Selecting the VLAN Mode

ChatGPT created the following explanation when asked about mapping VLAN design/mental models into netlab vlan.mode parameter (the final text was merged from multiple ChatGPT responses):

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Virtual LANs (VLANs) are used to segment Layer 2 broadcast domains within a network. The design models for VLANs typically fall into a few categories based on how traffic is forwarded and whether Layer 3 (routing) capabilities are introduced.

Bridge Mode (Layer 2 Switching)

Mental Model: VLANs are used to divide a physical switch into multiple Layer 2 (L2) broadcast domains. Devices in the same VLAN can communicate directly; devices in different VLANs need a router.

Analogy: VLANs are like separate rooms—devices in the same room can talk freely, but to talk to another room, you need to go through the hallway (router).

Design Model:

• VLANs act as isolated broadcast domains.

• Devices in the same VLAN can communicate at Layer 2 without routing.

• The switch simply forwards frames based on MAC addresses.

Use Cases:

• Basic network segmentation (e.g., separating departments).

• Enforcing isolation without needing Layer 3 services.

• Extending Layer 2 networks across locations via VLAN trunks.

Data Model Mapping:

mode = "bridge": Indicates the VLAN is used purely for Layer 2 switching without any IP routing associated with it.

Layer 3 Routed VLANs

Mental Model: VLANs are used more like tags or identifiers for routed subnets, with no actual L2 switching behavior.

Analogy: VLANs are like postcodes used to route traffic—there’s no local street behavior, just an address that routes packets elsewhere.

Design Model:

• VLAN interfaces are routed interfaces only, with no Layer 2 bridging.

• Typically, each VLAN maps to a separate IP subnet and is connected to a routed port or subinterface.

Use Cases:

• Pure Layer 3 networks, where each VLAN is terminated at a router.

• Spine-leaf architectures in data centers using Layer 3 ECMP (Equal-Cost Multi-Path) routing.

• Environments that use routing protocols like OSPF/BGP between VLANs/subnets.

Data Model Mapping:

mode = "route": Indicates the VLAN exists solely for routing—no Layer 2 forwarding occurs.

IRB Mode (Integrated Routing and Bridging)

Mental Model: VLANs support both L2 and L3 functionalities—local L2 switching within the VLAN and L3 routing via an interface in the VLAN.

Analogy: A room (VLAN) with a door (IRB interface) leading to other rooms (other VLANs) via a hallway (router).

Design Model:

• Also called Routed Bridge Interfaces or Switched Virtual Interfaces (SVIs).

• Allows a VLAN to have both Layer 2 bridging and Layer 3 routing on the same device.

• The VLAN is bridged for traffic within the VLAN and routed for inter-VLAN traffic.

Use Cases:

• Networks requiring communication both within and across VLANs.

• Data centers where virtual machines or services in the same VLAN need to talk to others in different VLANs.

• Smooth migration from Layer 2 to Layer 3.

Data Model Mapping:

mode = "irb": Indicates that a VLAN is both bridging at Layer 2 and associated with a Layer 3 interface (IP routing is enabled on the VLAN).

Summary Table

VLAN Mode	Layer	Forwarding Behavior	Typical Use Cases
bridge	Layer 2	MAC-based switching	Basic segmentation, isolation “bridge”
irb	L2 + L3	Bridge within VLAN, route between	Mixed L2/L3, inter-VLAN routing
route	Layer 3	IP routing only	Pure L3 networks, scalable DCs