Install this skill

npx skills add mukul975/Anthropic-Cybersecurity-Skills

Framework mappings

MITRE ATT&CK

NIST CSF 2.0

When to Use

Segmenting an enterprise network into isolated security zones (corporate, servers, DMZ, guest, IoT)
Meeting compliance requirements (PCI-DSS, HIPAA, SOC 2) that mandate network isolation for sensitive data
Reducing blast radius of security incidents by preventing lateral movement between network segments
Isolating high-risk devices (IoT, BYOD, legacy systems) from critical infrastructure
Implementing defense-in-depth by combining VLANs with firewall rules and access control lists

Do not use VLANs as the sole security control without Layer 3 filtering, for isolating networks that require air-gapping, or without proper switch hardening against VLAN hopping attacks.

Prerequisites

Managed switches supporting 802.1Q VLAN trunking (Cisco Catalyst, HP Aruba, Juniper EX, etc.)
Layer 3 switch or firewall for inter-VLAN routing and access control
Network design document specifying VLAN assignments, IP subnets, and traffic flow requirements
Console or SSH access to switches with privileged configuration mode
Understanding of 802.1Q trunking, STP, and inter-VLAN routing concepts

Workflow

Step 1: Design the VLAN Architecture

# Define VLANs based on security zones and function
 
VLAN Plan:
  VLAN 10  - CORPORATE    (10.10.10.0/24)  - Employee workstations
  VLAN 20  - SERVERS      (10.10.20.0/24)  - Internal servers
  VLAN 30  - DMZ          (10.10.30.0/24)  - Internet-facing servers
  VLAN 40  - GUEST        (10.10.40.0/24)  - Guest WiFi
  VLAN 50  - IOT          (10.10.50.0/24)  - IoT/OT devices
  VLAN 60  - VOIP         (10.10.60.0/24)  - VoIP phones
  VLAN 100 - MANAGEMENT   (10.10.100.0/24) - Switch/AP management
  VLAN 999 - QUARANTINE   (10.10.99.0/24)  - Isolated/compromised hosts
  VLAN 998 - NATIVE_UNUSED                  - Native VLAN (no traffic)
 
# Traffic flow matrix:
# CORPORATE -> SERVERS: Allowed (specific ports)
# CORPORATE -> DMZ: Allowed (HTTP/HTTPS only)
# CORPORATE -> GUEST: Denied
# CORPORATE -> IOT: Denied
# GUEST -> Any Internal: Denied
# IOT -> SERVERS: Allowed (specific ports to specific hosts only)
# DMZ -> SERVERS: Allowed (database ports only)
# MANAGEMENT -> All: Allowed (from management stations only)

Step 2: Configure VLANs on Cisco Catalyst Switch

! Enter configuration mode
enable
configure terminal
 
! Create VLANs
vlan 10
  name CORPORATE
  exit
vlan 20
  name SERVERS
  exit
vlan 30
  name DMZ
  exit
vlan 40
  name GUEST
  exit
vlan 50
  name IOT
  exit
vlan 60
  name VOIP
  exit
vlan 100
  name MANAGEMENT
  exit
vlan 998
  name NATIVE_UNUSED
  exit
vlan 999
  name QUARANTINE
  exit
 
! Configure access ports for workstations (VLAN 10)
interface range GigabitEthernet1/0/1-24
  switchport mode access
  switchport access vlan 10
  switchport nonegotiate
  spanning-tree portfast
  spanning-tree bpduguard enable
  no shutdown
  exit
 
! Configure access ports for servers (VLAN 20)
interface range GigabitEthernet1/0/25-36
  switchport mode access
  switchport access vlan 20
  switchport nonegotiate
  spanning-tree portfast
  spanning-tree bpduguard enable
  no shutdown
  exit
 
! Configure trunk ports to other switches
interface GigabitEthernet1/0/48
  switchport mode trunk
  switchport trunk encapsulation dot1q
  switchport trunk native vlan 998
  switchport trunk allowed vlan 10,20,30,40,50,60,100
  switchport nonegotiate
  no shutdown
  exit
 
