Ethernet Switching on EX Series Switches - Juniper Networks

<strong>Ethernet</strong> <strong>Switching</strong> on EX Series Switches
History of VLANs
<strong>Ethernet</strong> LANs were originally designed for small, simple networks that primarily carried
text. However, over time, the type of data carried by LANs grew to include voice, graphics,
and video. This more complex data, when combined with the ever-increasing speed of
transmission, eventually became too much of a load for the original <strong>Ethernet</strong> LAN design.
Multiple packet collisions were significantly slowing down the larger LANs.
The IEEE 802.1D-2004 standard helped evolve <strong>Ethernet</strong> LANs to cope with the higher
data and transmission requirements by defining the concept of transparent bridging
(generally called simply bridging). Bridging divides a single physical LAN (now called a
single broadcast domain) into two or more virtual LANs, or VLANs. Each VLAN is a
collection of some of the LAN nodes grouped together to form individual broadcast
domains.
How Bridging of VLAN Traffic Works
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When VLANs are grouped logically by function or organization, a significant percentage
of data traffic stays within the VLAN. This relieves the load on the LAN because all traffic
no longer has to be forwarded to all nodes on the LAN. A VLAN first transmits packets
within the VLAN, thereby reducing the number of packets transmitted on the entire LAN.
Because packets whose origin and destination are in the same VLAN are forwarded only
within the local VLAN, packets that are not destined for the local VLAN are the only ones
forwarded to other broadcast domains. This way, bridging and VLANs limit the amount
of traffic flowing across the entire LAN by reducing the possible number of collisions and
packet retransmissions within VLANs and on the LAN as a whole.
Because the objective of the IEEE 802.1D-2004 standard was to reduce traffic and
therefore reduce potential transmission collisions for <strong>Ethernet</strong> , a system was implemented
to reuse information. Instead of having a switch go through a location process every time
a frame is sent to a node, the transparent bridging protocol allows a switch to record the
location of known nodes. When packets are sent to nodes, those destination node
locations are stored in address-lookup tables called <strong>Ethernet</strong> switching tables. Before
sending a packet, a switch using bridging first consults the switching tables to see if that
node has already been located. If the location of a node is known, the frame is sent directly
to that node.
Transparent bridging uses five mechanisms to create and maintain <strong>Ethernet</strong> switching
tables on the switch:
• Learning
• Forwarding
• Flooding
• Filtering
• Aging
The key bridging mechanism used by LANs and VLANs is learning. When a switch is first
connected to an <strong>Ethernet</strong> LAN or VLAN, it has no information about other nodes on the
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