Segmenting a LAN with VLANs

To account for packet collisions that occur with broadcasting, Ethernet uses the carrier-sense multiple access with collection detection (CSMA/CD) protocol to detect frame collisions and to signal each device on the network to retransmit frames that aren't successfully sent the first time because of collisions.

Configuring the default VLAN

When you power on smaller 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:

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; } ...

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