! Configure trunk to firewall/router
interface GigabitEthernet1/0/47
  switchport mode trunk
  switchport trunk encapsulation dot1q
  switchport trunk native vlan 998
  switchport trunk allowed vlan 10,20,30,40,50,60,100
  switchport nonegotiate
  no shutdown
  exit
 
! Shutdown unused ports
interface range GigabitEthernet1/0/37-46
  shutdown
  switchport mode access
  switchport access vlan 999
  exit

Step 3: Harden Switch Against VLAN Hopping

! Disable DTP on all ports (prevents switch spoofing)
interface range GigabitEthernet1/0/1-46
  switchport nonegotiate
  exit
 
! Set native VLAN to unused VLAN on all trunks
interface range GigabitEthernet1/0/47-48
  switchport trunk native vlan 998
  exit
 
! Enable DHCP Snooping
ip dhcp snooping
ip dhcp snooping vlan 10,20,30,40,50,60
interface GigabitEthernet1/0/47
  ip dhcp snooping trust
  exit
 
! Enable Dynamic ARP Inspection
ip arp inspection vlan 10,20,30,40,50,60
interface GigabitEthernet1/0/47
  ip arp inspection trust
  exit
 
! Enable IP Source Guard (prevents IP spoofing)
interface range GigabitEthernet1/0/1-36
  ip verify source
  exit
 
! Enable Port Security
interface range GigabitEthernet1/0/1-24
  switchport port-security
  switchport port-security maximum 2
  switchport port-security violation restrict
  switchport port-security aging time 60
  exit
 
! Set VTP to transparent mode (prevents VTP attacks)
vtp mode transparent
 
! Enable BPDU Guard globally
spanning-tree portfast bpduguard default
 
! Enable Storm Control
interface range GigabitEthernet1/0/1-36
  storm-control broadcast level 10
  storm-control multicast level 10
  storm-control action shutdown
  exit

Step 4: Configure Inter-VLAN Routing with ACLs

! On the Layer 3 switch or firewall, configure SVIs
interface Vlan10
  ip address 10.10.10.1 255.255.255.0
  no shutdown
  exit
interface Vlan20
  ip address 10.10.20.1 255.255.255.0
  no shutdown
  exit
interface Vlan30
  ip address 10.10.30.1 255.255.255.0
  no shutdown
  exit
interface Vlan40
  ip address 10.10.40.1 255.255.255.0
  no shutdown
  exit
interface Vlan50
  ip address 10.10.50.1 255.255.255.0
  no shutdown
  exit
 
! ACL: Corporate to Servers (allow specific services)
ip access-list extended CORP-TO-SERVERS
  permit tcp 10.10.10.0 0.0.0.255 10.10.20.0 0.0.0.255 eq 80
  permit tcp 10.10.10.0 0.0.0.255 10.10.20.0 0.0.0.255 eq 443
  permit tcp 10.10.10.0 0.0.0.255 10.10.20.0 0.0.0.255 eq 445
  permit udp 10.10.10.0 0.0.0.255 10.10.20.0 0.0.0.255 eq 53
  permit icmp 10.10.10.0 0.0.0.255 10.10.20.0 0.0.0.255 echo
  deny ip any any log
  exit
 
! ACL: Guest to Internet only (deny all internal)
ip access-list extended GUEST-OUTBOUND
  deny ip 10.10.40.0 0.0.0.255 10.0.0.0 0.255.255.255
  deny ip 10.10.40.0 0.0.0.255 172.16.0.0 0.15.255.255
  deny ip 10.10.40.0 0.0.0.255 192.168.0.0 0.0.255.255
  permit tcp 10.10.40.0 0.0.0.255 any eq 80
  permit tcp 10.10.40.0 0.0.0.255 any eq 443
  permit udp 10.10.40.0 0.0.0.255 any eq 53
  deny ip any any log
  exit
 
