Inter-vlan Routing – ahmedhussein

What is Inter-VLAN Routing?

VLANs are used to segment switched Layer 2 networks for a variety of reasons. Regardless of the reason, hosts in one VLAN cannot communicate with hosts in another VLAN unless there is a router or a Layer 3 switch to provide routing services.
Inter-VLAN routing is the process of forwarding network traffic from one VLAN to another VLAN.
There are three inter-VLAN routing options:

Legacy Inter-VLAN routing– This is a legacy solution. It does not scale well.
Router-on-a-Stick– This is an acceptable solution for a small to medium-sized network.
Layer 3 switch using switched virtual interfaces (SVIs)– This is the most scalable solution for medium to large organizations

Legacy Inter-VLAN Routing

The first inter-VLAN routing solution relied on using a router with multiple Ethernet interfaces. Each router interface was connected to a switch port in different VLANs. The router interfaces served as the default gateways to the local hosts on the VLAN subnet.
Legacy inter-VLAN routing using physical interfaces works, but it has a significant limitation. It is not reasonably scalable because routers have a limited number of physical interfaces. Requiring one physical router interface per VLAN quickly exhausts the physical interface capacity of a router.
Note: This method of inter-VLAN routing is no longer implemented in switched networks and is included for explanation purposes only.

Router-on-a-Stick Inter-VLAN Routing

The ‘router-on-a-stick’ inter-VLAN routing method overcomes the limitation of the legacy inter-VLAN routing method. It only requires one physical Ethernet interface to route traffic between multiple VLANs on a network.

A Cisco IOS router Ethernet interface is configured as an 802.1Q trunk and connected to a trunk port on a Layer 2 switch. Specifically, the router interface is configured using subinterfaces to identify routable VLANs.
The configured subinterfaces are software-based virtual interfaces. Each is associated with a single physical Ethernet interface. Subinterfaces are configured in software on a router. Each subinterface is independently configured with an IP address and VLAN assignment. Subinterfaces are configured for different subnets that correspond to their VLAN assignment. This facilitates logical routing.
When VLAN-tagged traffic enters the router interface, it is forwarded to the VLAN subinterface. After a routing decision is made based on the destination IP network address, the router determines the exit interface for the traffic. If the exit interface is configured as an 802.1q subinterface, the data frames are VLAN-tagged with the new VLAN and sent back out the physical interface

Note: The router-on-a-stick method of inter-VLAN routing does not scale beyond 50 VLANs.

Inter-VLAN Routing on a Layer 3 Switch

The modern method of performing inter-VLAN routing is to use Layer 3 switches and switched virtual interfaces (SVI). An SVI is a virtual interface that is configured on a Layer 3 switch, as shown in the figure.

Note: A Layer 3 switch is also called a multilayer switch as it operates at Layer 2 and Layer 3. However, in this course we use the term Layer 3 switch

Inter-VLAN Routing on a Layer 3 advantages

The following are advantages of using Layer 3 switches for inter-VLAN routing:

They are much faster than router-on-a-stick because everything is hardware switched and routed.
There is no need for external links from the switch to the router for routing.
They are not limited to one link because Layer 2 EtherChannels can be used as trunk links between the switches to increase bandwidth.
Latency is much lower because data does not need to leave the switch in order to be routed to a different network.
They more commonly deployed in a campus LAN than routers.
The only disadvantage is that Layer 3 switches are more expensive.

Router-on-a-Stick Scenario

Routing Scenario on a Layer 3 Switch

Complete the following steps to configure D1 to route with R1:

Step 1. Configure the routed port. Use the no switchport command to convert the port to a routed port, then assign an IP address and subnet mask. Enable the port.
Step 2. Enable routing. Use the ip routing global configuration command to enable routing.
Step 3. Configure routing. Use an appropriate routing method. In this example, Single-Area OSPFv2 is configured
Step 4. Verify routing. Use the show ip route command.
Step 5. Verify connectivity. Use the ping command to verify reachability.

Common Inter-VLAN Issues

Troubleshoot Inter-VLAN Routing Scenario

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Created on December 20, 2022

Inter-VLAN Routing Quiz

Test Your Understanding

1 / 10

What is a disadvantage of using multilayer switches for inter-VLAN routing?

Multilayer switches are more expensive than router-on-a-stick implementations.

Multilayer switches have higher latency for Layer 3 routing.

Multilayer switches are limited to using trunk links for Layer 3 routing.

Spanning tree must be disabled in order to implement routing on a multilayer switch.

The main disadvantage of the multilayer switches is their higher cost. Because both routing and switching are done in hardware, multilayer switches are faster than router-on-a-stick.

