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Chapter 19 — Inter-VLAN Routing

A complete networking lesson based on the course chapter menu.

11 topicsPractical examplesTroubleshooting checksReview Q&A
Estimated reading time0% read

Chapter 19: Inter-VLAN Routing

This chapter follows the topics shown in the Networking chapter menu. Work through each section in order, then use the review questions to check recall and troubleshooting reasoning.

19.1 Why VLANs Need Routing

A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.

What to check

  • Check the destination prefix and the most-specific matching route.
  • Verify next-hop reachability, route source, preference, metric, and return path.
  • Confirm that ACLs, NAT, VPN policy, or upstream routing are not blocking an otherwise-correct route.
Practice: Practice: explain Why VLANs Need Routing without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.2 Router-on-a-Stick

Router-on-a-Stick affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.

Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.

What to check

  • Check the destination prefix and the most-specific matching route.
  • Verify next-hop reachability, route source, preference, metric, and return path.
  • Confirm that ACLs, NAT, VPN policy, or upstream routing are not blocking an otherwise-correct route.
Practice: Practice: explain Router-on-a-Stick without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.3 Router Subinterfaces

Router Subinterfaces affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.

Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.

What to check

  • Check the destination prefix and the most-specific matching route.
  • Verify next-hop reachability, route source, preference, metric, and return path.
  • Confirm that ACLs, NAT, VPN policy, or upstream routing are not blocking an otherwise-correct route.
Practice: Practice: explain Router Subinterfaces without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.4 Layer 3 Switching

Layer 3 Switching is one of the core topics in Inter-VLAN Routing. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Layer 3 Switching in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain Layer 3 Switching without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.5 SVI

SVI is one of the core topics in Inter-VLAN Routing. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place SVI in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain SVI without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.6 VLAN Gateway

A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain VLAN Gateway without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.7 Routing Between VLANs

A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.

What to check

  • Check the destination prefix and the most-specific matching route.
  • Verify next-hop reachability, route source, preference, metric, and return path.
  • Confirm that ACLs, NAT, VPN policy, or upstream routing are not blocking an otherwise-correct route.
Practice: Practice: explain Routing Between VLANs without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.8 ACLs Between VLANs

A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.

What to check

  • Identify the trust boundary and the traffic that should be permitted or denied.
  • Check authentication, authorization, encryption, policy order, logs, and time synchronization.
  • Verify the control with an allowed test and a denied test so policy behavior is observable.
Practice: Practice: explain ACLs Between VLANs without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.9 DHCP Across VLANs

DHCP automatically supplies IP configuration such as an address, mask or prefix, gateway, DNS servers, and lease information. IPv4 clients commonly use the Discover, Offer, Request, Acknowledge exchange.

Example: a laptop joins a LAN with no manual IP configuration. It broadcasts or multicasts the appropriate discovery traffic, receives an offer, requests the selected lease, and then applies the address, gateway, DNS settings, and lease timers.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain DHCP Across VLANs without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.10 SVI Troubleshooting

SVI Troubleshooting is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.

Example: place SVI Troubleshooting in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
  • Compare actual configuration and measurements with the intended design, baseline, or documentation.
  • Change one variable at a time, verify the result, and document both the cause and the final fix.
Practice: Practice: explain SVI Troubleshooting without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

19.11 Inter-VLAN Routing Troubleshooting

A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.

What to check

  • Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
  • Compare actual configuration and measurements with the intended design, baseline, or documentation.
  • Change one variable at a time, verify the result, and document both the cause and the final fix.
Practice: Practice: explain Inter-VLAN Routing Troubleshooting without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

Chapter 19 Review Questions

1. What should you remember about Why VLANs Need Routing?

Answer: A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

2. What should you remember about Router-on-a-Stick?

Answer: Router-on-a-Stick affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.

3. What should you remember about Router Subinterfaces?

Answer: Router Subinterfaces affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.

4. What should you remember about Layer 3 Switching?

Answer: Layer 3 Switching is one of the core topics in Inter-VLAN Routing. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

5. What should you remember about SVI?

Answer: SVI is one of the core topics in Inter-VLAN Routing. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

6. What should you remember about Routing Between VLANs?

Answer: A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

7. What should you remember about ACLs Between VLANs?

Answer: A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.

8. What should you remember about DHCP Across VLANs?

Answer: DHCP automatically supplies IP configuration such as an address, mask or prefix, gateway, DNS servers, and lease information. IPv4 clients commonly use the Discover, Offer, Request, Acknowledge exchange.

9. What should you remember about SVI Troubleshooting?

Answer: SVI Troubleshooting is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.

10. What should you remember about Inter-VLAN Routing Troubleshooting?

Answer: A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.