! ACL: IoT limited access
ip access-list extended IOT-OUTBOUND
  permit tcp 10.10.50.0 0.0.0.255 host 10.10.20.10 eq 443
  permit tcp 10.10.50.0 0.0.0.255 any eq 443
  permit udp 10.10.50.0 0.0.0.255 host 10.10.20.1 eq 53
  deny ip 10.10.50.0 0.0.0.255 10.10.50.0 0.0.0.255 log
  deny ip any any log
  exit
 
! Apply ACLs to VLAN interfaces
interface Vlan10
  ip access-group CORP-TO-SERVERS out
  exit
interface Vlan40
  ip access-group GUEST-OUTBOUND in
  exit
interface Vlan50
  ip access-group IOT-OUTBOUND in
  exit

Step 5: Configure DHCP and DNS per VLAN

! DHCP pools for each VLAN
ip dhcp pool CORPORATE
  network 10.10.10.0 255.255.255.0
  default-router 10.10.10.1
  dns-server 10.10.20.10
  domain-name corp.example.com
  lease 1
  exit
 
ip dhcp pool GUEST
  network 10.10.40.0 255.255.255.0
  default-router 10.10.40.1
  dns-server 1.1.1.1 8.8.8.8
  lease 0 4
  exit
 
ip dhcp pool IOT
  network 10.10.50.0 255.255.255.0
  default-router 10.10.50.1
  dns-server 10.10.20.10
  lease 7
  exit
 
! Exclude gateway and server IPs from DHCP pools
ip dhcp excluded-address 10.10.10.1 10.10.10.10
ip dhcp excluded-address 10.10.40.1 10.10.40.10
ip dhcp excluded-address 10.10.50.1 10.10.50.10

Step 6: Verify and Test Segmentation

# From a workstation on VLAN 10 (Corporate):
# Should succeed:
ping 10.10.20.10          # Server access
curl https://10.10.20.10  # HTTPS to server
 
# Should fail:
ping 10.10.40.100         # Guest VLAN - should be blocked
ping 10.10.50.100         # IoT VLAN - should be blocked
 
# From a device on VLAN 40 (Guest):
# Should succeed:
ping 8.8.8.8              # Internet access
curl https://www.google.com
 
# Should fail:
ping 10.10.10.1           # Corporate gateway - blocked
ping 10.10.20.10          # Server - blocked
 
# Verify switch configuration
show vlan brief
show interfaces trunk
show ip arp inspection statistics
show ip dhcp snooping binding
show port-security
show ip access-lists
 
# Run VLAN hopping tests (from authorized pentest)
# These should all fail if hardening is correct:
# 1. DTP negotiation - should fail (nonegotiate)
# 2. Double tagging - should fail (native VLAN 998)
# 3. ARP spoofing - should fail (DAI enabled)

Key Concepts

Term	Definition
VLAN (Virtual LAN)	Logical network partition at Layer 2 that groups switch ports into isolated broadcast domains, regardless of physical location
802.1Q Trunking	IEEE standard for VLAN tagging that adds a 4-byte header to Ethernet frames, identifying which VLAN a frame belongs to across trunk links
Inter-VLAN Routing	Layer 3 forwarding of traffic between VLANs using a router, Layer 3 switch, or firewall with access control lists
Native VLAN	VLAN assigned to untagged frames on trunk ports; should be set to an unused VLAN to prevent VLAN hopping attacks
DHCP Snooping	Switch feature that validates DHCP messages and builds a binding table of IP-MAC-port mappings, preventing rogue DHCP servers
Port Security	Switch feature that limits the number of MAC addresses per port and takes action (shutdown, restrict) when violated

Tools & Systems

Cisco Catalyst/Nexus: Enterprise managed switches with comprehensive VLAN, trunking, and security feature support
HP Aruba CX: Enterprise switches with REST API management and VLAN segmentation capabilities
pfSense/OPNsense: Open-source firewalls for inter-VLAN routing with stateful access control
NetBox: Open-source IPAM and DCIM tool for documenting VLAN assignments, IP addressing, and network topology
Nmap: Network scanner for verifying segmentation effectiveness by testing reachability across VLAN boundaries

Common Scenarios

Scenario: Implementing PCI-DSS Compliant Network Segmentation for Retail

Context: A retail chain must isolate their payment card processing systems from the general corporate network to meet PCI-DSS requirements. The current flat network has point-of-sale terminals, employee workstations, inventory servers, and guest WiFi on a single VLAN. The environment uses Cisco Catalyst 9300 switches.

