How Bridging Works - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

Complete Software Guide for Junos

How Bridging Works

1502
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OS for EX Series Ethernet Switches, Release 10.4
standardized by the IEEE in 1982, in IEEE 802.3. Ethernet is used to create LANs. The
network devices, called nodes, on the LAN transmit data in bundles that are generally
called frames or packets.
Each node on a LAN has a unique identifier so that it can be unambiguously located on
the network. Ethernet uses the Layer 2 media access control (MAC) address for this
purpose. MAC addresses are hardware addresses that are programmed ("burned") into
the Ethernet processor in the node.
A characteristic of Ethernet is that nodes on a LAN can transmit data frames at any time.
However, the physical connecting cable between the nodes—either coaxial, copper-based
(Category 5), or optical cable—can carry only a single stream of data at a time. One result
of this design is that when two nodes transmit at the same time, their frames can collide
on the cable and generate an error. Ethernet uses a protocol called carrier-sense multiple
access with collision detection (CSMA/CD) to detect frame collisions. If a node receives
a collision error message, it stops transmitting immediately and waits for a period of time
before trying to send the frame again. If the node continues to detect collisions, it
progressively increases the time between retransmissions in an attempt to find a time
when no other data is being transmitted on the LAN. The node uses a backoff algorithm
to calculate the increasing retransmission time intervals.
Ethernet LANs were originally implemented for small, simple networks that carried
primarily text. Over time, LANs have become larger and more complex; the type of data
they carry has grown to include voice, graphics, and video; and the increased speed of
Ethernet interfaces on LANs has resulted in exponential increases in traffic on the network.
The IEEE 802.1D-2004 standard addresses some of the problems caused by the increase
in LAN and complexity. This standard defines transparent bridging (generally called simply
bridging). Bridging divides a single physical LAN (a single broadcast domain) into two or
more virtual LANs, or VLANs. Each VLAN is a collection of network nodes that are grouped
together to form separate broadcast domains. On an Ethernet network that is a single
LAN, all traffic is forwarded to all nodes on the LAN. On VLANs, frames whose origin and
destination are in the same VLAN are forwarded only within the local VLAN. Frames that
are not destined for the local VLAN are the only ones forwarded to other broadcast
domains. VLANs thus limit the amount of traffic flowing across the entire LAN, reducing
the possible number of collisions and packet retransmissions within a VLAN and on the
LAN as a whole.
On an Ethernet LAN, all network nodes must be physically connected to the same network.
On VLANs, the physical location of the nodes is not important, so you can group network
devices in any way that makes sense for your organization, such as by department or
business function, types of network nodes, or even physical location. Each VLAN is
identified by a single IP subnetwork and by standardized IEEE 802.1Q encapsulation
(discussed below).
The transparent bridging protocol allows a switch to learn information about all the
nodes on the LAN, including nodes on all the different VLANs. The switch uses this
information to create address-lookup tables, called Ethernet switching tables that it
consults when forwarding traffic to or toward a destination on the LAN.
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