Trunks

Introduction

VLAN trunking is the practice of aggregating multiple network links into a single link. As each VLAN is relative to a switch Trunks can make two switches act like a single switch.

Trunking, with VLAN Aware bridging, is relatively simple as it builds on itself.

Example Configuration

Consider the topology:

Network Configuration

Let’s say we wanted to configure PC1 and PC3 to be on VLAN 10 while PC2 and PC4 would be on VLAN 20. By building a trunk, we can communicate over the two switches with one link.

First, ensure each switch has a bridge for the interfaces in use. In this example, enp0s4, enp0s5, and enp0s6 are in use on both switches.

On Switch 1 and Switch 2, run the following:

$ ip link add name br0 type bridge

$ ip link set dev enp0s4 master br0

$ ip link set dev enp0s5 master br0

$ ip link set dev enp0s6 master br0

Now Bring all devices up with

$ ip link set br0 up

$ ip link set enp0s4 up

$ ip link set enp0s5 up

$ ip link set enp0s6 up

Communication over the switches is now possible between all PCs.

To establish the VLANs toggle VLAN Aware mode on.

On Switch 1 and Switch 2, run the following:

$ ip link set dev br0 type bridge vlan_filtering 1

Next, add the desired VLAN tags to each PC interface. Then, add both tags to the two interfaces connecting the switches. In this example, the link for both switches is on their respective enp0s6.

On Switch 1 and Switch 2, run the following:

$ bridge vlan add dev enp0s4 vid 10 pvid untagged master

$ bridge vlan add dev enp0s5 vid 20 pvid untagged master

$ bridge vlan add dev enp0s6 vid 10

$ bridge vlan add dev enp0s6 vid 20

All four PCS can now only communicate over the trunk with the respective PCs who share their VLAN.

NOTE: The outputs below were tested on a Virtual Machine

PC1 : 192.168.0.1 255.255.255.0

PC1> ping 192.168.0.3

84 bytes from 192.168.0.3 icmp_seq=1 ttl=64 time=2.345 ms
84 bytes from 192.168.0.3 icmp_seq=2 ttl=64 time=1.093 ms
84 bytes from 192.168.0.3 icmp_seq=3 ttl=64 time=1.442 ms
^C
PC1> ping 192.168.0.2

host (192.168.0.2) not reachable

PC1> ping 192.168.0.4

host (192.168.0.4) not reachable

PC1>

PC2 : 192.168.0.2 255.255.255.0

PC2> ping 192.168.0.4

84 bytes from 192.168.0.4 icmp_seq=1 ttl=64 time=2.345 ms
84 bytes from 192.168.0.4 icmp_seq=2 ttl=64 time=1.093 ms
84 bytes from 192.168.0.4 icmp_seq=3 ttl=64 time=1.442 ms
^C
PC2> ping 192.168.0.1

host (192.168.0.1) not reachable

PC2> ping 192.168.0.3

host (192.168.0.3) not reachable

PC2>