Approach:

Design VLAN architecture: POS terminals on VLAN 50 (CDE), corporate on VLAN 10, servers on VLAN 20, guest on VLAN 40
Create VLANs on all access-layer switches and configure access ports by function
Configure trunk links between switches with explicit VLAN allowed lists (no "all" trunks)
Set native VLAN to 998 (unused) on all trunks and disable DTP on every port
Configure ACLs on the Layer 3 switch: CDE VLAN can only reach the payment processor's IP on port 443; no other inter-VLAN traffic to/from CDE
Enable DHCP snooping, DAI, and port security on all access ports
Verify segmentation with penetration testing from each VLAN, confirming CDE is fully isolated

Pitfalls:

Leaving DTP enabled on access ports, allowing VLAN hopping to reach the CDE
Using VLAN 1 as the native VLAN, enabling double-tagging attacks
Not restricting trunk allowed VLANs, carrying all VLANs including CDE to non-essential switches
Creating ACLs that allow "any" source to reach CDE servers instead of specific POS terminal IPs

Output Format

## Network Segmentation Implementation Report
 
**Network**: Retail Store #42
**Switch Platform**: Cisco Catalyst 9300
**VLANs Configured**: 8
 
### VLAN Summary
 
| VLAN ID | Name | Subnet | Ports | Purpose |
|---------|------|--------|-------|---------|
| 10 | CORPORATE | 10.10.10.0/24 | Gi1/0/1-24 | Employee workstations |
| 20 | SERVERS | 10.10.20.0/24 | Gi1/0/25-36 | Internal servers |
| 30 | DMZ | 10.10.30.0/24 | Gi2/0/1-4 | Internet-facing |
| 40 | GUEST | 10.10.40.0/24 | WiFi AP trunk | Guest WiFi |
| 50 | CDE | 10.10.50.0/24 | Gi2/0/5-12 | POS terminals |
| 100 | MGMT | 10.10.100.0/24 | Gi1/0/48 | Switch management |
| 998 | NATIVE | N/A | Trunks only | Unused native |
| 999 | QUARANTINE | 10.10.99.0/24 | Unused ports | Isolation |
 
### Security Hardening Status
 
| Control | Status |
|---------|--------|
| DTP Disabled (nonegotiate) | All ports |
| Native VLAN (998) | All trunks |
| DHCP Snooping | VLANs 10,20,40,50 |
| Dynamic ARP Inspection | VLANs 10,20,40,50 |
| Port Security | Access ports |
| BPDU Guard | Access ports |
| Unused Ports Shutdown | 10 ports in VLAN 999 |
| VTP Transparent Mode | Enabled |

Source materials

References and resources

Everything below is rendered for inspection. Script files are read-only and never run.

References 1

api-reference.md2.3 KB

API Reference: VLAN Network Segmentation Agent

Overview

Configures and audits VLAN-based network segmentation on Cisco and multi-vendor switches using Netmiko and NAPALM. Creates VLANs, configures access/trunk ports, hardens unused ports, and audits for VLAN hopping vulnerabilities.

Dependencies

Package	Version	Purpose
netmiko	>=4.0	SSH-based switch configuration
napalm	>=4.0	Multi-vendor network device management

CLI Usage

# Audit VLAN configuration
python agent.py --host 192.168.1.1 --username admin --password pass --audit-only
 
# Full configuration mode
python agent.py --host 192.168.1.1 --username admin --password pass --device-type cisco_ios

Key Functions

`connect_netmiko(host, username, password, device_type)`

Establishes SSH connection via Netmiko supporting cisco_ios, cisco_nxos, arista_eos, juniper_junos.

`get_vlan_config(conn)`

Retrieves current VLAN configuration with TextFSM parsing of show vlan brief.

`create_vlan(conn, vlan_id, vlan_name)`

Creates a new VLAN with name on the switch.