2 / 10

What is the meaning of the number 10 in the encapsulation dot1Q 10 native router subinterface command?

the subinterface number

the interface number

the subnet number

the VLAN ID

The administrator can use the encapsulation command to specify the encapsulation type (IEEE 802.1Q or ISL), the VLAN ID, and optionally the native VLAN.

3 / 10

A PC is to access a web server on another network. Which inter-VLAN method will provide the highest bandwidth at Layer 3 and also provide a default gateway for the PC?

trunked interface between the router and the switch

router on a stick

multilayer switch with routing enabled

multiple physical interfaces on the router, all connected to a Layer 2 switch

A router-on-a-stick design is the same as having a trunked interface between the router and the switch. This design works, but does not scale well because all VLANs must traverse the one connection between the router and the switch. Multiple physical interfaces on the router would be faster than the router-on-a-stick design, but a router has a limited number of physical interfaces. Layer 3 switches with routing enabled have more Ethernet ports as well as the ability to route.

4 / 10

A network administrator enters the following command sequence on a Cisco 3560 switch. What is the purpose of these commands?
Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# no switchport

to make the Gi0/1 port a routed port

to shut down the Gi0/1 port

to enable the Gi0/1 port as a switch virtual interface

to enable the Gi0/1 port as a bridge virtual interface

By default, the physical ports on a 3560 switch are Layer 2 interfaces. To make them routed ports, the interface command no switchport should be used. The other options do not describe the purpose of this command.

5 / 10

Which scalable method must be implemented in order to provide inter-VLAN routing on a switched network with more than 1000 VLANs?

connecting each physical router interface to a different physical switch port, with each switch port assigned to a different VLAN

connecting a router interface to a switch port that is configured in trunk mode to route packets between VLANs, with each VLAN assigned to a router subinterface

configuring static routes on a Layer 2 switch device

routing traffic internally to a Layer 3 switch device

Layer 2 switches are able to perform static routing, but this method is inefficient with a large number of VLANs. Multilayer switching is more scalable than any other inter-VLAN routing implementation, with traffic being routed internally to the switch device. In router-on-a-stick inter-VLAN routing, where a single physical interface routes traffic among multiple VLANs on a network, there is no practical scalability. The legacy inter-VLAN routing is very inefficient and is no longer used in switched networks, because each VLAN requires a physical router interface that is connected to a different physical switch port.

6 / 10

When configuring a router as part of a router-on-a-stick inter-VLAN routing topology, where should the IP address be assigned?

to the VLAN

to the SVI

to the interface

to the subinterface

The IP address and the encapsulation type should be assigned to each router subinterface in a router-on-a-stick inter-VLAN topology.

7 / 10

While configuring inter-VLAN routing on a multilayer switch, a network administrator issues the no switchport command on an interface that is connected to another switch. What is the purpose of this command?

to provide a static trunk link

to create a switched virtual interface

to create a routed port for a single network

to provide an access link that tags VLAN traffic

When a Layer 2 interface on a multilayer switch is configured with the no switchport command, it becomes a routed port. A routed port is configured with an IP address for a specific subnet.

8 / 10

Which type of inter-VLAN communication design requires the configuration of multiple subinterfaces?

legacy inter-VLAN routing

routing for the management VLAN

router on a stick

routing via a multilayer switch

The router-on-a-stick design always includes subinterfaces on a router. When a multilayer switch is used, multiple SVIs are created. When the number of VLANs equals the number of ports on a router, or when the management VLAN needs to be routed, any of the inter-VLAN design methods can be used.

9 / 10

What is a disadvantage of using router-on-a-stick inter-VLAN routing?

does not support VLAN-tagged packets

does not scale well beyond 50 VLANs

requires the use of more physical interfaces than legacy inter-VLAN routing

requires the use of multiple router interfaces configured to operate as access links

Router-on-a-stick inter-VLAN routing does not scale beyond 50 VLANs. The router can receive VLAN-tagged packets and send VLAN-tagged packets to a destination. Router-on-a-stick inter-VLAN routing can utilize a single router interface as a trunk link to receive and forward VLAN traffic and does not require multiple interfaces.

10 / 10

A small college uses VLAN 10 for the classroom network and VLAN 20 for the office network. What is needed to enable communication between these two VLANs while using legacy inter-VLAN routing?

A router with one VLAN interface is needed to connect to the SVI on a switch.

A router with at least two LAN interfaces should be used.

A switch with a port that is configured as trunk is needed to connect to a router.

Two groups of switches are needed, each with ports that are configured for one VLAN.

With legacy inter-VLAN routing, different physical router interfaces are connected to different physical switch ports. The switch ports that connect to the router are in access mode, each belonging to a different VLAN. Switches can have ports that are assigned to different VLANs, but communication between VLANs requires routing function from the router.

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