Switch Ports, VLANs and 802.1Q Trunks

Lesson plan

Level

Beginner

Time

About 40 minutes; the steps below add up to 42

Learners work out how one set of switches could act as two networks, and why a school would want that, before the coloured-sticker comparison names a VLAN. Predictions then precede how the second switch knows a frame’s VLAN, whether a new cable is a trunk, who reaches whom in the two-switch example and what joins two VLANs; checks follow the sticker rule and the frame walkthrough. The habit to build: VLANs separate, routing reconnects, and policy decides.

Objectives

• Explain what a VLAN is and why it is its own broadcast domain
• Tell an access port from a trunk, and explain 802.1Q tagging
• Follow a frame across a trunk between two switches
• Explain why traffic between VLANs needs a router, and where the real boundary is

Before the lesson

• Open this page on your own screen and use “Open the lesson window” to open the lesson in a second window. Share only that second window.
• Learners need the Ethernet switching and IP forwarding lessons first.
• Two colours of sticky note on a drawing of two switches make the walkthrough concrete.
• The lesson takes about 42 minutes.
• In presentation mode every step is shown in turns: Next reveals one moment, part or answer at a time, and the instructor view names what is on screen and what comes next.

Steps

1. Introduction Opening

   On screen: the lesson’s title, module and number; Next adds its objectives and key terms, as the lesson page lists them; Next again begins the first step.

   Objectives on screen

   • Explain what a VLAN is and why it is its own broadcast domain
   • Tell an access port from a trunk, and explain 802.1Q tagging
   • Follow a frame across a trunk between two switches
   • Explain why traffic between VLANs needs a router, and where the real boundary is

   Purpose: Learners work out how one set of switches could act as two networks, and why a school would want that, before the coloured-sticker comparison names a VLAN. Predictions then precede how the second switch knows a frame’s VLAN, whether a new cable is a trunk, who reaches whom in the two-switch example and what joins two VLANs; checks follow the sticker rule and the frame walkthrough. The habit to build: VLANs separate, routing reconnects, and policy decides.

2. 1. Two groups, one building Think 5 min

   Purpose: Raise the need for separate networks on shared equipment.

   Ask

   • How could one set of equipment act as two networks?
   • Why keep the groups apart?

   Responses to expect

   • Labelling sockets comes with a hint about the switch knowing each socket’s group.
   • On screen in turns: the opening question, its answer; then the question on why to keep the groups apart, and its answer.

   Discussion pause: Let learners propose a scheme before revealing.

   In the lesson window

   • Think it through: A school wants staff computers and visitors’ phones on separate networks, but it has only one set of switches and cables. How could one set of equipment behave like two separate networks?
   • Think it through: Why might the school want visitors’ phones unable to reach staff computers at all?

3. 2. Linking switches Explain 2 min

   Purpose: Show that larger networks join switches with trunk links.

   Ask

   • Why join switches at all?

   Responses to expect

   • A single switch runs out of ports.
   • On screen in turns: the opening; then the trunk link.

4. 3. One switch, several networks Explain 5 min

   Purpose: Introduce the VLAN with the sticker comparison, and the broadcast domain.

   Ask

   • Can a green and a blue socket talk?

   Responses to expect

   • Most answer correctly once the rule is stated.
   • On screen in turns: the opening; then the check question on the sticker rule, and its answer; the VLAN; each VLAN is its own broadcast domain; the managed switch.

   In the lesson window

   • Check your understanding: With the sticker rule, can a green socket and a blue socket on the same switch talk directly?

5. 4. Linked switches, access ports and VLAN trunks Predict 8 min

   Purpose: Introduce tags, access ports, trunks and 802.1Q.

   Ask

   • How could the second switch tell a frame’s colour?
   • Is a new cable a trunk?

   Responses to expect

   • Some think a cable between switches is automatically a trunk.
   • On screen in turns: the opening; then the question on telling the colours apart, and its answer; the tag; two kinds of port, starting with the access port; the trunk port; the 802.1Q standard; the question on plugging in a cable, and its answer; a trunk is configuration, not a cable; the native VLAN.

   Misconceptions to challenge

   • A learner may believe: “Any cable between two switches is a trunk.” Respond: Both ends must be configured as a trunk, with the VLANs it may carry.

