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Transparent bridging uses five mechanisms to create and maintain Ethernet switching
tables on the switch:
•
Learning
• Forwarding
• Flooding
•
Filtering
•
Aging
The first bridging mechanism is learning . When a switch is first connected to an Ethernet
LAN or VLAN, it has no information about other nodes on the network. The switch goes
through a learning process to obtain the MAC addresses of all the nodes on the network.
It stores these in the Ethernet switching table. To learn MAC addresses, the switch reads
all packets that it detects on the LAN or on the local VLAN, looking for MAC addresses
of sending nodes. It places these addresses into its Ethernet switching table, along with
two other pieces of information—the interface (or port) on which the traffic was received
and the time when the address was learned.
The second bridging mechanism is forwarding . Switches forward traffic, passing it from
an incoming interface to an outgoing interface that leads to or toward the destination.
To forward frames, the switch consults the Ethernet switching table to see whether the
table contains the MAC address corresponding to the frames' destination. If the Ethernet
switching table contains an entry for the desired destination address, the switch sends
the traffic out the interface associated with the MAC address. The switch also consults
the Ethernet switching table in the same way when transmitting frames that originate
on devices connected directly to the switch. If the Ethernet switching table does not
contain an entry for the desired destination address, the switch uses flooding, which is
the third bridging mechanism.
Flooding is how the switch learns about destinations not in its Ethernet switching table.
If this table has no entry for a particular destination MAC address, the switch floods the
traffic out all interfaces except the interface on which it was received. (If traffic originates
on the switch, the switch floods it out all interfaces.) When the destination node receives
the flooded traffic, it sends an acknowledgment packet back to the switch, allowing it
to learn the MAC address of the node and to add the address to its Ethernet switching
table.
Filtering , the fourth bridging mechanism, is how broadcast traffic is limited to the local
VLAN whenever possible. As the number of entries in the Ethernet switching table grows,
the switch pieces together an increasingly complete picture of the VLAN and the larger
LAN—of which nodes are in the local VLAN and which are on other network segments.
The switch uses this information to filter traffic. Specifically, for traffic whose source and
destination MAC addresses are in the local VLAN, filtering prevents the switch from
forwarding this traffic to other network segments.
Finally, the switch uses aging , the fifth bridging mechanism, to keep the entries in the
Ethernet switching table current. For each MAC address in the Ethernet switching table,
the switch records a timestamp of when the information about the network node was
learned. Each time the switch detects traffic from a MAC address, it updates the
1285 Copyright © 2010, Juniper Networks, Inc.
Chapter 57: Bridging and VLANs—Overview