I. What is VLAN?

VLAN (Virtual Local Area Network) is a technology that logically divides a physical LAN into multiple distinct broadcast domains. This effectively segments the LAN, isolates different broadcast domains, enhances network security, and improves network performance. In a large corporate network with different departments like R&D, Marketing, and Finance, without VLANs, all devices would be in the same broadcast domain, leading to excessive broadcast traffic and reduced network performance. By using VLANs, each department can form an independent virtual LAN. VLAN is a crucial technology in the networking world, contributing significantly to building efficient, secure, and flexible network environments.

II. Single-Arm Routing Topology Diagram

Network Requirements:

1. AR1 belongs to VLAN 4, and AR3 belongs to VLAN 8.
2. Implement communication between R1 and R3 across different subnets using single-arm routing technology.

III. VLAN Single-Arm Routing Configuration

SW1 Configuration:

1. Create VLANs 4 and 8
   plaintext

   vlan batch 4 8

2. Configure Interface E0/0/1:
   plaintext

   interface Ethernet0/0/1
   port link-type access
   port default vlan 4 // Assign to VLAN 4

3. Configure Interface E0/0/3:
   plaintext

   interface Ethernet0/0/3
   port link-type access
   port default vlan 8 // Assign to VLAN 8

4. Configure Interface G0/0/2:
   plaintext

   interface GigabitEthernet0/0/2
   port link-type trunk
   undo port trunk allow-pass vlan 1
   port trunk allow-pass vlan 4 8 // Configure as Trunk and allow VLANs 4 and 8 to pass

R2 Single-Arm Routing Configuration:

1. Create Sub-Interfaces: G0/0/1.1 and G0/0/1.3
2. Configure Sub-Interface G0/0/1.1:
   plaintext

   interface GigabitEthernet0/0/1.1 // Enter sub-interface
   dot1q termination vid 4 // Encapsulate dot1Q VLAN ID 4
   ip address 10.0.4.254 255.255.255.0
   arp broadcast enable // Remember to enable ARP broadcast function

3. Configure Sub-Interface G0/0/1.3:
   plaintext

   interface GigabitEthernet0/0/1.3
   dot1q termination vid 8 // Encapsulate dot1Q VLAN ID 8
   ip address 10.0.8.254 255.255.255.0
   arp broadcast enable

Explanation

VLAN Configuration on SW1

1. Creating VLANs:
   • The vlan batch 4 8 command creates VLANs 4 and 8.
2. Configuring Access Ports:
   • E0/0/1 is configured as an access port for VLAN 4.
   • E0/0/3 is configured as an access port for VLAN 8.
3. Configuring Trunk Port:
   • G0/0/2 is configured as a trunk port, allowing only VLANs 4 and 8 to pass through.

Single-Arm Routing Configuration on R2

1. Creating Sub-Interfaces:
   • Sub-interfaces G0/0/1.1 and G0/0/1.3 are created.
2. Configuring Sub-Interface G0/0/1.1:
   • The sub-interface is encapsulated with VLAN ID 4.
   • An IP address is assigned, and ARP broadcast is enabled.
3. Configuring Sub-Interface G0/0/1.3:
   • The sub-interface is encapsulated with VLAN ID 8.
   • An IP address is assigned, and ARP broadcast is enabled.

Conclusion

Single-arm routing is a powerful technique for enabling communication between different VLANs without the need for additional physical links. By configuring sub-interfaces on the router and properly setting up VLANs on the switch, network engineers can achieve efficient and secure inter-VLAN communication. This method is essential for managing complex network environments where multiple departments or groups need isolated yet interconnected network segments.