1. Inter VLAN routing
Ferry Astika Saputra

2. Understanding Access and Trunk Interfaces
Ethernet interfaces can be configured either as access ports or a trunk ports, as follows:
An access port can have only one VLAN configured on the interface; it can carry traffic for only one VLAN.
A trunk port can have two or more VLANs configured on the interface; it can carry traffic for several VLANs simultaneously.
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3. What is inter VLAN routing

4. Traditional Inter VLAN Routing (1)

5. Router on a stick Inter VLAN Routing (1)

6. Router on a stick Inter VLAN Routing (2)
Router-on-a-stick is a type of router configuration in which a single physical interface routes traffic between multiple VLANs on a network.
The router interface is configured to operate as a trunk link and is connected to a switch port configured in trunk mode.
The router performs the inter-VLAN routing by accepting VLAN tagged traffic on the trunk interface coming from the adjacent switch and internally routing between the VLANs using sub interfaces.

7. Router on a stick Inter VLAN Routing (3)
Subinterfaces are multiple virtual interfaces, associated with one physical interface.
These subinterfaces are configured independently configured with an IP address and VLAN assignment to operate on a specific VLAN.

8. Traditional Vs Router-on-stick (1)
Port Limits
Physical interfaces are configured to have one interface per VLAN on the network.
On networks with many VLANs, using a single router to perform inter-VLAN routing is not possible.
Instead, you could use multiple routers to perform inter-VLAN routing for all VLANs if avoiding the use of subinterfaces is a priority.
Subinterfaces allow a router to scale to accommodate more VLANs than the physical interfaces permit.

9. Traditional Vs Router-on-stick (2)
Performance
Because there is no contention for bandwidth on separate physical interfaces, physical interfaces have better performance when compared to using subinterfaces.
Traffic from each connected VLAN has access to the full bandwidth of the physical router interface connected to that VLAN for inter-VLAN routing.
When subinterfaces are used for inter-VLAN routing, the traffic being routed competes for bandwidth on the single physical interface.
On a busy network, this could cause a bottleneck for communication.

10. Traditional Vs Router-on-stick (3)
Access Ports and Trunk Ports
Connecting physical interfaces for inter-VLAN routing requires that the switch ports be configured as access ports.
Subinterfaces require the switch port to be configured as a trunk port so that it can accept VLAN tagged traffic on the trunk link.

11. Traditional Vs Router-on-stick (4)
Cost
Financially, it is more cost-effective to use sub interfaces over separate physical interfaces.
Routers that have many physical interfaces cost more than routers with a single interface.
Additionally, if you have a router with many physical interfaces, each interface is connected to a separate switch port, consuming extra switch ports on the network.
Switch ports are an expensive resource on high performance switches.

12. Traditional Vs Router-on-stick (5)
Complexity
Using subinterfaces for inter-VLAN routing results in a less complex physical configuration than using separate physical interfaces, because there are fewer physical network cables interconnecting the router to the switch.
On the other hand, using sub interfaces with a trunk port results in a more complex software configuration, which can be difficult to troubleshoot.
If one VLAN is having trouble routing to other VLANs, you need to check to see if the switch port is configured to be a trunk and verify that the VLAN is not being filtered on any of the trunk links before it reaches the router interface.
You also need to check that the router subinterface is configured to use the correct VLAN ID and IP address for the subnet associated with that VLAN.

13. Traditional Vs Router-on-stick (6)

14. Configuring Traditional Inter VLAN routing (1)

15. Configuring Traditional Inter VLAN routing (2)
Switch configuration

16. Configuring Traditional Inter VLAN routing (3)
Router configuration

17. Configuring Traditional Inter VLAN routing (4)

18. Configure router-on-a-stick inter VLAN routing (1)

19. Configure router-on-a-stick inter VLAN routing (2)

20. Configure router-on-a-stick inter VLAN routing (3)

21. Troubleshooting inter VLAN routing – switch issues (1)
Incorrect VLAN

22. Troubleshooting inter VLAN routing – switch issues (2)
Not configure as a trunk

23. Troubleshooting inter VLAN routing – switch issues (3)
No Redundant link

24. Troubleshooting inter VLAN routing – router issues
Incorrect VLAN on a stick

25. Troubleshooting inter VLAN routing – IP addressing issues (1)
Incorrect IP Addressing

26. Troubleshooting inter VLAN routing – IP addressing issues (2)
Incorrect subnet mask