How to Trunk Together VLANs on Junos Switches

By Walter J. Goralski, Cathy Gadecki, and Michael Bushong from JUNOS OS For Dummies, 2nd Edition

Access ports are simply ports that connect to network devices. By default, all switch ports are in access mode, so you don’t need to specify this mode in the configuration. When you connect one switch to another, or to a router, they are usually connected with trunk ports.

On trunk ports, the devices add a short header to each Ethernet frame, which includes the VLAN ID. The receiving device reads the VLAN ID and puts the traffic into the correct VLAN. This information in the header lets the two devices exchange traffic for multiple VLANs, while keeping all the data straight.

You convert a port into a trunk port simply by configuring it to be a trunk port. Although you can make any port a trunk port, you generally connect switches together using the uplink ports, which are numbered starting at ge-0/1/0 or xe-0/1/0. (Depending on the uplink module, it will have either two or four ports.)

Suppose that the physics and chemistry departments have two separate switches and you've connected them by plugging in a cable to ge-0/1/0 on the physics side and to ge-0/1/1 on the chemistry side. Here’s how you configure the trunk port on the physics switch:

[edit interfaces]
user@physics# set ge-0/1/0 unit 0 family ethernet-switching port-mode trunk
 
[edit interfaces]
user@physics# set ge-0/1/0 unit 0 family ethernet-switching vlan members
  [ physics chemistry ]

The configuration on the chemistry switch is similar:

[edit interfaces]
user@chemistry# set ge-0/1/1 unit 0 family ethernet-switching port-mode trunk
user@chemistry# set ge-0/1/1 unit 0 family ethernet-switching vlan members
  [ physics chemistry ]