Inter-VLAN Routing

Time: 30 min

Needs: one Ethernet switch, one OL9 PC, 4 VPCS

GNS3 project: w04-a4-intervlan

Build it

Drag an Ethernet Switch on to GNS3 canvas, name it SW1. Right-click it → Configure, and configure the ports:

Cable PC1 and PC2 to the VLAN 10 ports; assign them 192.168.10.11/24 and 192.168.10.12/24 respectively.

Cable PC3 and PC4 to the VLAN 20 ports; assign them 192.168.10.13/24 and 192.168.10.14/24 respectively.

Confirm PC1 can reach PC2 but not the other two.

Part 1 — Change the addressing

On PC1:

ip 192.168.10.11/24 192.168.10.1
save

On PC2:

ip 192.168.10.12/24 192.168.10.1
save

On PC3:

ip 192.168.20.13/24 192.168.20.1
save

On PC4:

ip 192.168.20.14/24 192.168.20.1
save

What just happened

• Routing works at Layer 3. A router forwards between networks. If the hosts are in the same subnet, they will never consult a gateway at all.
• Thus, VLAN 10 and VLAN 20 now map to two separate subnets.
• Note the direction of the dependency: the layer-2 decision (which VLAN) forces the layer-3 decision (which subnet), and not the other way round.

Part 2 — Configure routing

Add an OL9 PC (we are using a PC node because we require only one physical interface) named R1 and cable it to SW1:Ethernet4. That port has to carry both VLANs, so set it configure it accordingly:

Log on to R1 as root, then:

nmcli con mod 'System eth0' con-name eth0 \
ipv4.method disabled ipv6.method disabled

nmcli con up eth0

nmcli con add type vlan ifname eth0.10 con-name eth0.10 dev eth0 id 10 \
ipv4.method manual ipv4.addresses 192.168.10.1/24

nmcli con add type vlan ifname eth0.20 con-name eth0.20 dev eth0 id 20 \
ipv4.method manual ipv4.addresses 192.168.20.1/24

nmcli con up eth0.10
nmcli con up eth0.20

echo 'net.ipv4.ip_forward=1' | tee -a /etc/sysctl.d/99-ipv4-routing.conf

ip addr show
ip route show

Start three captures simultaneously on PC1, R1 and PC3 cables.

From PC1: ping 192.168.20.13.

Fill in the following table:

What just happened

• R1 was able to forward the packets across both VLANs.
• eth0.10 and eth0.20 are two sub-interfaces on a single physical port, distinguished entirely by their 802.1Q (VLAN) tag. The router can tag the frames just as the switch does. It needs to be able to do so because routing involves re-framing the packet.

If it went wrong