`configure_access_port(conn, interface, vlan_id)`

Configures port as access with port-security, portfast, and BPDU guard.

`configure_trunk_port(conn, interface, allowed_vlans)`

Configures trunk port with explicit allowed VLANs, native VLAN 999, and DTP disabled.

`harden_unused_ports(conn, interfaces)`

Assigns unused ports to quarantine VLAN 999 and shuts them down.

`configure_inter_vlan_acl(conn, acl_name, rules)`

Creates extended ACLs for inter-VLAN routing access control.

`audit_vlan_security(conn)`

Checks for: default native VLAN, unhardened unused ports, and DTP negotiation enabled.

`get_napalm_config(host, username, password, driver)`

Retrieves device facts, interfaces, and VLANs using NAPALM for multi-vendor support.

Security Checks

Check	Severity	Issue
Native VLAN	Medium	Default VLAN 1 on trunks enables VLAN hopping
Unused Ports	Low	Unhardened ports allow unauthorized network access
DTP Negotiation	High	Dynamic trunking enables VLAN hopping attacks
Port Security	Medium	Missing MAC address limiting

Supported Device Types

Netmiko: cisco_ios, cisco_nxos, arista_eos, juniper_junos, hp_procurve NAPALM: ios, nxos, eos, junos

Scripts 1

agent.py7.0 KB

Display-only source. This catalog never executes bundled scripts.

#!/usr/bin/env python3
"""VLAN Network Segmentation Agent - Configures and audits VLAN segmentation on managed switches."""

import json
import logging
import argparse
from datetime import datetime

from netmiko import ConnectHandler
from napalm import get_network_driver

logging.basicConfig(level=logging.INFO, format="%(asctime)s [%(levelname)s] %(message)s")
logger = logging.getLogger(__name__)


def connect_netmiko(host, username, password, device_type="cisco_ios"):
    """Establish SSH connection to network device via Netmiko."""
    device = {
        "device_type": device_type,
        "host": host,
        "username": username,
        "password": password,
        "timeout": 30,
    }
    conn = ConnectHandler(**device)
    logger.info("Connected to %s (%s)", host, device_type)
    return conn


def get_vlan_config(conn):
    """Retrieve current VLAN configuration from the switch."""
    output = conn.send_command("show vlan brief", use_textfsm=True)
    if isinstance(output, list):
        vlans = []
        for entry in output:
            vlans.append({
                "vlan_id": entry.get("vlan_id", ""),
                "name": entry.get("name", ""),
                "status": entry.get("status", ""),
                "interfaces": entry.get("interfaces", []),
            })
        logger.info("Retrieved %d VLANs", len(vlans))
        return vlans
    return []


def create_vlan(conn, vlan_id, vlan_name):
    """Create a new VLAN on the switch."""
    commands = [
        f"vlan {vlan_id}",
        f"name {vlan_name}",
    ]
    output = conn.send_config_set(commands)
    logger.info("Created VLAN %s (%s)", vlan_id, vlan_name)
    return output


def configure_access_port(conn, interface, vlan_id):
    """Configure a switch port as an access port in a specific VLAN."""
    commands = [
        f"interface {interface}",
        "switchport mode access",
        f"switchport access vlan {vlan_id}",
        "switchport port-security",
        "switchport port-security maximum 2",
        "switchport port-security violation restrict",
        "spanning-tree portfast",
        "spanning-tree bpduguard enable",
    ]
    output = conn.send_config_set(commands)
    logger.info("Configured %s as access port in VLAN %s", interface, vlan_id)
    return output


def configure_trunk_port(conn, interface, allowed_vlans):
    """Configure a switch port as a trunk port with specific allowed VLANs."""
    vlan_list = ",".join(str(v) for v in allowed_vlans)
    commands = [
        f"interface {interface}",
        "switchport mode trunk",
        "switchport trunk encapsulation dot1q",
        f"switchport trunk allowed vlan {vlan_list}",
        "switchport trunk native vlan 999",
        "switchport nonegotiate",
    ]
    output = conn.send_config_set(commands)
    logger.info("Configured %s as trunk with VLANs %s", interface, vlan_list)
    return output


