Cisco ISR4351/K9 vs ISR4331/K9: Mid-Range vs Entry-Level Router Comparison

When evaluating Cisco's ISR 4000 series for branch office deployments, the ISR4351/K9 and ISR4331/K9 represent two tiers of routing solutions with distinct performance characteristics. This detailed comparison examines their technical specifications, operational capabilities, and value propositions to help network administrators make informed purchasing decisions.

Technical Specifications Comparison

The ISR4351/K9 offers 50-100% better performance metrics across key categories while maintaining similar physical dimensions to the ISR4331/K9.

Functional Capabilities Analysis

1. Performance Benchmarks

• Routing Performance:

• ISR4351: 1.2 million pps

• ISR4331: 500,000 pps

• Application Handling:

• 50 VoIP calls

• 15 HD video streams

• Basic traffic shaping

• 150+ concurrent VoIP calls

• 30+ HD video streams

• Advanced QoS policies

• ISR4351 manages:

• ISR4331 optimal for:

2. Advanced Feature Support

Shared Capabilities:

• Cisco IOS-XE operating system

• SD-WAN functionality

• Basic threat detection

• Cisco DNA-ready

ISR4351 Advantages:

• Enhanced application visibility

• Greater VPN tunnel capacity (3,000 vs 1,500)

• Improved traffic engineering

• Better handling of encrypted traffic

Physical Design Comparison

Chassis Architecture:

• ISR4351/K9:

• 1RU form factor (slightly deeper)

• Enhanced cooling system

• Optional redundant power

• Tool-less service module access

• ISR4331/K9:

• Compact 1RU design

• Fixed power supply

• Basic cooling solution

• Space-optimized for small racks

Environmental Specifications:

• Operating temperature:

• ISR4351: 0 to 45°C

• ISR4331: -5 to 45°C

• Noise levels:

• ISR4351: 50 dBA

• ISR4331: 45 dBA

User Experience Evaluation

Administration & Management:

• Both utilize:

• Cisco DNA Center management

• Identical web interface

• Same CLI structure

• API access

Operational Differences:

• ISR4351 Advantages:

• More detailed analytics

• Better traffic visualization

• Enhanced troubleshooting tools

• ISR4331 Strengths:

• Simpler initial setup

• Lower learning curve

• Faster basic configuration

Real-World Performance:

• Latency under load:

• ISR4351: <1ms

• ISR4331: 2-3ms

• Configuration commit times:

• ISR4351: 30% faster

Pricing & Total Cost Analysis

Acquisition Costs:

• ISR4331/K9: 6,500−8,500

• ISR4351/K9: 10,000−12,500

Operational Expenditures:

• Power consumption difference: ~$40/year

• Identical rack space requirements

• Similar cooling needs

Five-Year TCO Considerations:

• ISR4351 offers better ROI for:

• Growing branch offices

• Sites adding services

• Locations needing headroom

• ISR4331 economical for:

• Static small offices

• Basic connectivity needs

• Cost-sensitive deployments

Power Efficiency Metrics

Power Specifications:

• Input options:

• Both support AC/DC power

• ISR4351 offers optional redundancy

• Energy monitoring:

• Both provide power usage metrics

Efficiency Comparison:

• Performance per watt:

• ISR4351: 66 Mbps/W

• ISR4331: 54 Mbps/W

• Idle power draw:

• ISR4351: 45W

• ISR4331: 30W

Compatibility & Ecosystem Integration

Third-Party Interoperability:

• Both support:

• Standard SNMP monitoring

• NETCONF/YANG models

• Syslog reporting

• REST API access

ISR4351 Enhanced Integration:

• Better Cisco SD-Access support

• Improved AVC integration

• More extensive API capabilities

Module Support:

• ISR4351 accommodates:

• Two SM-X service modules

• More interface options

• ISR4331 limited to:

• Single SM-X module

• Basic interface cards

Software Support & Lifecycle

Current Software Features:

• ISR4351 receives:

• More frequent feature updates

• Enhanced analytics

• Better SD-WAN capabilities

• ISR4331 gets:

• Stable core features

• Basic SD-WAN functionality

Security Updates:

• Both receive:

• Regular vulnerability patches

• 5+ years of maintenance

• ISR4351 qualifies for:

• Some premium security features

• Advanced crypto options

Future Roadmap:

• ISR4351 positioned for:

• Enhanced SD-WAN features

• Better cloud integration

• Application optimization

• ISR4331 focuses on:

• Core routing stability

• Basic security updates

• Essential features

Decision Framework

Select ISR4331/K9 When:
✔ Deploying to small branches (<50 users)
✔ Budget under $9,000 required
✔ Basic routing needs only
✔ No significant growth anticipated

Choose ISR4351/K9 When:
✔ Supporting medium branches (50-150 users)
✔ Planning service additions
✔ Needing performance headroom
✔ Can justify 25-30% premium

Key Selection Criteria:

1. Current and projected user counts

2. Application performance requirements

3. Planned network services

4. Available budget

For most growing branch offices, the ISR4351/K9 represents the smarter investment with its better performance headroom and enhanced capabilities. The ISR4331/K9 remains a viable choice for truly small, static locations with basic connectivity needs.

Implementation Tip: Use Cisco's Performance Advisor tool to analyze your specific traffic patterns before deciding. The performance difference between these models becomes most noticeable when implementing multiple services concurrently or during peak usage periods. Consider your three-year growth projections when evaluating these options.