Juniper JUNOS OS 10.3 - SOFTWARE, Manual, Page: 1382 / 3946

timestamp. A timer on the switch periodically checks the timestamp, and if it is older
than a user-configured value, the switch removes the node's MAC address from the
Ethernet switching table. This aging process ensures that the switch tracks only active
nodes on the network and that it is able to flush out network nodes that are no longer
available.
Types of Switch Ports
The ports, or interfaces, on a switch operate in either access mode or trunk mode.
An interface in access mode connects to a network device, such as a desktop computer,
an IP telephone, a printer, a file server, or a security camera. The interface itself belongs
to a single VLAN. The frames transmitted over an access interface are normal Ethernet
frames. By default, when you boot a switch and use the factory-default configuration,
or when you boot the switch and do not explicitly configure a port mode, all interfaces
on the switch are in access mode.
Trunk interfaces handle traffic for multiple VLANs, multiplexing the traffic for all those
VLANs over the same physical connection. Trunk interfaces are generally used to
interconnect switches to one another.
IEEE 802.1Q Encapsulation and Tags
To identify which VLAN traffic belongs to, all frames on an Ethernet VLAN are identified
by a tag, as defined in the IEEE 802.1Q standard. These frames are
tagged and are
encapsulated with 802.1Q tags.
For a simple network that has only a single VLAN, all traffic has the same 802.1Q tag.
When an Ethernet LAN is divided into VLANs, each VLAN is identified by a unique 802.1Q
tag. The tag is applied to all frames so that the network nodes receiving the frames know
which VLAN the frames belong to. Trunk ports, which multiplex traffic among a number
of VLANs, use the tag to determine to origin of frames and where to forward them.
VLANs 0 and 4095 are reserved by the Juniper Networks Junos operating system (Junos
OS), so you cannot use them in your network.
Assignment of Traffic to VLANs
You assign traffic to a particular VLAN in one of the following ways:
•
By interface (port) on the switch. You specify that all traffic received on a particular
interface on the switch is assigned to a specific VLAN. If you use the default factory
switch settings, all traffic received on an access interface is untagged. This traffic is
part of a default VLAN, but it is not tagged with an 802.1Q tag. When configuring the
switch, you specify which VLAN to assign the traffic to. You configure the VLAN either
by using a VLAN number (called a VLAN ID) or by using a name, which the switch
translates into a numeric VLAN ID.
•
By MAC address. You can specify that all traffic received from a specific MAC address
be forwarded to a specific egress interface (next hop) on the switch. This method is
administratively cumbersome to configure manually, but it can be useful when you are
using automated databases to manage the switches on your network.
Copyright © 2010, Juniper Networks, Inc. 1286
Complete Software Guide for Junos
®
OS for EX Series Ethernet Switches, Release 10.3