def harden_unused_ports(conn, interfaces):
    """Shut down and assign unused ports to a quarantine VLAN."""
    commands = []
    for iface in interfaces:
        commands.extend([
            f"interface {iface}",
            "switchport mode access",
            "switchport access vlan 999",
            "shutdown",
        ])
    output = conn.send_config_set(commands)
    logger.info("Hardened %d unused ports", len(interfaces))
    return output


def configure_inter_vlan_acl(conn, acl_name, rules):
    """Configure access control lists for inter-VLAN routing."""
    commands = [f"ip access-list extended {acl_name}"]
    for rule in rules:
        commands.append(rule)
    output = conn.send_config_set(commands)
    logger.info("Configured ACL %s with %d rules", acl_name, len(rules))
    return output


def audit_vlan_security(conn):
    """Audit VLAN configuration for common security issues."""
    findings = []
    vlan_output = conn.send_command("show vlan brief")
    if "VLAN0001" in vlan_output:
        trunk_output = conn.send_command("show interfaces trunk")
        if "1" in trunk_output:
            findings.append({
                "check": "Native VLAN",
                "finding": "Default VLAN 1 may be used as native VLAN on trunks",
                "severity": "Medium",
                "remediation": "Change native VLAN to unused VLAN (e.g., 999)",
            })

    port_output = conn.send_command("show interfaces status")
    if "notconnect" in port_output.lower():
        findings.append({
            "check": "Unused Ports",
            "finding": "Ports in notconnect state may not be hardened",
            "severity": "Low",
            "remediation": "Assign to quarantine VLAN and shut down",
        })

    dtp_output = conn.send_command("show dtp")
    if "DESIRABLE" in dtp_output or "AUTO" in dtp_output:
        findings.append({
            "check": "DTP Negotiation",
            "finding": "DTP negotiation enabled - VLAN hopping risk",
            "severity": "High",
            "remediation": "Set all access ports to 'switchport nonegotiate'",
        })

    logger.info("Security audit: %d findings", len(findings))
    return findings


def get_napalm_config(host, username, password, driver="ios"):
    """Retrieve device configuration using NAPALM for multi-vendor support."""
    Driver = get_network_driver(driver)
    device = Driver(host, username, password, timeout=30)
    device.open()
    facts = device.get_facts()
    interfaces = device.get_interfaces()
    vlans = device.get_vlans()
    device.close()
    return {"facts": facts, "interfaces": interfaces, "vlans": vlans}


def generate_report(host, vlans, audit_findings, actions):
    """Generate VLAN segmentation audit report."""
    report = {
        "device": host,
        "timestamp": datetime.utcnow().isoformat(),
        "vlans": vlans,
        "security_findings": audit_findings,
        "configuration_actions": actions,
    }
    print(f"VLAN SEGMENTATION REPORT: {len(vlans)} VLANs, {len(audit_findings)} findings")
    return report


def main():
    parser = argparse.ArgumentParser(description="VLAN Network Segmentation Agent")
    parser.add_argument("--host", required=True, help="Switch management IP")
    parser.add_argument("--username", required=True, help="SSH username")
    parser.add_argument("--password", required=True, help="SSH password")
    parser.add_argument("--device-type", default="cisco_ios", help="Netmiko device type")
    parser.add_argument("--audit-only", action="store_true", help="Audit without changes")
    parser.add_argument("--output", default="vlan_report.json")
    args = parser.parse_args()

    conn = connect_netmiko(args.host, args.username, args.password, args.device_type)
    vlans = get_vlan_config(conn)
    findings = audit_vlan_security(conn)

    actions = []
    if not args.audit_only:
        logger.info("Audit-only mode not set - configuration changes require explicit commands")

    report = generate_report(args.host, vlans, findings, actions)
    conn.disconnect()

    with open(args.output, "w") as f:
        json.dump(report, f, indent=2)
    logger.info("Report saved to %s", args.output)


if __name__ == "__main__":
    main()

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