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17. VLANs Part 2

4 min read

Review: Basic VLAN Topology

Let’s start with the basic VLAN topology from Part 1.

Expanded Network Topology

Now, consider a more complex network topology:

In this setup, we have two switches (SW1 and SW2), with the Engineering department (VLAN 10) located at two different network locations.

Trunk Ports

  • In a small network with only a few VLANs, it’s possible to use a separate interface for each VLAN when connecting switches to switches, or switches to routers.

  • However, as the number of VLANs increases, this method becomes inefficient. It leads to wasted interfaces, and often routers won’t have enough interfaces to support all VLANs.

  • Trunk Ports solve this problem by allowing multiple VLANs to share a single interface, effectively carrying traffic from multiple VLANs over a single connection.

Example: A Trunk Port Carrying Multiple VLAN Connections

How Does Trunking Work?

When a packet travels over a trunk port, how does it know which VLAN to use? This is managed through VLAN Tags.

  • VLAN Tagging: Switches tag frames with VLAN information when they are sent over a trunk link, allowing the receiving switch to know which VLAN the frame belongs to.

  • Trunk Port: This port type uses “tagged” frames.

  • Access Port: This port type uses “untagged” frames.

VLAN Tagging Protocols

There are two main trunk protocols:

  1. ISL (Inter-Switch Link): A Cisco proprietary protocol that is now outdated.
  2. IEEE 802.1Q (dot1q): An industry-standard protocol developed by the IEEE.

ISL is rarely used today, even on Cisco equipment. For the CCNA exam, you need to focus on 802.1Q.

Ethernet Header with 802.1Q

  • The 802.1Q Tag is inserted between the Source MAC and Type/Length fields in the Ethernet frame.
  • The tag is 4 bytes (32 bits) long and consists of two main fields:
    1. Tag Protocol Identifier (TPID): Identifies the frame as tagged with 802.1Q.
    2. Tag Control Information (TCI): Contains three sub-fields:

TPID (Tag Protocol Identifier)

  • 16 bits (2 bytes) long, always set to 0x8100 to indicate 802.1Q tagging.

TCI (Tag Control Information)

  1. PCP (Priority Code Point):
    • 3 bits, used for Class of Service (CoS) to prioritize important traffic.
  2. DEI (Drop Eligible Indicator):
    • 1 bit, used to mark frames that can be dropped if the network is congested.
  3. VID (VLAN ID):
    • 12 bits, identifies the VLAN the frame belongs to.
    • Allows for 4096 VLANs (0-4095), but VLANs 0 and 4095 are reserved, so the usable range is 1-4094.

VLAN Ranges

Here’s a breakdown of VLAN ranges:

Native VLAN

The Native VLAN is the default VLAN for untagged traffic on a trunk port.

Configuring Trunk Ports

Step-by-Step Trunk Configuration:

  1. Select Interface:

    • Begin by selecting the interface you wish to configure as a trunk.

  2. Set Trunk Encapsulation:

    • Use #switchport trunk encapsulation dot1q to set the encapsulation to 802.1Q.

  3. Enable Trunk Mode:

    • Use #switchport mode trunk to manually configure the interface as a trunk port.

Verification

  • Use the #show interfaces trunk command to confirm which interfaces are set as trunk ports.

Allow Specific VLANs on a Trunk

  • You can specify which VLANs are allowed to traverse a trunk link.

Change the Native VLAN

  • The Native VLAN can be modified if needed.

Setting Up Trunks for This Network

For our example network, we’ll need to configure the following:

  • SW1: g0/0 interface (already configured in the previous section).
  • SW2: g0/0 and g0/1 interfaces.

Configuring SW2 g0/0

Configuring SW2 g0/1

What About the Router (R1)?

Router on a Stick (ROAS)

Router on a Stick (ROAS) is a method used to route traffic between multiple VLANs using a single physical interface on a router, leveraging sub-interfaces.

Sub-Interface Configuration

  • Sub-interfaces are named (e.g., 0.10, 0.20, 0.30) according to the VLAN they serve.

  • Assign IP addresses to sub-interfaces just like you would with regular interfaces, using the last usable IP address in each VLAN subnet.

  • Show IP Interface Brief: Sub-interfaces will appear in the output of this command.

  • Show IP Route: Sub-interfaces will also appear in the router’s route table.

ROAS Functionality

  • Routing Between VLANs: ROAS allows the router to route between VLANs using a single interface and its associated sub-interfaces.
  • Trunk Configuration: The switch interface connected to the router is configured as a trunk port.
  • Sub-Interface Behavior: The router uses VLAN tags to determine which sub-interface to process incoming frames on and tags outgoing frames accordingly.

This method is highly efficient in VLAN routing, particularly in environments where conserving physical interfaces is necessary.

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