   To go further

   • What happens to an untagged frame that arrives on a trunk? (See the native VLAN note.)

   In the lesson window

   • Think it through: Both colours share the one cable between the switches. How could the second switch tell which colour each frame belongs to?
   • Think it through: A technician joins two managed switches with a new cable. Is the link now a trunk carrying every VLAN?

6. 5. Fictional example: staff and guest networks across two switches Predict 10 min

   Purpose: Follow the fictional two-switch example, a frame across the trunk, and routing between VLANs.

   Ask

   • Who reaches whom?
   • What joins VLAN 10 and VLAN 30?
   • Where was the tag?

   Responses to expect

   • Learners often think devices on the same switch can always talk.
   • On screen in turns: the opening; then the question on who reaches whom, and its answer; the two switches and the shared trunk; who shares which VLAN; switching inside each VLAN; the question on staff and printers, and its answer; routing between VLANs, and the policy that decides; following one frame across the trunk, starting at Switch A; onto the trunk; into Switch B; out to Staff B; guest traffic, and the result; the check question on where the tag is, and its answer; VLANs and the security boundary.

   Misconceptions to challenge

   • A learner may believe: “Devices in different VLANs can never communicate.” Respond: They can through a router, if routing and policy allow it.

   Discussion pause: Ask who reaches whom from the description alone; the diagram of the two switches appears only after the answers. Then reveal the frame’s journey one stage at a time, asking where the tag is at each.

   If the explanation does not land: Walk two learners holding sticky notes across a drawn trunk, adding and removing a tag.

   In the lesson window

   • Think it through: From that description, can Staff A reach Staff B, on the other switch? Can Staff A reach Guest A, on the same switch?
   • Think it through: The school puts its printers in VLAN 30, and staff in VLAN 10 need to print. What has to connect the two?
   • Check your understanding: In that walkthrough, where did the frame carry an 802.1Q tag?

7. 6. Apply it in a new situation Apply 5 min

   Purpose: Set up independent application, ideally on a later day.

   Ask

   • Who will try these now, and who later in the week?

   Responses to expect

   • The printers question tests where the boundary is; VLAN 10 tests numbering; the broadcast tests broadcast domains across a trunk.
   • On screen in turns: the opening; then the question on guests and printers, and its answer; the question on VLAN 10 and port 10, and its answer; the question on who hears a broadcast, and its answer.

   In the lesson window

   • Apply it: Guests (VLAN 20) and staff (VLAN 10) are joined by a Layer 3 switch so that both can reach the printers. A colleague says guests are still kept away from staff computers “because they are in a different VLAN”. What do you check?
   • Apply it: A new colleague assumes a device on VLAN 10 must be plugged into switch port 10. What do you say?
   • Apply it: A staff computer in VLAN 10 sends a broadcast. Staff and guest devices are on both switches. Which devices receive it?

8. 7. Check your understanding Check 4 min

   Purpose: Check the lesson against the approved revision-bank questions.

   Ask

   • Answer individually, then compare before revealing.

   Responses to expect

   • The routing question links to the two-switch step.
   • On screen in turns: the opening; then the revision questions one at a time, each followed by its answer; where to practise next.

9. 8. What to remember Consolidate 3 min

   Purpose: Summarise the five points, the key terms and the misunderstandings.

   Ask

   • Without looking: what does a trunk add to each frame?

   Responses to expect

   • An 802.1Q tag naming its VLAN.
   • On screen in turns: the opening; then the summary; the key terms; the common misunderstandings.

10. Lesson recap Closing

    On screen: the lesson’s objectives again; Next adds the closing question, which asks the class to explain each one in their own words without looking back. It says nothing about what anyone has learnt: use the answers to decide what to revisit.

    Points to listen for

    • One set of switches, several networks.
    • Access ports: one VLAN, untagged.
    • Trunks: several VLANs, tagged, configured at both ends.
    • Between VLANs: a router.
    • The router’s policy is the boundary, not the VLAN alone.

Consolidation

• One set of switches, several networks.
• Access ports: one VLAN, untagged.
• Trunks: several VLANs, tagged, configured at both ends.
• Between VLANs: a router.
• The router’s policy is the boundary, not the VLAN alone.