How to Configure the Default VLAN on Junos Switches

When you power on smaller Juniper EX models (or an individual EX 4200 that isn’t part of a Virtual Chassis unit), bridging is enabled on all the interfaces (ports) on the switch, and all the ports are part of a preconfigured VLAN named default.

So, for a small office or research lab, all you need to do to get the Juniper switch up and running is to power on the device, perform the initial configuration to give the switch a name and time, set the root password, and then connect your laptops, PCs, WAPs, printers, file servers, and other devices to the ports on the front of the switch.

The great thing about this setup is that you don’t need to open the switch’s configuration file to set up anything. The JUNOS software handles everything automatically. You can at some point look in the configuration file to see the results of the automatic configuration. The automatic switching portion of the configuration will look as follows:

[edit]
user@junos-switch# show
[...]
interfaces {
 ge-0/0/0 {
 unit 0 {
  family ethernet-switching;
 }
 }
 ge-0/0/1 {
 unit 0 {
  family ethernet-switching;
 }
 }
 [...]
}
protocols {
 lldp {
 interface all;
 }
 rstp;
}
poe {
 interface all;
}

Look at the configuration to see that the ge-0/0/1 interface supports bridging. First, the interface is configured with the ethernet-switching keyword, which enables the JUNOS Ethernet switch protocol family. Second, the protocols section of the configuration shows that two bridging protocols are enabled, rstp and lldp. The switch uses the Link Layer Discovery Protocol (LLDP) to learn which network devices are attached to each port on the switch.

A switch may find multiple paths to another device, or the path that it finds may go in circles, creating a loop. The second protocol, RSTP, is a rapid version of the original Spanning Tree Protocol (STP) that prevents loops in a bridged LAN or within a VLAN.

Spanning tree protocols also determine new paths to devices on the VLAN after the topology in the VLAN changes — for example, when you add or move a laptop or a printer. The convergence times for RSTP are faster than those for STP.

The last protocol in the configuration is Power over Ethernet (PoE), which is enabled on all switch interfaces. This protocol ensures that PoE is enabled on all the ports that support PoE.

Another way to check that the default VLAN is configured on the switch is to use the show vlans command to list all the VLANs.

user@junos-switch> show vlans
Name   Tag  Interfaces
default
      ge-0/0/0.0, ge-0/0/1.0, [...]
mgmt
      me0.0

The output shows two VLANs: default, which contains the ge-0/0/1 interface that you have plugged into the laptop, and mgmt, which is a switch management interface.

The default VLAN has no VLAN ID, or tag, to identify which VLAN arriving packets originated from, so no tag name is listed in the Tag column. All packets sent on this VLAN are untagged, which is fine because the network has only one VLAN.

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Source:http://www.dummies.com/how-to/content/how-to-configure-the-default-vlan-on-junos-switche.html