Enterprise-Grade Fortinet VPN Configuration: Multi-Site IPSec with VLAN Segmentation

Introduction: The Critical Role of VPNs in Modern Enterprise Networks

In today’s distributed business environment, organizations with multiple locations require secure, high-performance connectivity between sites while maintaining strict network segmentation. Fortinet’s FortiGate firewalls provide an industry-leading solution through IPSec VPN tunnels with VLAN-aware routing, enabling:

✔ Secure site-to-site communications over public internet
✔ Isolated traffic flows between departments (VLAN segregation)
✔ Centralized management of distributed network policies
✔ QoS-aware routing for voice/video prioritization

Lab Architecture: Multi-Site Enterprise with VLAN Requirements

Key Requirements:

• VLAN-to-VLAN communication: Only IT VLANs (10.x.10.0/24) can intercommunicate
• Encryption: AES-256-GCM with PFS (DH Group 20)
• High Availability: VPN failover to backup ISP links

Phase 1: FortiGate IPSec VPN Configuration

Step 1: Building the Base Tunnel (HQ to Branch-1)

Navigate to VPN > IPsec Tunnels and use the CLI for granular control:

config vpn ipsec phase1-interface
    edit "HQ-to-Branch1"
        set interface "wan1"
        set ike-version 2
        set peertype any
        set net-device disable
        set proposal aes256gcm-prfsha384-ecp384
        set dhgrp 20
        set remote-gw 198.51.100.2
        set psksecret "Str0ngP@ssw0rd!"
        set dpd on-idle
    next
end

Critical Parameters Explained:

• aes256gcm-prfsha384-ecp384: NSA Suite B compliant encryption
• dhgrp 20: 384-bit ECP (Elliptic Curve) for faster PFS than traditional DH groups
• dpd on-idle: Dead Peer Detection to automatically reconnect stalled tunnels

Step 2: Phase 2 Configuration (Traffic Selectors)

config vpn ipsec phase2-interface
    edit "HQ-to-Branch1-P2"
        set phase1name "HQ-to-Branch1"
        set proposal aes256gcm-ecp384
        set src-addr-name "HQ_IT_VLAN"
        set dst-addr-name "Branch1_IT_VLAN"
        set auto-negotiate enable
    next
end

Security Note:
Always use narrow traffic selectors (specific VLANs instead of 0.0.0.0/0) to reduce attack surface.

Phase 2: VLAN-Aware Policy Routing

Inter-VLAN Communication Rules

Create policies to allow only IT-to-IT VLAN communication while blocking other combinations:

config firewall policy
    edit 100
        set name "IT-VLAN-to-IT-VLAN"
        set srcintf "vlan.10"
        set dstintf "ipsec.HQ-to-Branch1"
        set srcaddr "HQ_IT_VLAN"
        set dstaddr "Branch1_IT_VLAN"
        set action accept
        set schedule "always"
        set service "ALL"
        set utm-status enable
        set ssl-ssh-profile "deep-inspection"
        set av-profile "strict"
    next
end

Advanced Security:

• UTM inspection for encrypted traffic (SSL/TLS decryption)
• Application Control to block high-risk apps (e.g., Tor, P2P)

Phase 3: Dynamic Routing with BGP

For large-scale deployments, static routes become unmanageable. Implement BGP:

config router bgp
    set as 64512
    config neighbor
        edit "198.51.100.2"
            set remote-as 64513
            set update-source "ipsec.HQ-to-Branch1"
            set ebgp-enforce-multihop enable
            set route-map-out "VLAN10-ONLY"
        next
    end
end

Route Map to Filter VLANs:

config router route-map
    edit "VLAN10-ONLY"
        config rule
            edit 1
                set match-ip-address "IT_VLANs"
            next
        end
    next
end

Troubleshooting: Real-World Scenarios

Case 1: Intermittent VPN Drops

Symptoms: Tunnel flaps every 30 minutes
Diagnosis:

diagnose debug application ike -1
diagnose vpn ike log-filter dst-addr 198.51.100.2

Solution: Adjust DPD timers or replace unstable ISP link

Case 2: VLAN Traffic Not Routing

Check:

get router info routing-table all
diagnose sniffer packet any 'host 198.51.100.2 and proto 50' 4

Common Fixes:

• Verify Phase 2 selectors match VLAN subnets
• Check firewall policies for implicit denies

Going Beyond: SD-WAN Integration

For enterprises with dual ISP links, integrate IPSec tunnels into SD-WAN:

config system sdwan
    config service
        edit 1
            set name "Branch1_IPSec_Primary"
            set mode priority
            set member "ipsec.HQ-to-Branch1"
        next
        edit 2
            set name "Branch1_IPSec_Backup"
            set mode priority
            set member "ipsec.HQ-to-Branch1-backup"
            set priority 10
        next
    end
end

Performance Benefit: Automatic failover with <1s downtime

Conclusion: Building Future-Proof VPN Architectures

This guide extends basic multi-site VPN concepts with enterprise-grade features:

• NSA-grade encryption with Suite B compliance
• Zero Trust segmentation through VLAN-aware policies
• Carrier-class reliability via BGP and SD-WAN

For ongoing management:
✅ Monitor tunnels with FortiManager
✅ Audit policies with FortiAnalyzer
✅ Automate with FortiGate APIs

Need customized configurations? Contact our network security team for a free consultation.

Would you like me to elaborate on any specific section? For example:
🔹 Deep dive into Suite B cryptography
🔹 Step-by-step VLAN troubleshooting workflow
🔹 IPSec vs. SSL-VPN performance benchmarks