KB 20KB 80% reduction
SPF Time 50ms 10ms 80% improvement
Convergence 5 seconds 2 seconds 60% improvement
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